[THIN] Re: We shall see
- From: "Jim Kenzig http://thethin.net" <jimkenz@xxxxxxxxxxxxxx>
- To: <thin@xxxxxxxxxxxxx>
- Date: Thu, 5 Jun 2003 22:30:33 -0400
More on this
http://support.microsoft.com/default.aspx?scid=kb;EN-US;332023
Slow Disk Performance When Write Caching Is Enabled
The information in this article applies to:
Microsoft Windows XP Professional
Microsoft Windows XP Home Edition
Microsoft Windows 2000 Server SP3
Microsoft Windows 2000 Server SP2
Microsoft Windows 2000 Professional SP3
Microsoft Windows 2000 Professional SP2
Microsoft Windows 2000 Advanced Server SP3
Microsoft Windows 2000 Advanced Server SP2
This article was previously published under Q332023
SYMPTOMS
The performance of certain hard-disk write operations may be slower than
expected if the hard disk implements write caching and your computer uses
any of the following operating systems:
Windows 2000 Service Pack 3 (SP3)
Windows 2000 Service Pack 2 (SP2) on which you have installed the hotfix
that is described in the following Microsoft Knowledge Base article:
281672 Possible Data Loss After You Enable the "Write Cache Enabled" Feature
Windows XP
Specifically, the hard-disk write performance may be slower than that
experienced with versions of Windows 2000 earlier than SP3. Also, on a
cluster computer, hard-disk write operations to shared disks may cause large
differences in write performance between each node.
CAUSE
Versions of Windows 2000 earlier than SP3 contain a bug that prevents
certain disk commands from being issued. These commands are only issued to
disks that have write caching enabled. These commands force critical data to
be written to disk devices immediately instead of being held temporarily in
the disk's write-behind cache.
For additional information about this bug, click the following article
number to view the article in the Microsoft Knowledge Base:
281672 Possible Data Loss After You Enable the "Write Cache Enabled" Feature
The hotfix that is described in Knowledge Base article 281672 corrects this
bug and causes these commands to be issued to the disk as expected when
write caching is enabled. This fix is included in Windows 2000 SP3. A
similar fix is also included in Windows XP.
By design, these commands sacrifice some performance for the sake of the
safety of critical data. They make sure that critical data is written to the
disk immediately. These commands are only issued if write caching is enabled
for a specific disk device and if it can be detected by using standard
mechanisms. When this safe behavior is enforced for disks that implement
write caching, you will experience loss of performance during certain disk
operations. See the "More Information" section of this article for more
details. Also, on a cluster computer, hard-disk information may not be
delivered to a node that takes over a cluster resource when the resource is
failed over. Therefore, differences in hard-disk write operation performance
between each node occurs.
RESOLUTION
For Computers That Run Windows 2000
Warning: If you implement this fix (by installing the updated drivers and
turning on the Power Protected write cache option), protections that are
built in to the Windows 2000 disk drivers to make sure that critical data is
written to the disk immediately instead of being written to the disk later
from the disk's built-in cache are removed. Do not implement this fix unless
your computer and any connected hard disks are protected against accidental
or inadvertent power loss through hardware design that may include such
features as redundant battery-backed power supplies. Contact the vendors of
your computer and storage devices to determine the degree of protection
afforded by your hardware. Do not implement this fix unless you understand
and accept the level of risk that may be involved, and are confident that
this risk has been mitigated through appropriate hardware power protection.
Also, on a cluster computer, differences in performance of hard-disk write
operations to shared disks between nodes can be solved by setting the Power
Protected write cache option on each node.
For additional information, click the following article number to view the
article in the Microsoft Knowledge Base:
281672 Possible Data Loss After You Enable the "Write Cache Enabled" Feature
Part 1: Install Windows 2000 Update
A supported feature that modifies the product's default behavior is now
available from Microsoft, but it is only intended to modify the behavior
that is described in this article. Apply it only to systems that
specifically need it. This feature may receive additional testing.
Therefore, if your system is not severely affected by the lack of this
feature, Microsoft recommends that you wait for the next Windows 2000
Service Pack that contains this feature.
To obtain this feature immediately, contact Microsoft Product Support
Services. For a complete list of Microsoft Product Support Services phone
numbers and information on support costs, visit the following Microsoft Web
site:
http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS
The English version of this fix has the file attributes (or later) that are
listed in the following table. The dates and times for these files are
listed in coordinated universal time (UTC). When you view the file
information, it is converted to local time. To find the difference between
UTC and local time, use the Time Zone tab in the Date and Time tool in
Control Panel.
Date Time Version Size File name
-------------------------------------------------------
15-Jan-2003 19:42 5.0.2195.6655 34,832 Classpnp.sys
15-Jan-2003 19:43 5.0.2195.6655 30,768 Disk.sys
Note The files that are included in this hotfix have post-SP3 version
numbers, but they can be used on both Windows 2000 SP2-based and Windows
SP3-based computers. The hotfix package will install on both Windows 2000
SP2-based and Windows 2000 SP3-based computers.
Part 2: Configure the "Power Protected" Write Cache Option
After you install the Windows 2000 update and then restart your computer so
that the updated drivers are loaded, you must also turn on the Power
Protected write cache option for each disk that has write caching enabled
and has its write cache protected against accidental loss of power. If the
Power Protected write cache option is not turned on, this update does not
implement any change in disk behavior or performance.
To configure the Power Protected write cache option on the selected disks,
use the Dskcache.exe tool. Dskcache.exe is available separately. For
additional information about the Dskcache.exe tool and how to enable the
"power protected" write cache option, click the following article number to
view the article in the Microsoft Knowledge Base:
811392 Obtain the Dskcache.exe Tool to Configure the "Power Protected" Write
Cache Option
Note The Power Protected write cache option can only be enabled for disks
that are currently present. By default, the Power Protected write cache
option is disabled. As a result, this option is disabled for any disks that
may be added after you apply this setting. If additional disks are added,
enable the Power Protected write cache option for those added disks for the
setting to take effect.
For Computers That Run Windows XP
A supported update for this behavior is not currently available from
Microsoft. If you are not severely affected by this behavior, Microsoft
recommends that you wait for the next service pack that contains this fix.
STATUS
This behavior is by design.
MORE INFORMATION
Many disk devices provide enhanced performance through the use of an onboard
cache, which provides read-ahead caching for data that is being read from
the disk, and write-behind caching (or delayed writes or "lazy" writes) for
data that is being written to disk. In some cases, it is important for data
to be written to the physical disk immediately, and not retained in the
disk's onboard write cache to be written later during an otherwise idle
moment. This prevents loss or corruption of this data if the disk or
controller (wherever the write cache is implemented) suddenly loses power,
for example.
There are two commands that are typically used to force cached data to be
written to the physical disk immediately:
A flush buffers command, which is available for SCSI and IDE/ATAPI disk
devices, instructs the disk to write all cached data to the disk
immediately. For SCSI disks, this is implemented by issuing the SYNCHRONIZE
CACHE command to the disk. For IDE/ATAPI disks, the FLUSH CACHE command is
sent to the disk. This command is typically issued as a result of a Windows
program calling the FlushFileBuffers API. Writing to the registry is an
example of an operation that results in the FlushFileBuffers API being
called and the SYNCHRONIZE CACHE command being sent to the disk.
A Write Through command, which is only available for SCSI disk devices, is
implemented by issuing a WRITE command to the disk with the ForceUnitAccess
(FUA) bit set. This type of command instructs the disk to write the current
packet of data to the disk immediately, bypassing the onboard write cache.
This typically occurs as a result of a Windows program that calls the
WriteFile API to write to a file it has opened by calling the CreateFile API
with the FILE_FLAG_WRITE_THROUGH flag set. Virus-scanning software typically
opens files with the FILE_FLAG_WRITE_THROUGH flag set. Support for the FUA
bit is optional, and only some SCSI and Fiber Channel (FC) devices,
typically individual drives (as opposed to RAID arrays), implement this
functionality.
By design, these commands sacrifice a measure of performance (which would
typically be provided by the disk's onboard write cache) to maintain the
safety of the data that is being written to the disk. If such data is not
written to the disk immediately by these commands, and if the disk loses
power suddenly, that data is lost, and the file that is being written may
become corrupted.
In Windows 2000 versions earlier than SP3, these commands are not correctly
issued to disk devices when expected. For example, the SYNCHRONIZE CACHE
command is not issued to a disk when a program calls FlushFileBuffers, and
the FUA bit is not set for WRITE commands when the file is opened with
FILE_FLAG_WRITE_THROUGH set. The hotfix that is described in Knowledge Base
article 281672 corrects this problem, so that these commands are issued as
intended and, as a result, provide the expected protection against loss of
such critical data.
When the Write Cache Enabled option is configured for a disk device in
Device Manager, two things occur:
A command to turn on or turn off Write Caching is sent to the disk device.
A registry value is saved, which the disk driver uses to determine whether
or not to issue write-cache-management commands (Flush, Write-Through) to
the disk.
The hotfix that is described in this article provides an additional
configuration option, which is the Power Protected write cache option. When
both Write Caching and Power Protected options are turned on, the disk's
write cache is enabled and recognized as such by the disk driver, but the
disk driver does not issue write-cache-management commands (Flush,
Write-Through) to the disk. This avoids the performance penalty that would
otherwise be incurred by flushing data from the write cache to the disk
media, but exposes a risk of loss or corruption of disk data if power is
lost while there is still data in the disk's write cache that has not been
written to disk.
The following table summarizes the effects of the Write Caching and Power
Protected options: Write Caching Power Protected
Effect
Disabled N/A Disk does not cache writes.
Driver does not issue Flush/Write-Through commands.
(Only behavior possible for drives that do not have write caching enabled in
hardware.)
Enabled Disabled Disk caches writes.
Driver issues Flush/Write-Through commands.
(Default behavior as of SP3, and with this update applied, for drives that
have write caching enabled in hardware.)
Enabled Enabled Disk caches writes.
Driver does not issue Flush/Write-Through commands.
(Behavior resulting from pre-SP3 bug, for drives that have write caching
enabled in hardware.)
Last Reviewed: 5/14/2003
Keywords: kbfix kbbug kbQFE KB332023 kbAudOEM kbAudITPRO kbAudDeveloper
kbAudEndUser
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