PERC 5i integrated

Question

Just purchased PowerEdge Server 2900. It comes with 4 SAS HD in RAID5 configuration using PERC 5i integrated controller. This server will be used as SQL server. I've been looking for an information on how to disable write caching on the controller but I can't find it. The reason why I want to disable caching is that if the battery backup on the controller dies (or can't hold that much charge any longer), I don't want to lose SQL data.   Does anyone know how to disable caching on PERC5i controller? Thanks.

pcmeiners · Answer

Rj1127, with raid 5 you really do not want to disable the cache, the performance will be incredibly dismal. If you really worried about the onboard battery, change it out after 3 years, if super paranoid, get a large UPS unit. As a note,  years ago few used on-board battery backup ( early 90s), I had about 10 raid servers without on-board raid battery units (did have UPS units), never any data loss...today the disks are gigantic and would never run a raid without a battery.

Mark_A_Smith · Answer

Change the write policy on the virtual disk to write-through.  This will disable the write cache.  You can do this in Server Administrator or from the Ctrl-R Controller BIOS.

rj1127 · Answer

pcmeiners,

Some SQL admins recommend against using a cached controller even if it's battery backed (on a SQL server). Some say something about a "smart" controller that recognizes SQL data and lets it bypass the cache (but cached the rest of data). Not sure if that's true or not. That's why I'm a bit worried about enabling the cache. Thanks for your reply.

rj1127 · Answer

Mark_A_Smith,

Thanks for your reply. I did see that option in the SCSI BIOS. It's set to write-back at the moment. Read/write settings are grayed out and inaccessible. Read/write settings can be selected when I enable "Advanced" check box right above them. Not sure why that is. I'm also not sure that if "Advanced" check box isn't checked (rw settings inaccessible), does that mean read/write settings are ignored?

pcmeiners · Answer

Look,  if your going to kill any performance you have you should have a reason. In a computer you have data cached in ram, you have a large cache on the hard drives, machine info cached in the bios, device info cached in ram. Failure of any component in a CPU can cause data loss. I been using LSI based raid cards since the very early 90s, and only had one card die with data loss, out of roughly 125 raid servers. The soothsayer who blame a raid card for data loss, not counting disk failures likely have loses due to program glitches, poor programing, poor setup, user intervention, or other hardware. Yes, I have had a few memory sticks die on raid adapters, running SQL, and yes I have had some tractions lost as a result, but in none of the cases has database corruption occurred. The last stick which died cause SQL transaction to 'backout' with the loss of an hours work. If you turn off the cache, likely you will cut performance by >50%, multiple that over years of a servers life, and the largest data loss due to data lost in cache would be negligible to the lost productivity, even if you had to restore from a backup. Do you drive 5 miles an hour for a 3000 mile trip, just to avoid an accident?

rj1127 · Answer

yeah... why would I drive my ferrari enzo at such low speed? Pedal to the metal baby... Yeah!...

PowerEdge HDD/SCSI/RAID

PERC 5i integrated

Was this post helpful?