Extremely poor software RAID5 performance

Inaba · n00b Joined: 18 Jun 2002 Posts: 16

Hello folks,

I've seen the other thread about poor raid5 performance. While it's similar to my problem, it's not exact, so that's why I'm creating a new thread. Besides, there was no solution in that thread, either.

I have two problems, which are probably related:

The first problem is my write performance on my RAID5 array is horrendous. It's about 8MB/Sec. Single drive write performance is anywhere from 40MB/sec - 60MB/sec. I would be happy to get single drive performance out of this raid5 array. Anything beyond that is just gravy.

The second problem is that when writing to the array, the system appears to buffer up 80 - 100MB of data (I have 512MB RAM in this system) and then dump it all at once to the array, halting ALL other IO on that device until it's done. Needless to say, this causes all sorts of havoc to anyone doing anything on that drive, and makes streaming video or audio off that drive impossible.

My configuration is:

I've tried Gentoo Sources 2.6.12 kernel, and am currently using an external 2.6.13 kernel. Both produce the same results in terms of performance (I had heard there were /dev/md improvements in .13).
I have 512 MB RAM in the system
I have 2 External PCI Silicon Image PATA controllers with 2 channels each. I have 1 250GB Western Digital drive hooked up to each channel in master mode.

At one point, I was able to get my write performance up to about 15MB/sec, but that dropped back down to 8MB/Sec after I filled up about half of the array with files.

I'm desparate for a solution here, as this array is just basically unusuable, and I've spent the last week playing with parameters and recreating the array ad nauseum. Any help or clue would be so greatly apprecaited.

Thank you!

Here is my raidtab:

hazza · Posted: Thu Sep 22, 2005 5:01 pm Post subject:

Hi Inaba,

Run quite a few systems here with software RAID and on one system experienced a similar problem to the one you describe.

Long story short, it boiled down to the slots the controller cards were in! As a start, pull any unnecessary cards, disable onboard services etc. and build a RAID-0 array with just ONE of the cards. Run bonnie++ or whatever you're using to benchmark it again and see if you get an improvement.

If that works, add the second card (but in a different slot to your original RAID-5 setup), build a RAID-0 with four drives and re-bench. If that's good, tear the array down and build a RAID5. Bench that, then add all your cards back in bit by bit. It's a pain, but it solved my performance issue...

Following results are from a PIII/1GHz with three ATA/100 drives hung off the primary and secondary on-board controllers and an external PCI controller - 26Meg/sec write, 35Meg/sec read...