path: root/docs/docbook/projdoc/locking.xml

<chapter id="locking">
<chapterinfo>
	&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>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>
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
a range of bytes in a open file.  The second is the <emphasis>deny modes</emphasis>
that are specified when a file is open.
</para>

<para>
Record locking semantics under Unix are 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.
</para>

<para>
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.
</para>

<para>
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 <command>rpc.lockd</command>. 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 <parameter>strict locking = yes</parameter> then it
will make lock checking calls on every read and write.
</para>

<para>
You can also disable byte range locking completely using <parameter>locking = no</parameter>.
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.
</para>

<para>
The second class of locking is the <parameter>deny modes</parameter>. 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
<constant>DENY_NONE</constant>, <constant>DENY_READ</constant>, 
<constant>DENY_WRITE</constant> or <constant>DENY_ALL</constant>. There are also special compatibility
modes called <constant>DENY_FCB</constant> and <constant>DENY_DOS</constant>.
</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/exclusive
                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 local 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 <parameter>force user</parameter> 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>
	<parameter>force user</parameter> 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: 
<parameter>oplock break wait time</parameter>,
<parameter>oplock contention limit</parameter>.
</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.
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.
</para>

<para>
Level1 Oplocks (aka just plain "oplocks") is another term for opportunistic locking.
</para>

<para>
Level2 Oplocks provides opportunistic locking for a file that will be treated as
<emphasis>read only</emphasis>. 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.
</para>

<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

<sect2>
<title>Example Configuration</title>

<para>
In the following we examine two distinct 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>
</para>

<para>
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.
</para>

</sect3>

<sect3>
<title>Disabling 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>
[global]
kernel oplocks = yes
</programlisting>
The default is "no".
</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>
<title>MS Windows Opportunistic Locking and Caching Controls</title>

<para>
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 <emphasis>Opportunistic Locking</emphasis>. 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 <errorname>Access Denied</errorname>
 error message being displayed during network operations.
</para>

<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

<para>
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.
</para>

<para>
The location of the client registry entry for opportunistic locking has changed in
Windows 2000 from the earlier location in Microsoft Windows NT.
</para>

<note><para>
Windows 2000 will still respect the EnableOplocks registry value used to disable oplocks
in earlier versions of Windows.
</para></note>

<para>
You can also deny the granting of opportunistic locks by changing the following registry entries:
</para>

<para>
<programlisting>
	HKEY_LOCAL_MACHINE\System\
		CurrentControlSet\Services\MRXSmb\Parameters\

		OplocksDisabled REG_DWORD 0 or 1
		Default: 0 (not disabled)
</programlisting>
</para>

<note><para>
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.
</para></note>

<para>
<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)
</programlisting>
</para>

<note><para>
The EnableOplocks value configures Windows-based servers (including Workstations sharing
files) to allow or deny opportunistic locks on local files.
</para></note>

<para>
To force closure of open oplocks on close or program exit EnableOpLockForceClose must be set to 1.
</para>

<para>
An illustration of how level II oplocks work:
</para>

<itemizedlist>
	<listitem><para>
	Station 1 opens the file, requesting oplock.
	</para></listitem>
	<listitem><para>
	Since no other station has the file open, the server grants station 1 exclusive oplock.
	</para></listitem>
	<listitem><para>
	Station 2 opens the file, requesting oplock.
	</para></listitem>
	<listitem><para>
	Since station 1 has not yet written to the file, the server asks station 1 to Break
	to Level II Oplock.
	</para></listitem>
	<listitem><para>
	Station 1 complies by flushing locally buffered lock information to the server.
	</para></listitem>
	<listitem><para>
	Station 1 informs the server that it has Broken to Level II Oplock (alternatively,
	station 1 could have closed the file).
	</para></listitem>
	<listitem><para>
	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.
	</para></listitem>
	<listitem><para>
	Station 2 (or any station that has the file open) sends a write request SMB.
	The server returns the write response.
	</para></listitem>
	<listitem><para>
	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.
	</para></listitem>
</itemizedlist>

<sect2>
<title>Workstation Service Entries</title>

<para><programlisting>
	\HKEY_LOCAL_MACHINE\System\
		CurrentControlSet\Services\LanmanWorkstation\Parameters

	UseOpportunisticLocking   REG_DWORD   0 or 1
	Default: 1 (true)
</programlisting></para>

<para>
Indicates whether the redirector should use opportunistic-locking (oplock) performance
enhancement. This parameter should be disabled only to isolate problems.
</para>

</sect2>
<sect2>
<title>Server Service Entries</title>

<para><programlisting>
	\HKEY_LOCAL_MACHINE\System\
		CurrentControlSet\Services\LanmanServer\Parameters

	EnableOplocks   REG_DWORD   0 or 1
	Default: 1 (true)
</programlisting></para>

<para>
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.
</para>

<para><programlisting>
	MinLinkThroughput   REG_DWORD   0 to infinite bytes per second
	Default: 0
</programlisting></para>

<para>
Specifies the minimum link throughput allowed by the server before it disables
raw and opportunistic locks for this connection.
</para>

<para><programlisting>
	MaxLinkDelay   REG_DWORD   0 to 100,000 seconds
	Default: 60
</programlisting></para>

<para>
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.
</para>

<para><programlisting>
	OplockBreakWait   REG_DWORD   10 to 180 seconds
	Default: 35
</programlisting></para>

<para>
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.
</para>

</sect2>
</sect1>

<sect1>
<title>Persistent Data Corruption</title>

<para>
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:
</para>

<para>
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.
</para>

</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 exception, when a locking
problem does surface it will cause embarrassment 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>

	<sect2>
	<title>locking.tdb error messages</title>

	<para>
	<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?
	</screen>
	</para>

	<para>
	Corrupted tdb.  Stop all instances of smbd, delete locking.tdb, restart smbd.
	</para>

	</sect2>

</sect1>

<sect1>
<title>Additional Reading</title>

<para>
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.
</para>

<para>
Section of the Microsoft MSDN Library on opportunistic locking: 
</para>

<para>
Opportunistic Locks, Microsoft Developer Network (MSDN), Windows Development &gt;
Windows Base Services &gt; Files and I/O &gt; SDK Documentation &gt; File Storage &gt; File Systems
&gt; About File Systems &gt; Opportunistic Locks, Microsoft Corporation.
<ulink url="http://msdn.microsoft.com/library/en-us/fileio/storage_5yk3.asp">http://msdn.microsoft.com/library/en-us/fileio/storage_5yk3.asp</ulink>
</para>

<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>.
</para>

<para>
Microsoft Knowledge Base Article Q296264 "Configuring Opportunistic Locking in Windows 2000",
Microsoft Corporation, April 2001, <ulink url="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q296264">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q296264</ulink>.
</para>

<para>
Microsoft Knowledge Base Article Q129202 "PC Ext: Explanation of Opportunistic Locking on Windows NT",
 Microsoft Corporation, April 1995, <ulink url="http://support.microsoft.com/default.aspx?scid=kb;en-us;Q129202">http://support.microsoft.com/default.aspx?scid=kb;en-us;Q129202</ulink>.
</para>

</sect1>
</chapter>