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| author | Gerald Carter <jerry@samba.org> | 2003-07-16 05:42:34 +0000 |
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| committer | Gerald Carter <jerry@samba.org> | 2003-07-16 05:42:34 +0000 |
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diff --git a/docs/htmldocs/locking.html b/docs/htmldocs/locking.html new file mode 100644 index 0000000000..a128cac7ce --- /dev/null +++ b/docs/htmldocs/locking.html @@ -0,0 +1,656 @@ +<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 14. File and Record Locking</title><link rel="stylesheet" href="samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.60.1"><link rel="home" href="index.html" title="SAMBA Project Documentation"><link rel="up" href="optional.html" title="Part III. Advanced Configuration"><link rel="previous" href="AccessControls.html" title="Chapter 13. File, Directory and Share Access Controls"><link rel="next" href="securing-samba.html" title="Chapter 15. Securing Samba"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 14. File and Record Locking</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="AccessControls.html">Prev</a> </td><th width="60%" align="center">Part III. Advanced Configuration</th><td width="20%" align="right"> <a accesskey="n" href="securing-samba.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="locking"></a>Chapter 14. File and Record Locking</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Jeremy</span> <span class="surname">Allison</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><tt class="email"><<a href="mailto:jra@samba.org">jra@samba.org</a>></tt></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Jelmer</span> <span class="othername">R.</span> <span class="surname">Vernooij</span></h3><div class="affiliation"><span class="orgname">The Samba Team<br></span><div class="address"><p><tt class="email"><<a href="mailto:jelmer@samba.org">jelmer@samba.org</a>></tt></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">John</span> <span class="othername">H.</span> <span class="surname">Terpstra</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><tt class="email"><<a href="mailto:jht@samba.org">jht@samba.org</a>></tt></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">Eric</span> <span class="surname">Roseme</span></h3><div class="affiliation"><span class="orgname">HP Oplocks Usage Recommendations Whitepaper<br></span><div class="address"><p><tt class="email"><<a href="mailto:eric.roseme@hp.com">eric.roseme@hp.com</a>></tt></p></div></div></div></div></div><div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><a href="locking.html#id2928216">Features and Benefits</a></dt><dt><a href="locking.html#id2928272">Discussion</a></dt><dd><dl><dt><a href="locking.html#id2928403">Opportunistic Locking Overview</a></dt></dl></dd><dt><a href="locking.html#id2929049">Samba Opportunistic Locking Control</a></dt><dd><dl><dt><a href="locking.html#id2929159">Example Configuration</a></dt></dl></dd><dt><a href="locking.html#id2929419">MS Windows Opportunistic Locking and Caching Controls</a></dt><dd><dl><dt><a href="locking.html#id2929649">Workstation Service Entries</a></dt><dt><a href="locking.html#id2929676">Server Service Entries</a></dt></dl></dd><dt><a href="locking.html#id2929755">Persistent Data Corruption</a></dt><dt><a href="locking.html#id2929785">Common Errors</a></dt><dd><dl><dt><a href="locking.html#id2929859">locking.tdb error messages</a></dt></dl></dd><dt><a href="locking.html#id2929890">Additional Reading</a></dt></dl></div><p> +One area which causes trouble for many network administrators is locking. +The extent of the problem is readily evident from searches over the internet. +</p><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2928216"></a>Features and Benefits</h2></div></div><div></div></div><p> +Samba provides all the same locking semantics that MS Windows clients expect +and that MS Windows NT4 / 200x servers provide also. +</p><p> +The term <span class="emphasis"><em>locking</em></span> has exceptionally broad meaning and covers +a range of functions that are all categorized under this one term. +</p><p> +Opportunistic locking is a desirable feature when it can enhance the +perceived performance of applications on a networked client. However, the +opportunistic locking protocol is not robust, and therefore can +encounter problems when invoked beyond a simplistic configuration, or +on extended, slow, or faulty networks. In these cases, operating +system management of opportunistic locking and/or recovering from +repetitive errors can offset the perceived performance advantage that +it is intended to provide. +</p><p> +The MS Windows network administrator needs to be aware that file and record +locking semantics (behaviour) can be controlled either in Samba or by way of registry +settings on the MS Windows client. +</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p> +Sometimes it is necessary to disable locking control settings BOTH on the Samba +server as well as on each MS Windows client! +</p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2928272"></a>Discussion</h2></div></div><div></div></div><p> +There are two types of locking which need to be performed by a SMB server. +The first is <span class="emphasis"><em>record locking</em></span> which allows a client to lock +a range of bytes in a open file. The second is the <span class="emphasis"><em>deny modes</em></span> +that are specified when a file is open. +</p><p> +Record locking semantics under Unix is very different from record locking under +Windows. Versions of Samba before 2.2 have tried to use the native fcntl() unix +system call to implement proper record locking between different Samba clients. +This can not be fully correct due to several reasons. The simplest is the fact +that a Windows client is allowed to lock a byte range up to 2^32 or 2^64, +depending on the client OS. The unix locking only supports byte ranges up to 2^31. +So it is not possible to correctly satisfy a lock request above 2^31. There are +many more differences, too many to be listed here. +</p><p> +Samba 2.2 and above implements record locking completely independent of the +underlying unix system. If a byte range lock that the client requests happens +to fall into the range 0-2^31, Samba hands this request down to the Unix system. +All other locks can not be seen by unix anyway. +</p><p> +Strictly a SMB server should check for locks before every read and write call on +a file. Unfortunately with the way fcntl() works this can be slow and may overstress +the <b class="command">rpc.lockd</b>. It is also almost always unnecessary as clients are supposed to +independently make locking calls before reads and writes anyway if locking is +important to them. By default Samba only makes locking calls when explicitly asked +to by a client, but if you set <i class="parameter"><tt>strict locking = yes</tt></i> then it +will make lock checking calls on every read and write. +</p><p> +You can also disable by range locking completely using <i class="parameter"><tt>locking = no</tt></i>. +This is useful for those shares that don't support locking or don't need it +(such as cdroms). In this case Samba fakes the return codes of locking calls to +tell clients that everything is OK. +</p><p> +The second class of locking is the <i class="parameter"><tt>deny modes</tt></i>. These +are set by an application when it opens a file to determine what types of +access should be allowed simultaneously with its open. A client may ask for +<tt class="constant">DENY_NONE</tt>, <tt class="constant">DENY_READ</tt>, +<tt class="constant">DENY_WRITE</tt> or <tt class="constant">DENY_ALL</tt>. There are also special compatibility +modes called <tt class="constant">DENY_FCB</tt> and <tt class="constant">DENY_DOS</tt>. +</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2928403"></a>Opportunistic Locking Overview</h3></div></div><div></div></div><p> +Opportunistic locking (Oplocks) is invoked by the Windows file system +(as opposed to an API) via registry entries (on the server AND client) +for the purpose of enhancing network performance when accessing a file +residing on a server. Performance is enhanced by caching the file +locally on the client which allows: +</p><div class="variablelist"><dl><dt><span class="term">Read-ahead:</span></dt><dd><p> + The client reads the local copy of the file, eliminating network latency + </p></dd><dt><span class="term">Write caching:</span></dt><dd><p> + The client writes to the local copy of the file, eliminating network latency + </p></dd><dt><span class="term">Lock caching:</span></dt><dd><p> + The client caches application locks locally, eliminating network latency + </p></dd></dl></div><p> +The performance enhancement of oplocks is due to the opportunity of +exclusive access to the file - even if it is opened with deny-none - +because Windows monitors the file's status for concurrent access from +other processes. +</p><div class="variablelist"><p class="title"><b>Windows defines 4 kinds of Oplocks:</b></p><dl><dt><span class="term">Level1 Oplock:</span></dt><dd><p> + The redirector sees that the file was opened with deny + none (allowing concurrent access), verifies that no + other process is accessing the file, checks that + oplocks are enabled, then grants deny-all/read-write/exclusive + access to the file. The client now performs + operations on the cached local file. + </p><p> + If a second process attempts to open the file, the open + is deferred while the redirector "breaks" the original + oplock. The oplock break signals the caching client to + write the local file back to the server, flush the + local locks, and discard read-ahead data. The break is + then complete, the deferred open is granted, and the + multiple processes can enjoy concurrent file access as + dictated by mandatory or byte-range locking options. + However, if the original opening process opened the + file with a share mode other than deny-none, then the + second process is granted limited or no access, despite + the oplock break. + </p></dd><dt><span class="term">Level2 Oplock:</span></dt><dd><p> + Performs like a level1 oplock, except caching is only + operative for reads. All other operations are performed + on the server disk copy of the file. + </p></dd><dt><span class="term">Filter Oplock:</span></dt><dd><p> + Does not allow write or delete file access + </p></dd><dt><span class="term">Batch Oplock:</span></dt><dd><p> + Manipulates file openings and closings - allows caching + of file attributes + </p></dd></dl></div><p> +An important detail is that oplocks are invoked by the file system, not +an application API. Therefore, an application can close an oplocked +file, but the file system does not relinquish the oplock. When the +oplock break is issued, the file system then simply closes the file in +preparation for the subsequent open by the second process. +</p><p> +<span class="emphasis"><em>Opportunistic Locking</em></span> is actually an improper name for this feature. +The true benefit of this feature is client-side data caching, and +oplocks is merely a notification mechanism for writing data back to the +networked storage disk. The limitation of opportunistic locking is the +reliability of the mechanism to process an oplock break (notification) +between the server and the caching client. If this exchange is faulty +(usually due to timing out for any number of reasons) then the +client-side caching benefit is negated. +</p><p> +The actual decision that a user or administrator should consider is +whether it is sensible to share amongst multiple users data that will +be cached locally on a client. In many cases the answer is no. +Deciding when to cache or not cache data is the real question, and thus +"opportunistic locking" should be treated as a toggle for client-side +caching. Turn it "ON" when client-side caching is desirable and +reliable. Turn it "OFF" when client-side caching is redundant, +unreliable, or counter-productive. +</p><p> +Opportunistic locking is by default set to "on" by Samba on all +configured shares, so careful attention should be given to each case to +determine if the potential benefit is worth the potential for delays. +The following recommendations will help to characterize the environment +where opportunistic locking may be effectively configured. +</p><p> +Windows Opportunistic Locking is a lightweight performance-enhancing +feature. It is not a robust and reliable protocol. Every +implementation of Opportunistic Locking should be evaluated as a +tradeoff between perceived performance and reliability. Reliability +decreases as each successive rule above is not enforced. Consider a +share with oplocks enabled, over a wide area network, to a client on a +South Pacific atoll, on a high-availability server, serving a +mission-critical multi-user corporate database, during a tropical +storm. This configuration will likely encounter problems with oplocks. +</p><p> +Oplocks can be beneficial to perceived client performance when treated +as a configuration toggle for client-side data caching. If the data +caching is likely to be interrupted, then oplock usage should be +reviewed. Samba enables opportunistic locking by default on all +shares. Careful attention should be given to the client usage of +shared data on the server, the server network reliability, and the +opportunistic locking configuration of each share. +n mission critical high availability environments, data integrity is +often a priority. Complex and expensive configurations are implemented +to ensure that if a client loses connectivity with a file server, a +failover replacement will be available immediately to provide +continuous data availability. +</p><p> +Windows client failover behavior is more at risk of application +interruption than other platforms because it is dependant upon an +established TCP transport connection. If the connection is interrupted +- as in a file server failover - a new session must be established. +It is rare for Windows client applications to be coded to recover +correctly from a transport connection loss, therefore most applications +will experience some sort of interruption - at worst, abort and +require restarting. +</p><p> +If a client session has been caching writes and reads locally due to +opportunistic locking, it is likely that the data will be lost when the +application restarts, or recovers from the TCP interrupt. When the TCP +connection drops, the client state is lost. When the file server +recovers, an oplock break is not sent to the client. In this case, the +work from the prior session is lost. Observing this scenario with +oplocks disabled, and the client was writing data to the file server +real-time, then the failover will provide the data on disk as it +existed at the time of the disconnect. +</p><p> +In mission critical high availability environments, careful attention +should be given to opportunistic locking. Ideally, comprehensive +testing should be done with all affected applications with oplocks +enabled and disabled. +</p><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928695"></a>Exclusively Accessed Shares</h4></div></div><div></div></div><p> +Opportunistic locking is most effective when it is confined to shares +that are exclusively accessed by a single user, or by only one user at +a time. Because the true value of opportunistic locking is the local +client caching of data, any operation that interrupts the caching +mechanism will cause a delay. +</p><p> +Home directories are the most obvious examples of where the performance +benefit of opportunistic locking can be safely realized. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928719"></a>Multiple-Accessed Shares or Files</h4></div></div><div></div></div><p> +As each additional user accesses a file in a share with opportunistic +locking enabled, the potential for delays and resulting perceived poor +performance increases. When multiple users are accessing a file on a +share that has oplocks enabled, the management impact of sending and +receiving oplock breaks, and the resulting latency while other clients +wait for the caching client to flush data, offset the performance gains +of the caching user. +</p><p> +As each additional client attempts to access a file with oplocks set, +the potential performance improvement is negated and eventually results +in a performance bottleneck. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928748"></a>Unix or NFS Client Accessed Files</h4></div></div><div></div></div><p> +Local Unix and NFS clients access files without a mandatory +file locking mechanism. Thus, these client platforms are incapable of +initiating an oplock break request from the server to a Windows client +that has a file cached. Local Unix or NFS file access can therefore +write to a file that has been cached by a Windows client, which +exposes the file to likely data corruption. +</p><p> +If files are shared between Windows clients, and either local Unix +or NFS users, then turn opportunistic locking off. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928775"></a>Slow and/or Unreliable Networks</h4></div></div><div></div></div><p> +The biggest potential performance improvement for opportunistic locking +occurs when the client-side caching of reads and writes delivers the +most differential over sending those reads and writes over the wire. +This is most likely to occur when the network is extremely slow, +congested, or distributed (as in a WAN). However, network latency also +has a very high impact on the reliability of the oplock break +mechanism, and thus increases the likelihood of encountering oplock +problems that more than offset the potential perceived performance +gain. Of course, if an oplock break never has to be sent, then this is +the most advantageous scenario to utilize opportunistic locking. +</p><p> +If the network is slow, unreliable, or a WAN, then do not configure +opportunistic locking if there is any chance of multiple users +regularly opening the same file. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928808"></a>Multi-User Databases</h4></div></div><div></div></div><p> +Multi-user databases clearly pose a risk due to their very nature - +they are typically heavily accessed by numerous users at random +intervals. Placing a multi-user database on a share with opportunistic +locking enabled will likely result in a locking management bottleneck +on the Samba server. Whether the database application is developed +in-house or a commercially available product, ensure that the share +has opportunistic locking disabled. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928829"></a>PDM Data Shares</h4></div></div><div></div></div><p> +Process Data Management (PDM) applications such as IMAN, Enovia, and +Clearcase, are increasing in usage with Windows client platforms, and +therefore SMB data stores. PDM applications manage multi-user +environments for critical data security and access. The typical PDM +environment is usually associated with sophisticated client design +applications that will load data locally as demanded. In addition, the +PDM application will usually monitor the data-state of each client. +In this case, client-side data caching is best left to the local +application and PDM server to negotiate and maintain. It is +appropriate to eliminate the client OS from any caching tasks, and the +server from any oplock management, by disabling opportunistic locking on +the share. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928870"></a>Beware of Force User</h4></div></div><div></div></div><p> +Samba includes an <tt class="filename">smb.conf</tt> parameter called <i class="parameter"><tt>force user</tt></i> that changes +the user accessing a share from the incoming user to whatever user is +defined by the smb.conf variable. If opportunistic locking is enabled +on a share, the change in user access causes an oplock break to be sent +to the client, even if the user has not explicitly loaded a file. In +cases where the network is slow or unreliable, an oplock break can +become lost without the user even accessing a file. This can cause +apparent performance degradation as the client continually reconnects +to overcome the lost oplock break. +</p><p> +Avoid the combination of the following: +</p><div class="itemizedlist"><ul type="disc"><li><p> + <i class="parameter"><tt>force user</tt></i> in the <tt class="filename">smb.conf</tt> share configuration. + </p></li><li><p> + Slow or unreliable networks + </p></li><li><p> + Opportunistic Locking Enabled + </p></li></ul></div></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928948"></a>Advanced Samba Opportunistic Locking Parameters</h4></div></div><div></div></div><p> +Samba provides opportunistic locking parameters that allow the +administrator to adjust various properties of the oplock mechanism to +account for timing and usage levels. These parameters provide good +versatility for implementing oplocks in environments where they would +likely cause problems. The parameters are: +<i class="parameter"><tt>oplock break wait time</tt></i>, +<i class="parameter"><tt>oplock contention limit</tt></i>. +</p><p> +For most users, administrators, and environments, if these parameters +are required, then the better option is to simply turn oplocks off. +The samba SWAT help text for both parameters reads "DO NOT CHANGE THIS +PARAMETER UNLESS YOU HAVE READ AND UNDERSTOOD THE SAMBA OPLOCK CODE." +This is good advice. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2928991"></a>Mission Critical High Availability</h4></div></div><div></div></div><p> +In mission critical high availability environments, data integrity is +often a priority. Complex and expensive configurations are implemented +to ensure that if a client loses connectivity with a file server, a +failover replacement will be available immediately to provide +continuous data availability. +</p><p> +Windows client failover behavior is more at risk of application +interruption than other platforms because it is dependant upon an +established TCP transport connection. If the connection is interrupted +- as in a file server failover - a new session must be established. +It is rare for Windows client applications to be coded to recover +correctly from a transport connection loss, therefore most applications +will experience some sort of interruption - at worst, abort and +require restarting. +</p><p> +If a client session has been caching writes and reads locally due to +opportunistic locking, it is likely that the data will be lost when the +application restarts, or recovers from the TCP interrupt. When the TCP +connection drops, the client state is lost. When the file server +recovers, an oplock break is not sent to the client. In this case, the +work from the prior session is lost. Observing this scenario with +oplocks disabled, and the client was writing data to the file server +real-time, then the failover will provide the data on disk as it +existed at the time of the disconnect. +</p><p> +In mission critical high availability environments, careful attention +should be given to opportunistic locking. Ideally, comprehensive +testing should be done with all affected applications with oplocks +enabled and disabled. +</p></div></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2929049"></a>Samba Opportunistic Locking Control</h2></div></div><div></div></div><p> +Opportunistic Locking is a unique Windows file locking feature. It is +not really file locking, but is included in most discussions of Windows +file locking, so is considered a defacto locking feature. +Opportunistic Locking is actually part of the Windows client file +caching mechanism. It is not a particularly robust or reliable feature +when implemented on the variety of customized networks that exist in +enterprise computing. +</p><p> +Like Windows, Samba implements Opportunistic Locking as a server-side +component of the client caching mechanism. Because of the lightweight +nature of the Windows feature design, effective configuration of +Opportunistic Locking requires a good understanding of its limitations, +and then applying that understanding when configuring data access for +each particular customized network and client usage state. +</p><p> +Opportunistic locking essentially means that the client is allowed to download and cache +a file on their hard drive while making changes; if a second client wants to access the +file, the first client receives a break and must synchronise the file back to the server. +This can give significant performance gains in some cases; some programs insist on +synchronising the contents of the entire file back to the server for a single change. +</p><p> +Level1 Oplocks (aka just plain "oplocks") is another term for opportunistic locking. +</p><p> +Level2 Oplocks provides opportunistic locking for a file that will be treated as +<span class="emphasis"><em>read only</em></span>. Typically this is used on files that are read-only or +on files that the client has no initial intention to write to at time of opening the file. +</p><p> +Kernel Oplocks are essentially a method that allows the Linux kernel to co-exist with +Samba's oplocked files, although this has provided better integration of MS Windows network +file locking with the under lying OS, SGI IRIX and Linux are the only two OS's that are +oplock aware at this time. +</p><p> +Unless your system supports kernel oplocks, you should disable oplocks if you are +accessing the same files from both Unix/Linux and SMB clients. Regardless, oplocks should +always be disabled if you are sharing a database file (e.g., Microsoft Access) between +multiple clients, as any break the first client receives will affect synchronisation of +the entire file (not just the single record), which will result in a noticeable performance +impairment and, more likely, problems accessing the database in the first place. Notably, +Microsoft Outlook's personal folders (*.pst) react very badly to oplocks. If in doubt, +disable oplocks and tune your system from that point. +</p><p> +If client-side caching is desirable and reliable on your network, you will benefit from +turning on oplocks. If your network is slow and/or unreliable, or you are sharing your +files among other file sharing mechanisms (e.g., NFS) or across a WAN, or multiple people +will be accessing the same files frequently, you probably will not benefit from the overhead +of your client sending oplock breaks and will instead want to disable oplocks for the share. +</p><p> +Another factor to consider is the perceived performance of file access. If oplocks provide no +measurable speed benefit on your network, it might not be worth the hassle of dealing with them. +</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2929159"></a>Example Configuration</h3></div></div><div></div></div><p> +In the following we examine two distinct aspects of Samba locking controls. +</p><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2929172"></a>Disabling Oplocks</h4></div></div><div></div></div><p> +You can disable oplocks on a per-share basis with the following: +</p><p> +</p><pre class="programlisting"> +[acctdata] + oplocks = False + level2 oplocks = False +</pre><p> +</p><p> +The default oplock type is Level1. Level2 Oplocks are enabled on a per-share basis +in the <tt class="filename">smb.conf</tt> file. +</p><p> +Alternately, you could disable oplocks on a per-file basis within the share: +</p><p> +</p><pre class="programlisting"> + veto oplock files = /*.mdb/*.MDB/*.dbf/*.DBF/ +</pre><p> +</p><p> +If you are experiencing problems with oplocks as apparent from Samba's log entries, +you may want to play it safe and disable oplocks and level2 oplocks. +</p></div><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="id2929235"></a>Disabling Kernel OpLocks</h4></div></div><div></div></div><p> +Kernel OpLocks is an <tt class="filename">smb.conf</tt> parameter that notifies Samba (if +the UNIX kernel has the capability to send a Windows client an oplock +break) when a UNIX process is attempting to open the file that is +cached. This parameter addresses sharing files between UNIX and +Windows with Oplocks enabled on the Samba server: the UNIX process +can open the file that is Oplocked (cached) by the Windows client and +the smbd process will not send an oplock break, which exposes the file +to the risk of data corruption. If the UNIX kernel has the ability to +send an oplock break, then the kernel oplocks parameter enables Samba +to send the oplock break. Kernel oplocks are enabled on a per-server +basis in the <tt class="filename">smb.conf</tt> file. +</p><p> +</p><pre class="programlisting"> +[global] +kernel oplocks = yes +</pre><p> +The default is "no". +</p><p> +Veto OpLocks is an <tt class="filename">smb.conf</tt> parameter that identifies specific files for +which Oplocks are disabled. When a Windows client opens a file that +has been configured for veto oplocks, the client will not be granted +the oplock, and all operations will be executed on the original file on +disk instead of a client-cached file copy. By explicitly identifying +files that are shared with UNIX processes, and disabling oplocks for +those files, the server-wide Oplock configuration can be enabled to +allow Windows clients to utilize the performance benefit of file +caching without the risk of data corruption. Veto Oplocks can be +enabled on a per-share basis, or globally for the entire server, in the +<tt class="filename">smb.conf</tt> file: +</p><p> +</p><pre class="programlisting"><font color="red"><title>Example Veto OpLock Settings</title></font> +[global] + veto oplock files = /filename.htm/*.txt/ + +[share_name] + veto oplock files = /*.exe/filename.ext/ +</pre><p> +</p><p> +<span class="emphasis"><em>Oplock break wait time</em></span> is an <tt class="filename">smb.conf</tt> parameter that adjusts the time +interval for Samba to reply to an oplock break request. Samba +recommends "DO NOT CHANGE THIS PARAMETER UNLESS YOU HAVE READ AND +UNDERSTOOD THE SAMBA OPLOCK CODE." Oplock Break Wait Time can only be +configured globally in the <tt class="filename">smb.conf</tt> file: +</p><p> +</p><pre class="programlisting"> +[global] + oplock break wait time = 0 (default) +</pre><p> +</p><p> +<span class="emphasis"><em>Oplock break contention limit</em></span> is an <tt class="filename">smb.conf</tt> parameter that limits the +response of the Samba server to grant an oplock if the configured +number of contending clients reaches the limit specified by the +parameter. Samba recommends "DO NOT CHANGE THIS PARAMETER UNLESS YOU +HAVE READ AND UNDERSTOOD THE SAMBA OPLOCK CODE." Oplock Break +Contention Limit can be enable on a per-share basis, or globally for +the entire server, in the <tt class="filename">smb.conf</tt> file: +</p><p> +</p><pre class="programlisting"> +[global] + oplock break contention limit = 2 (default) + +[share_name] + oplock break contention limit = 2 (default) +</pre><p> +</p></div></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2929419"></a>MS Windows Opportunistic Locking and Caching Controls</h2></div></div><div></div></div><p> +There is a known issue when running applications (like Norton Anti-Virus) on a Windows 2000/ XP +workstation computer that can affect any application attempting to access shared database files +across a network. This is a result of a default setting configured in the Windows 2000/XP +operating system known as <span class="emphasis"><em>Opportunistic Locking</em></span>. When a workstation +attempts to access shared data files located on another Windows 2000/XP computer, +the Windows 2000/XP operating system will attempt to increase performance by locking the +files and caching information locally. When this occurs, the application is unable to +properly function, which results in an <span class="errorname">Access Denied</span> + error message being displayed during network operations. +</p><p> +All Windows operating systems in the NT family that act as database servers for data files +(meaning that data files are stored there and accessed by other Windows PCs) may need to +have opportunistic locking disabled in order to minimize the risk of data file corruption. +This includes Windows 9x/Me, Windows NT, Windows 200x and Windows XP. +</p><p> +If you are using a Windows NT family workstation in place of a server, you must also +disable opportunistic locking (oplocks) on that workstation. For example, if you use a +PC with the Windows NT Workstation operating system instead of Windows NT Server, and you +have data files located on it that are accessed from other Windows PCs, you may need to +disable oplocks on that system. +</p><p> +The major difference is the location in the Windows registry where the values for disabling +oplocks are entered. Instead of the LanManServer location, the LanManWorkstation location +may be used. +</p><p> +You can verify (or change or add, if necessary) this Registry value using the Windows +Registry Editor. When you change this registry value, you will have to reboot the PC +to ensure that the new setting goes into effect. +</p><p> +The location of the client registry entry for opportunistic locking has changed in +Windows 2000 from the earlier location in Microsoft Windows NT. +</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p> +Windows 2000 will still respect the EnableOplocks registry value used to disable oplocks +in earlier versions of Windows. +</p></div><p> +You can also deny the granting of opportunistic locks by changing the following registry entries: +</p><p> +</p><pre class="programlisting"> + HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\MRXSmb\Parameters\ + + OplocksDisabled REG_DWORD 0 or 1 + Default: 0 (not disabled) +</pre><p> +</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p> +The OplocksDisabled registry value configures Windows clients to either request or not +request opportunistic locks on a remote file. To disable oplocks, the value of + OplocksDisabled must be set to 1. +</p></div><p> +</p><pre class="programlisting"> + HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\LanmanServer\Parameters + + EnableOplocks REG_DWORD 0 or 1 + Default: 1 (Enabled by Default) + + EnableOpLockForceClose REG_DWORD 0 or 1 + Default: 0 (Disabled by Default) +</pre><p> +</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p> +The EnableOplocks value configures Windows-based servers (including Workstations sharing +files) to allow or deny opportunistic locks on local files. +</p></div><p> +To force closure of open oplocks on close or program exit EnableOpLockForceClose must be set to 1. +</p><p> +An illustration of how level II oplocks work: +</p><div class="itemizedlist"><ul type="disc"><li><p> + Station 1 opens the file, requesting oplock. + </p></li><li><p> + Since no other station has the file open, the server grants station 1 exclusive oplock. + </p></li><li><p> + Station 2 opens the file, requesting oplock. + </p></li><li><p> + Since station 1 has not yet written to the file, the server asks station 1 to Break + to Level II Oplock. + </p></li><li><p> + Station 1 complies by flushing locally buffered lock information to the server. + </p></li><li><p> + Station 1 informs the server that it has Broken to Level II Oplock (alternatively, + station 1 could have closed the file). + </p></li><li><p> + The server responds to station 2's open request, granting it level II oplock. + Other stations can likewise open the file and obtain level II oplock. + </p></li><li><p> + Station 2 (or any station that has the file open) sends a write request SMB. + The server returns the write response. + </p></li><li><p> + The server asks all stations that have the file open to Break to None, meaning no + station holds any oplock on the file. Because the workstations can have no cached + writes or locks at this point, they need not respond to the break-to-none advisory; + all they need do is invalidate locally cashed read-ahead data. + </p></li></ul></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2929649"></a>Workstation Service Entries</h3></div></div><div></div></div><pre class="programlisting"> + \HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\LanmanWorkstation\Parameters + + UseOpportunisticLocking REG_DWORD 0 or 1 + Default: 1 (true) +</pre><p> +Indicates whether the redirector should use opportunistic-locking (oplock) performance +enhancement. This parameter should be disabled only to isolate problems. +</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2929676"></a>Server Service Entries</h3></div></div><div></div></div><pre class="programlisting"> + \HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\LanmanServer\Parameters + + EnableOplocks REG_DWORD 0 or 1 + Default: 1 (true) +</pre><p> +Specifies whether the server allows clients to use oplocks on files. Oplocks are a +significant performance enhancement, but have the potential to cause lost cached +data on some networks, particularly wide-area networks. +</p><pre class="programlisting"> + MinLinkThroughput REG_DWORD 0 to infinite bytes per second + Default: 0 +</pre><p> +Specifies the minimum link throughput allowed by the server before it disables +raw and opportunistic locks for this connection. +</p><pre class="programlisting"> + MaxLinkDelay REG_DWORD 0 to 100,000 seconds + Default: 60 +</pre><p> +Specifies the maximum time allowed for a link delay. If delays exceed this number, +the server disables raw I/O and opportunistic locking for this connection. +</p><pre class="programlisting"> + OplockBreakWait REG_DWORD 10 to 180 seconds + Default: 35 +</pre><p> +Specifies the time that the server waits for a client to respond to an oplock break +request. Smaller values can allow detection of crashed clients more quickly but can +potentially cause loss of cached data. +</p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2929755"></a>Persistent Data Corruption</h2></div></div><div></div></div><p> +If you have applied all of the settings discussed in this paper but data corruption problems +and other symptoms persist, here are some additional things to check out: +</p><p> +We have credible reports from developers that faulty network hardware, such as a single +faulty network card, can cause symptoms similar to read caching and data corruption. +If you see persistent data corruption even after repeated reindexing, you may have to +rebuild the data files in question. This involves creating a new data file with the +same definition as the file to be rebuilt and transferring the data from the old file +to the new one. There are several known methods for doing this that can be found in +our Knowledge Base. +</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2929785"></a>Common Errors</h2></div></div><div></div></div><p> +In some sites locking problems surface as soon as a server is installed, in other sites +locking problems may not surface for a long time. Almost without exception, when a locking +problem does surface it will cause embarrassment and potential data corruption. +</p><p> +Over the past few years there have been a number of complaints on the samba mailing lists +that have claimed that samba caused data corruption. Three causes have been identified +so far: +</p><div class="itemizedlist"><ul type="disc"><li><p> + Incorrect configuration of opportunistic locking (incompatible with the application + being used. This is a VERY common problem even where MS Windows NT4 or MS Windows 200x + based servers were in use. It is imperative that the software application vendors' + instructions for configuration of file locking should be followed. If in doubt, + disable oplocks on both the server and the client. Disabling of all forms of file + caching on the MS Windows client may be necessary also. + </p></li><li><p> + Defective network cards, cables, or HUBs / Switched. This is generally a more + prevalent factor with low cost networking hardware, though occasionally there + have been problems with incompatibilities in more up market hardware also. + </p></li><li><p> + There have been some random reports of samba log files being written over data + files. This has been reported by very few sites (about 5 in the past 3 years) + and all attempts to reproduce the problem have failed. The Samba-Team has been + unable to catch this happening and thus has NOT been able to isolate any particular + cause. Considering the millions of systems that use samba, for the sites that have + been affected by this as well as for the Samba-Team this is a frustrating and + a vexing challenge. If you see this type of thing happening please create a bug + report on https://bugzilla.samba.org without delay. Make sure that you give as much + information as you possibly can to help isolate the cause and to allow reproduction + of the problem (an essential step in problem isolation and correction). + </p></li></ul></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2929859"></a>locking.tdb error messages</h3></div></div><div></div></div><p> + </p><pre class="screen"> + > We are seeing lots of errors in the samba logs like: + > + > tdb(/usr/local/samba_2.2.7/var/locks/locking.tdb): rec_read bad magic + > 0x4d6f4b61 at offset=36116 + > + > What do these mean? + </pre><p> + </p><p> + Corrupted tdb. Stop all instances of smbd, delete locking.tdb, restart smbd. + </p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2929890"></a>Additional Reading</h2></div></div><div></div></div><p> +You may want to check for an updated version of this white paper on our Web site from +time to time. Many of our white papers are updated as information changes. For those papers, +the Last Edited date is always at the top of the paper. +</p><p> +Section of the Microsoft MSDN Library on opportunistic locking: +</p><p> +Opportunistic Locks, Microsoft Developer Network (MSDN), Windows Development > +Windows Base Services > Files and I/O > SDK Documentation > File Storage > File Systems +> About File Systems > Opportunistic Locks, Microsoft Corporation. +<a href="http://msdn.microsoft.com/library/en-us/fileio/storage_5yk3.asp" target="_top">http://msdn.microsoft.com/library/en-us/fileio/storage_5yk3.asp</a> +</p><p> +Microsoft Knowledge Base Article Q224992 "Maintaining Transactional Integrity with OPLOCKS", +Microsoft Corporation, April 1999, <a href="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224992" target="_top">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224992</a>. +</p><p> +Microsoft Knowledge Base Article Q296264 "Configuring Opportunistic Locking in Windows 2000", +Microsoft Corporation, April 2001, <a href="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q296264" target="_top">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q296264</a>. +</p><p> +Microsoft Knowledge Base Article Q129202 "PC Ext: Explanation of Opportunistic Locking on Windows NT", + Microsoft Corporation, April 1995, <a href="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q129202" target="_top">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q129202</a>. +</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="AccessControls.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="optional.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="securing-samba.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 13. 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