Re: 3Ware 9500S-12 RAID controller -- poor performance

From: Mikael Abrahamsson
Date: Mon Oct 10 2005 - 06:09:00 EST

On Mon, 10 Oct 2005, Dr. David Alan Gilbert wrote:

Nice. Have you tried Software RAID5 on top of that? I would be very interested to know how software RAID5 goes relative to the 3Ware hardware.

There have been hundreds of email regarding this on the linux-raid@xxxxxxxxxxxxxxx list. Please look in the archives.

It's well known that 3ware hw raid is slow when writing, current theory is that this is due to the lack of buffering meaning that any write makes it read a lot as well, destroying performance. Generally, the performance numbers advertised by 3ware when writing is a dd to the drive itself (I got this number after doing a support request on it a few years back), without a filesystem. This goes very quickly, but writing files on a filesystem is usually very slow (10 megabyte/s or so). When doing SW raid the SW layer has access to the memory block cache and can thus avoid a lot of physical reads on the drives.

I never had any problems getting good read speeds on the HW raid.

My experience is with the 7500 series, the 9500 series has cache as well but this doesn't seem to have solved a lot of the performance problems seen with the 7500 series.

Mikael Abrahamsson email: swmike@xxxxxxxxx
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