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File
Corruption associated to Opportunistic Locking and Read Caching
on Microsoft Windows Networks
Summary
Improperly configured Windows networks
can lead to data corruption in any file system database that uses ISAM
technology, including ManageMore. Two Windows networking behaviors,
opportunistic locking (on Windows servers) and read caching (on Windows
clients) are the source of the corruption potential.
This paper discusses these behaviors, their
effects and what can be done to minimize the chances of data corruption on
Windows networks when running ManageMore Business Software.
The information in this paper is compiled
from the latest information regarding these issues from Microsoft's
documentation, our own in-house testing and customer reports.
Contents
What is
Opportunistic Locking?
Opportunistic locking is a Windows-specific
mechanism for client/server databases to allow multiple processes to lock
the same file while allowing for local (client) data caching to improve
performance over Windows networks. Unfortunately, the default setting of the
opportunistic locking mechanism that enhances the performance of one type of
database also introduces data integrity issues for other database types.
Microsoft's documentation states "An
opportunistic lock (also called an oplock) is a lock placed by a client
on a file residing on a server. In most cases, a client requests an
opportunistic lock so it can cache data locally, thus reducing network
traffic and improving apparent response time. Opportunistic locks are used
by network redirectors on clients with remote servers, as well as by client
applications on local servers" and "Opportunistic locks are requests from
the client to the server. From the point of view of the client, they are
opportunistic. In other words, the server grants such locks whenever other
factors make the locks possible.".
You can read more about opportunistic locking
in Microsoft's documentation. Please see the
Additional Reading section below for more information.
What is Read
Caching?
Read caching is local caching of read
data, sometimes referred to as read-ahead caching, on client
(workstation) PCs. This type of caching is the default behavior of the
Windows operating system to improve performance of client/server databases
over Windows networks. The way it works
is that workstations can read data from the server and keep a local copy of
that data to reduce network traffic. As soon as that workstation writes any
data, it should flush file buffers at that time, reread the data from the
server and then commit the changes.
Problems with read caching usually occur if
something unforeseen happens, such as a workstation crash, where data is not
properly flushed from the workstation, which can lead to data corruption.
Microsoft's documentation states that "Under
extreme conditions, some multi-user database applications that use a common
data store over a network connection on a file server may experience
transactional integrity issues or corruption of the database files and/or
indexes stored on the server. This typically applies to some so-called "ISAM
style", or "record oriented" multi-user database applications, not to a
client/server relational system like SQL Server." and "A hazard of local
caching is that written data only has as much integrity as the client itself
for as long as the data is cached on the client. In general, locally cached
data should be flushed to the server as soon as possible."
You can read more about read caching in
Microsoft's documentation. Please see the
Additional Reading section below for more information.
How Does This
Affect Me?
Certain versions of the
ManageMore Software use an
ISAM database and thus are more susceptible to the
effects of the default Windows read caching and opportunistic locking
settings than client/server databases.
Reliable
database operation on Windows Networks can be achieved using the provided
that the network is properly configured. You can use the information
in this paper to set up your Windows network's opportunistic locking and
read caching parameters or contact Intellisoft Technical Support.
Disabling opportunistic locking and read
caching will have a performance impact on any database application, whether
an ISAM database or client/server database is used, but it will not affect
the data integrity of any database.
What
Operating Systems are Affected?
All server computers running Windows
operating systems in the NT/2000 family that host data files accessed by
other Windows PCs need to have opportunistic locking disabled in order to
minimize the chances of database corruption. The NT/2000 family OS list
includes Windows NT, Windows 2000, Windows XP and Windows 2003 Server.
All workstation computers running Windows
operating systems in the 9X family running applications that use data files
hosted on NT/2000 family servers need to have read caching disabled in order
to minimize the chances of database corruption. This includes Windows 98 and
Windows ME.
Disabling Read
Caching on Windows Workstations
All Windows operating systems in the 9X
family that act as database clients for data files (meaning that they access
data files stored on other Windows PCs) need to have read caching disabled
in order to minimize the chances of database corruption. This includes
Windows 98 and Windows ME.
The Windows registry entry that controls read
caching on Windows network clients is:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VxD\VREDIR
DiscardCacheOnOpen
REG_BINARY 0 or 1
Default: 0 (not disabled)
To disable read caching, the value
of DiscardCacheOnOpen must be set to 1.
You can verify (or change or add, if
necessary) this Registry value using the Windows Registry Editor (regedit.exe).
If you do change this Registry value, you will have to reboot the PC to
ensure that the new setting goes into effect.
STEPS:
1. Start > Run > Regedit.exe
2. Click on the + (plus sign) next to HKey_Local_Machine
3. Click on the + (plus sign) next to System
4. Click on the + (plus sign) next to CurrentControlSet
5. Click on the + (plus sign) next to Services
6. Click on the + (plus sign) next to VxD
7. Click on the VREDIR entry on the left-hand side of Registry
Editor
8. If the DiscardCacheOnOpen
registry value already exists (on the right-hand side of Registry
Editor), ensure that its value is 1
9. If the DiscardCacheOnOpen value
already exists but its value is not 1, double-click on
DiscardCacheOnOpen to change its value to 1
10. If the DiscardCacheOnOpen
entry does not exist, right-click in the white space of the right-hand
side of Registry Editor
11. Select New > Binary value
12. Rename the value to DiscardCacheOnOpen
13. Double-click on DiscardCacheOnOpen to change its value to
1
Disabling
Opportunistic Locking on Windows Servers
All Windows operating systems in the NT/2000
family that act as database servers for data files (meaning that data files
are stored there and accessed by other Windows PCs) need to have
opportunistic locking disabled in order to minimize the chances of database
corruption. This includes Windows NT, Windows 2000, Windows XP and Windows
2003 Server.
There are 2 Windows registry entries
that control opportunistic locking (oplocks) on Windows network servers:
1. HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
EnableOpLockForceClose
2. HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
EnableOplocks
1. EnableOpLockForceClose
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
EnableOpLockForceClose
REG_DWORD 0 or 1
Default: 0 (not disabled)
To disable oplocks, the value of
EnableOpLockForceClose must be set to 1.
You can verify (or change or add, if
necessary) this Registry value using the Windows Registry Editor (regedit.exe).
If you do change this Registry value, you will have to reboot the PC to
ensure that the new setting goes into effect.
STEPS:
1. Start > Run > Regedit.exe
2. Click on the + (plus sign) next to HKey_Local_Machine
3. Click on the + (plus sign) next to System
4. Click on the + (plus sign) next to CurrentControlSet
5. Click on the + (plus sign) next to Services
6. Click on the + (plus sign) next to LanManServer
7. Click on the Parameters entry on the left-hand side of
Registry Editor
8. If the EnableOpLockForceClose
registry value already exists (on the right-hand side of Registry
Editor), ensure that its value is 1
9. If the EnableOpLockForceClose
value already exists but its value is not 1, double-click on
EnableOpLockForceClose to change its value to 1
10. If the EnableOpLockForceClose
entry does not exist, right-click in the white space of the
right-hand side of Registry Editor
11. Select New > DWORD value
12. Rename the value to EnableOpLockForceClose
13. Double-click on EnableOpLockForceClose to change its
value to 1
2. EnableOplocks
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters
EnableOplocks
REG_DWORD 0 or 1
Default: 1 (true)
To disable oplocks, the value of
EnableOplocks must be set to 0.
You can verify (or change or add, if
necessary) this Registry value using the Windows Registry Editor (regedit.exe).
If you do change this Registry value, you will have to reboot the PC to
ensure that the new setting goes into effect.
STEPS:
1. Start > Run > Regedit.exe
2. Click on the + (plus sign) next to HKey_Local_Machine
3. Click on the + (plus sign) next to System
4. Click on the + (plus sign) next to CurrentControlSet
5. Click on the + (plus sign) next to Services
6. Click on the + (plus sign) next to LanManServer
7. Click on the Parameters entry on the left-hand side of
Registry Editor
8. If the EnableOplocks registry
value already exists (on the right-hand side of Registry Editor),
ensure that its value is 0
9. If the EnableOplocks value
already exists but its value is not 0, double-click on
EnableOplocks to change its value to 0
10. If the EnableOplocks entry
does not exist, right-click in the white space of the right-hand side
of Registry Editor
11. Select New > DWORD value
12. Rename the value to EnableOplocks
13. Double-click on EnableOplocks to change its value to
0
Note: The
location of the registry entry for opportunistic locking has changed in
Windows 2000 from the earlier location in Microsoft Windows NT. In Windows
2000, the registry entry that disables opportunistic locking is:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MRXSmb\Parameters\
OplocksDisabled
REG_DWORD 0 or 1
Default: 0 (not disabled)
To disable oplocks, the value of
OplocksDisabled must be set to 1.
Note:
Windows 2000 will still respect the EnableOplocks registry value used
to disable oplocks in earlier versions of Windows.
Disabling
Opportunistic Locking on Windows Workstations
All Windows operating systems in the NT/2000
family that act as database servers for data files (meaning that data files
are stored there and accessed by other Windows PCs) need to have locking
disabled in order to minimize the chances of database corruption. This
includes Windows NT, Windows 2000 and Windows XP.
If you use a Windows NT/2000 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, Windows 2000 Professional instead of Windows 2000
Server, or Windows XP Home instead of Windows XP Professional,
and you have data files located on it that are accessed from other Windows
PCs, you will need to disable oplocks on that system.
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
is used here.
There are 2 Windows registry entries
that control opportunistic locking (oplocks) on Windows network
workstations:
1.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters
EnableOpLockForceClose
2.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters
EnableOplocks
1. EnableOpLockForceClose
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters
EnableOpLockForceClose
REG_DWORD 0 or 1
Default: 0 (not disabled)
To disable oplocks, the value of
EnableOpLockForceClose must be set to 1.
You can verify (or change or add, if
necessary) this Registry value using the Windows Registry Editor (regedit.exe).
If you do change this Registry value, you will have to reboot the PC to
ensure that the new setting goes into effect.
STEPS:
1. Start > Run > Regedit.exe
2. Click on the + (plus sign) next to HKey_Local_Machine
3. Click on the + (plus sign) next to System
4. Click on the + (plus sign) next to CurrentControlSet
5. Click on the + (plus sign) next to Services
6. Click on the + (plus sign) next to LanManWorkstation
7. Click on the Parameters entry on the left-hand side of
Registry Editor
8. If the EnableOpLockForceClose
registry value already exists (on the right-hand side of Registry
Editor), ensure that its value is 1
9. If the EnableOpLockForceClose
value already exists but its value is not 1, double-click on
EnableOpLockForceClose to change its value to 1
10. If the EnableOpLockForceClose
entry does not exist, right-click in the white space of the
right-hand side of Registry Editor
11. Select New > DWORD value
12. Rename the value to EnableOpLockForceClose
13. Double-click on EnableOpLockForceClose to change its
value to 1
2. EnableOplocks
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanManWorkstation\Parameters
EnableOplocks
REG_DWORD 0 or 1
Default: 1 (true)
To disable oplocks, the value of
EnableOplocks must be set to 0.
You can verify (or change or add, if
necessary) this Registry value using the Windows Registry Editor (regedit.exe).
If you do change this Registry value, you will have to reboot the PC to
ensure that the new setting goes into effect.
STEPS:
1. Start > Run > Regedit.exe
2. Click on the + (plus sign) next to HKey_Local_Machine
3. Click on the + (plus sign) next to System
4. Click on the + (plus sign) next to CurrentControlSet
5. Click on the + (plus sign) next to Services
6. Click on the + (plus sign) next to LanManWorkstation
7. Click on the Parameters entry on the left-hand side of
Registry Editor
8. If the EnableOplocks registry
value already exists (on the right-hand side of Registry Editor),
ensure that its value is 0
9. If the EnableOplocks value
already exists but its value is not 0, double-click on
EnableOplocks to change its value to 0
10. If the EnableOplocks entry
does not exist, right-click in the white space of the right-hand side
of Registry Editor
11. Select New > DWORD value
12. Rename the value to EnableOplocks
13. Double-click on EnableOplocks to change its value to
0
Persistent
Data Corruption
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:
- Faulty network hardware, such as a single
faulty network card, network cable, or network hub/switch, can cause
symptoms similar to read caching and data corruption.
- Consider upgrading workstations and
servers which contain outdated versions of Microsoft's OS. Some
older service packs in certain versions of OS's have had known issues with
file corruption that have since been corrected by Microsoft.
Editing the Windows Registry
Caution:
The following warning appears in every
Microsoft Knowledge Base article that discusses editing the Windows
registry:
WARNING
: You can edit the registry by using Registry
Editor (Regedit.exe or Regedt32.exe). If you use Registry Editor
incorrectly, you can cause serious problems that may require you to
reinstall your operating system. Microsoft does not guarantee that
problems that you cause by using Registry Editor incorrectly can be
resolved. Use Registry Editor at your own risk.
Terms
- ISAM
Indexed Sequential Access Method is a file management system developed at
IBM that allows records to be accessed either sequentially (in the order
they were entered) or randomly (with an index).
Additional Reading
- Section of the
Microsoft MSDN Library on opportunistic locking:
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.
URL=http://msdn.microsoft.com/library/en-us/fileio/base/opportunistic_locks.asp
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