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

<chapter id="locking">
<chapterinfo>
	&author.jeremy;
	&author.jelmer;
	&author.jht;
</chapterinfo>
<title>File and Record Locking</title>

<sect1>
<title>Discussion</title>

<para>
One area which sometimes causes trouble is locking.
</para>

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

<para>
You can also disable by range locking completely using <emphasis>locking = no</emphasis>.
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 <emphasis>deny modes</emphasis>. 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
DENY_NONE, DENY_READ, DENY_WRITE or DENY_ALL. There are also special compatibility
modes called DENY_FCB and  DENY_DOS.
</para>
</sect1>

<sect1>
<title>Samba Opportunistic Locking Control</title>

<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 provids 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 noticable 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>

<para>
You can disable oplocks on a per-share basis with the following:

<programlisting>
	oplocks = False
	level2 oplocks = False
</programlisting>

Alternately, you could disable oplocks on a per-file basis within the share:

<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>

</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 <emphasis>Access Denied</emphasis>
 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>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>