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diff --git a/docs/docbook/projdoc/locking.xml b/docs/docbook/projdoc/locking.xml index adb4356497..437f7756d9 100644 --- a/docs/docbook/projdoc/locking.xml +++ b/docs/docbook/projdoc/locking.xml @@ -3,16 +3,57 @@ &author.jeremy; &author.jelmer; &author.jht; + &author.eroseme; </chapterinfo> <title>File and Record Locking</title> +<para> +One area which causes trouble for many network administrators is locking. +The extent of the problem is readily evident from searches over the internet. +</para> + <sect1> -<title>Discussion</title> +<title>Features and Benefits</title> + +<para> +Samba provides all the same locking semantics that MS Windows clients expect +and that MS Windows NT4 / 200x servers provide also. +</para> + +<para> +The term <emphasis>locking</emphasis> has exceptionally broad meaning and covers +a range of functions that are all categorized under this one term. +</para> + +<para> +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. +</para> <para> -One area which sometimes causes trouble is locking. +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. </para> +<note> +<para> +Sometimes it is necessary to disable locking control settings BOTH on the Samba +server as well as on each MS Windows client! +</para> +</note> + +</sect1> + +<sect1> +<title>Discussion</title> + <para> There are two types of locking which need to be performed by a SMB server. The first is <emphasis>record locking</emphasis> which allows a client to lock @@ -62,12 +103,432 @@ access should be allowed simultaneously with its open. A client may ask for DENY_NONE, DENY_READ, DENY_WRITE or DENY_ALL. There are also special compatibility modes called DENY_FCB and DENY_DOS. </para> + +<sect2> +<title>Opportunistic Locking Overview</title> + +<para> +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: +</para> + +<variablelist> + <varlistentry><term>Read-ahead:</term> + <listitem><para> + The client reads the local copy of the file, eliminating network latency + </para></listitem> + </varlistentry> + + <varlistentry><term>Write caching:</term> + <listitem><para> + The client writes to the local copy of the file, eliminating network latency + </para></listitem> + </varlistentry> + + <varlistentry><term>Lock caching:</term> + <listitem><para> + The client caches application locks locally, eliminating network latency + </para></listitem> + </varlistentry> +</variablelist> + +<para> +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. +</para> + +<variablelist> +<title>Windows defines 4 kinds of Oplocks:</title> + <varlistentry><term>Level1 Oplock:</term> + <listitem><para> + 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/ex- + clusive access to the file. The client now performs + operations on the cached local file. + </para> + + <para> + 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. + </para></listitem> + </varlistentry> + + <varlistentry><term>Level2 Oplock:</term> + <listitem><para> + 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. + </para></listitem> + </varlistentry> + + <varlistentry><term>Filter Oplock:</term> + <listitem><para> + Does not allow write or delete file access + </para></listitem> + </varlistentry> + + <varlistentry><term>Batch Oplock:</term> + <listitem><para> + Manipulates file openings and closings - allows caching + of file attributes + </para></listitem> + </varlistentry> +</variablelist> + +<para> +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. +</para> + +<para> +<emphasis>Opportunistic Locking</emphasis> 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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<sect3> +<title>Exclusively Accessed Shares</title> + +<para> +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. +</para> + +<para> +Home directories are the most obvious examples of where the performance +benefit of opportunistic locking can be safely realized. +</para> + +</sect3> + +<sect3> +<title>Multiple-Accessed Shares or Files</title> + +<para> +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. +</para> + +<para> +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. +</para> + +</sect3> + +<sect3> +<title>Unix or NFS Client Accessed Files</title> + +<para> +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. +</para> + +<para> +If files are shared between Windows clients, and either loca Unix +or NFS users, then turn opportunistic locking off. +</para> + +</sect3> + +<sect3> +<title>Slow and/or Unreliable Networks</title> + +<para> +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. +</para> + +<para> +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. +</para> + +</sect3> + +<sect3> +<title>Multi-User Databases</title> + +<para> +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. +</para> + +</sect3> + +<sect3> +<title>PDM Data Shares</title> + +<para> +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. +</para> + +</sect3> + +<sect3> +<title>Beware of Force User</title> + +<para> +Samba includes an smb.conf parameter called "force user" 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. +</para> + +<para> +Avoid the combination of the following: +</para> + +<itemizedlist> + <listitem><para> + <emphasis>force user</emphasis> in the &smb.conf; share configuration. + </para></listitem> + + <listitem><para> + Slow or unreliable networks + </para></listitem> + + <listitem><para> + Opportunistic Locking Enabled + </para></listitem> +</itemizedlist> + +</sect3> + +<sect3> +<title>Advanced Samba Opportunistic Locking Parameters</title> + +<para> +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: <emphasis>oplock break wait time, +oplock contention limit</emphasis>. +</para> + +<para> +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. +</para> + +</sect3> + +<sect3> +<title>Mission Critical High Availability</title> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +<para> +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. +</para> + +</sect3> +</sect2> </sect1> <sect1> <title>Samba Opportunistic Locking Control</title> <para> +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. +</para> + +<para> +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. +</para> + +<para> 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. @@ -116,16 +577,38 @@ Another factor to consider is the perceived performance of file access. If oploc measurable speed benefit on your network, it might not be worth the hassle of dealing with them. </para> +<sect2> +<title>Example Configuration</title> + +<para> +In the following we examine two destinct aspects of samba locking controls. +</para> + +<sect3> +<title>Disabling Oplocks</title> + <para> You can disable oplocks on a per-share basis with the following: +</para> +<para> <programlisting> +[acctdata] oplocks = False level2 oplocks = False </programlisting> +</para> + +<para> +The default oplock type is Level1. Level2 Oplocks are enabled on a per-share basis +in the &smb.conf; file. +</para> +<para> Alternately, you could disable oplocks on a per-file basis within the share: +</para> +<para> <programlisting> veto oplock files = /*.mdb/*.MDB/*.dbf/*.DBF/ </programlisting> @@ -136,6 +619,96 @@ If you are experiencing problems with oplocks as apparent from Samba's log entri you may want to play it safe and disable oplocks and level2 oplocks. </para> +</sect3> + +<sect3> +<title>Diabling Kernel OpLocks</title> + +<para> +Kernel OpLocks is an &smb.conf; 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 &smb.conf; file. +</para> + +<para> +<programlisting><title>Example:</title> +[global] + kernel oplocks = yes + +The default is "no". +</programlisting> +</para> + +<para> +Veto OpLocks is an &smb.conf; 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 +&smb.conf; file: +</para> + +<para> +<programlisting><title>Example Veto OpLock Settings</title> +[global] + veto oplock files = /filename.htm/*.txt/ + +[share_name] + veto oplock files = /*.exe/filename.ext/ +</programlisting> +</para> + +<para> +<emphasis>Oplock break wait time</emphasis> is an &smb.conf; 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 smb.conf file: +</para> + +<para> +<programlisting> +[global] + oplock break wait time = 0 (default) +</programlisting> +</para> + +<para> +<emphasis>Oplock break contention limit</emphasis> is an &smb.conf; 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 &smb.conf; file: +</para> + +<para> +<programlisting> +[global] + oplock break contention limit = 2 (default) + + [share_name] + oplock break contention limit = 2 (default) +</programlisting> +</para> + +</sect3> +</sect2> + </sect1> <sect1> @@ -196,7 +769,8 @@ You can also deny the granting of opportunistic locks by changing the following <para> <programlisting> - HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MRXSmb\Parameters\ + HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\MRXSmb\Parameters\ OplocksDisabled REG_DWORD 0 or 1 Default: 0 (not disabled) @@ -211,7 +785,8 @@ request opportunistic locks on a remote file. To disable oplocks, the value of <para> <programlisting> - HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters + HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\LanmanServer\Parameters EnableOplocks REG_DWORD 0 or 1 Default: 1 (Enabled by Default) @@ -275,7 +850,8 @@ An illustration of how level II oplocks work: <title>Workstation Service Entries</title> <para><programlisting> - \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters + \HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\LanmanWorkstation\Parameters UseOpportunisticLocking REG_DWORD 0 or 1 Default: 1 (true) @@ -291,7 +867,8 @@ enhancement. This parameter should be disabled only to isolate problems. <title>Server Service Entries</title> <para><programlisting> - \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters + \HKEY_LOCAL_MACHINE\System\ + CurrentControlSet\Services\LanmanServer\Parameters EnableOplocks REG_DWORD 0 or 1 Default: 1 (true) @@ -358,6 +935,53 @@ our Knowledge Base. </sect1> <sect1> +<title>Common Errors</title> + +<para> +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 exeception, when a locking +problem does surface it will cause embarassment and potential data corruption. +</para> + +<para> +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: +</para> + +<itemizedlist> + <listitem><para> + 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. + </para></listitem> + + <listitem><para> + 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. + </para></listitem> + + <listitem><para> + 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). + </para></listitem> +</itemizedlist> + +</sect1> + +<sect1> <title>Additional Reading</title> <para> @@ -379,7 +1003,7 @@ Windows Base Services > Files and I/O > SDK Documentation > File Storag <para> Microsoft Knowledge Base Article Q224992 "Maintaining Transactional Integrity with OPLOCKS", -Microsoft Corporation, April 1999, <ulink url="=http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224992">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224992</ulink>. +Microsoft Corporation, April 1999, <ulink url="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224992">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224992</ulink>. </para> <para> |