ok, or else just leave it as it is i presume :-)
Thanks for the comment.
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RAID Controller policy on Dell 2950-III PERC 6/i
Hi All,
Im currently in the process of rebuilding my Dell 2950-III server which has got these specs:
1x Quadcore Xeon 5420 @ 2.33 GHz
5x 400 GB SAS 10k rpm HDD
RAID connector 0: 4 HDD
RAID connector 1: 1 HDD
Dell PERC 6/I RAID Controllers
16 GB RAM
While inside the Dell RAID configuration utility apps, I saw the following configuration:
Stripe Element Size: 64k
Read Policy: No Read Ahead*
Write Policy: Write Back
Disk Cache Policy: Disabled
" From: http://g4u0420c.houston.hp
As Im about to use this server as the RAID-5 for Exchange Server 2007, is there anything that I could tune for the best performance?
Thanks,
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Answers
Re: Disk Cache Policy: Disabled
This means the internal cache inside the hard drive is disabled. If you enable it, certain operations will be dramatically improved (writes). When I load large (100g) databases with "Disk Cache Policy: Enabled", I see a 5 times performance improvement.
But some Dell 2950's use Hitachi disk drives (HUA72101 disks, aka Ultrastar A7K1000). These disks can use their own internal write cache (the default). If a power loss occurs during heavy write activity, writes will be lost, possibly corrupting your disk (even though the drive tells the operating system that the writes had completed!)
Just halting the operating system won't cause this corruption, it's a power failure problem. With a Sybase database, we could get database corruption more than 80% of the time by pulling the plug during heavy write activity with internal hard drive caching enabled. We had no corruption in this scenario with internal hard drive caching disabled.
I don't know what other brands of disks do for this option.
There are commands in the megacli (MegaRaid disk controller (aka Perc) command line interpreter) for turning on "hard disk cache" (bad wording, it should say "hard disk internal cache" to separate it from the disk controller hard disk cache)
Command to turn hard drive internal disk cache on (power failure can corrupt the disks):
/opt/MegaRAID/MegaCli/Mega
Command to turn hard drive internal disk cache off (slower writes):
/opt/MegaRAID/MegaCli/Mega
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by: mastooPosted on 2009-04-27 at 07:31:12ID: 24242060
Assuming you have servers on ups and configured to properly shutdown on a power loss, I would think:
Read policy = adaptive read ahead, Write Policy = Write Back, and Disk Cache = Enabled