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RAID Controller policy on Dell 2950-III PERC 6/i

Posted on 2009-04-26
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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.com/en/B7961-90025/ch12s02.html  You cannot set the read policy on a RAID-5 data plex. RAID-5 plexes have their own read policy (RAID).

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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Question by:jjoz
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mastoo earned 2000 total points
ID: 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
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by:jjoz
ID: 24246356
ok, or else just leave it as it is i presume :-)
Thanks for the comment.
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by:BenSlade
ID: 26081942
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.
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by:jjoz
ID: 26105416
Hi all, thanks for the reply.

Is there a way to enable this option when the hard drive RAID has been created ?

Cheers.
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by:BenSlade
ID: 26106446
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/MegaCli -LDSetProp -EnDskCache -LALL -aALL

Command to turn hard drive internal disk cache off (slower writes):

  /opt/MegaRAID/MegaCli/MegaCli -LDSetProp -DisDskCache -LALL -aALL

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