Suggestions for write-caching settings for Windows 2008R2 and EVA4400

[Randy Voss]

We are using an EVA4400 and presenting the LUNs to Windows Server 2008 R2. I noticed a setting for enabling write-caching for the drives in Windows and I was wondering if anyone had a suggestion where it is worth turning it on or not?

[4dmir]

good question, can anyone help? yes I understand that we can manage write\reade cache policy in CV for every LUN, but regarding to microsoft's manual
"To understand the caching features offered by a given device and determine which settings best suit your needs, consult the documentation provided by the manufacturer.
"
pls, help to find manual with given info. thanx in advance

[Rajiv Kulkarni]

Hi,

Perhaps this two articles would give you more information about this:

This explains how to turn on the feature:
http://support.microsoft.com/kb/324805

This document gives all the necessary information about tuning performance on Windows machines:
http://download.microsoft.com/download/9/c/5/9c5b2167-8017-4bae-9fde-d599bac8184a/Perf-tun-srv.docx

As quoted from the above document:

"There are two performance-related options for every disk under Disk > Properties > Policies:
â ¢ Enable write caching.
â ¢ Enable an â advancedâ performance mode that assumes that the storage is protected against power failures.

Enabling write caching means that the storage subsystem can indicate to the operating system that a write request is complete even though the data has not been flushed from its volatile intermediate hardware cache(s) to its final nonvolatile storage location, such as a disk drive. Note that with this action a period of time passes during which a power failure or other catastrophic event could result in data loss. However, this period is typically fairly short because write caches in the storage subsystem are usually flushed during any period of idle activity. Caches are also flushed frequently by the operating system (or some applications) to force write operations to be written to the final storage medium in a specific order. Alternately, hardware timeouts at the cache level might force dirty data out of the cache.
The â advanced performanceâ disk policy option is available only when write caching is enabled. This option strips all write-through flags from disk requests and removes all flush-cache commands. If you have power protection for all hardware write caches along the I/O path, you do not need to worry about those two pieces of functionality. By definition, any dirty data that resides in a power-protected write cache is safe and appears to have occurred â in-orderâ from the softwareâ s viewpoint. If power is lost to the final storage location (for example, a disk drive) while the data is being flushed from a write cache, the cache manager can retry the write operation after power has been restored to the relevant storage components."

[CLEB]

Surely this setting should be off so it writes data to the EVA ASAP?. The EVA cache will be vastly superior to anything Windows can do, being it is optimised for that sort of work.

[Rajiv Kulkarni]

Yes, I'd agree to keep this setting to OFF. Note that if the windows host looses power the cache is lost and so could be data. On the other hand all storage arrays are equipped with battery backed up cache so that in case of power loss data in cache is safe.

[4dmir]

whatever, impossible to set this option ON (get error), because as told guys from support there no option to manage cache settings of EVA from host, only from CV.
So it looks this option lose sense for servers with hardware raids or external storages, wich has owm management software.

thank you for replies