Questions/confusions on enabling write cache

Question

I've read over various threads out there regarding write caching, and seem to be more confused.   Background, I have a PowerEdge 2900 with a PERC 5/i Integrated controller, and do have a working good battery backup.   I recently applied the latest updates from Dell, which included the latest PERC 5/i firmware & driver, version 5.2.1-0067 and 2.14.00.32 respectively.  After completing the updates and rebooting I received the following warning in the system event log 'The driver detected that the device \Device\Harddisk0\DR0 has its write cache enabled. Data corruption may occur.'   If I go to the properties of the PERC 5/i controller in Device manager, the following is checked under the policies tab 'Enable write caching on the disk.  This device does not allow its write cache setting to be modified.'  The following option is not checked  'Enable advanced performance.  Recommended only for disks with a backup power supply.  This setting further improves disk performance, but it also increases the risk of data loss if the disk lose power.'   Under Server Administrator, I have the following set Read/Write Policy:  Read Policy - Adaptive Read Ahead, for Write Policy - Write Back.   I believe I have these settings set correctly, but am curious about the settings in the windows device manager.  Do they have any bering on the controller if I check or uncheck them?  Should I check Enable advanced performance?  Is there anyway to disable the warnings in the event log if everything is setup correctly, or is that just how things will be?  Also, did the default settings change between the old firmware and the new firmware to enable write back caching?   Lastly, I've read about using the Dell SAS Raid Storage Manager to check/modify the individual physical disk cache.  I haven't found that, but did find some additional information/options on the virtual disk. Disk Cache Policy:  Unchanged Current Write Policy:  Write Back Write Through for failed/missing battery:  No Access Policy: Read Write Read Policy:  Adaptive Read Ahead Default Write Policy:  Write Back IO Policy:  Direct IO   Would I want to change any of the above items?   Sorry for the long-winded post, but any input/comments would be greatly appreciated   Thanks in advance Josh

Dennis Smith · Answer

Legacy777,   First, on the error 'The driver detected that the device \Device\Harddisk0\DR0 has its write cache enabled. Data corruption may occur.'   This could happen if a computer or power failure occurs that would prevent your computer from shutting down properly. This is because until the cached data is written to disk, the last few write operations were reported to the operating system as being complete by the hardware, but the hardware still contains the data in its physical memory. Should a power loss occur, those last few write operations may not take place, possibly leading to data loss or file system damage. However, the perc has a battery backup so the cache can be written to the disks if a power outage does occur.   As for the settings on the Virtual Disk, I would leave them unless you are seeing performance issues then you could change the Write policy to see if increases any.

Legacy777 · Answer

Dennis,

Thanks for the comments. What about if the system get's powered down incorrectly, ie hit the power button or reset button while still connected to power? Would that present a problem with the cache?

Also, quick question about these settings in the SAS Raid Storage Manager

Disk Cache Policy: Unchanged

Write Through for failed/missing battery: No

Would I want to make the "Disk Cache Policy" "Disabled", or just leave it alone as "Unchanged"?

Also, would I want to check the box for "Use Write Through for failed/missing battery"?

Or should I just leave everything alone?

Thanks again.

Josh

Legacy777 · Answer

Yeah I agree I wouldn't want to 'hard' reboot/shutdown the server frequently, but it can happen once in a while.   I do not have frequent power outages, and do have a good UPS on the server, so I'll probably just leave those settings alone.   Again, thanks for the comments Dennis. Josh

Dennis Smith · Answer

If it is being powered down incorrectly the battery should provide enough time to write the data to disk.  Obviously, I don't recommend doing this on a regular basis though.   Disk Cache Policy, i would leave alone.  As for the Write through for failed/missing battery, You could change this if you do have frequent power outages, incorrect shutdowns.  This way it does not use cache when there is not a battery backup.

PowerEdge HDD/SCSI/RAID

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