RAID 5 on x346 server has very slow write I/O

Posted on 2007-08-08
Medium Priority
Last Modified: 2008-01-09
I have an IBM x346 with five 320gig drives (FastSCSI) and two volumes defined on top of this RAID (c: and e:)  When I put nbench on this (in response to complaints of slow disk access) I see that the write speed is 15mbs versus 60mbs on a server with a cheap SATA RAID 5. The read speed on the IBM is actually superior.  Any ideas or performance tips for improving the write speed on this RAID 5, or ideas why it may be so dismal? I used the single large RAID 5 because we needed the disk space. These are high-end disks, I would have thought that they would outperform generic SATA-based RAIDs on other servers. The machine itself is 64-bit Windows Server 2003 Ent with 8gigs ram, and not under general stress.

I know it's kind of a longshot: just looking for ideas how to diagnose, solve such an issue. There are no errors being logged by the RAID software and apparently all the disks are fine.  
Question by:rzup
LVL 56

Accepted Solution

andyalder earned 1000 total points
ID: 19655978
http://www-304.ibm.com/jct01004c/systems/support/supportsite.wss/docdisplay?brandind=5000008&lndocid=MIGR-58752 might apply (sorry about the long URL).

If the firmware detects the cache battery as faulty it disables write caching and that really slows down RAID 5. Of course you may have one of the superior add-in cards with more cache than you get on the onboard one.

The cheap SATA RAID5 you're comparing it to probably has the write cache turned on even without a battery, might have enabled the on-disk write cache as well, they do nasty tricks like that to improve the speed at the expense of data integrity.

Author Comment

ID: 19656312
Hmm, I do have that exact model of card and server. This is certainly worth a try. Thanks, and I'll let you know the outcome.

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