On Fri December 18 2009, Bernd Schubert wrote:

On Saturday 19 December 2009, Matt Tehonica wrote:

quoted

I have a 4 disk RAID5 using a 2048K chunk size and using XFS

4 disks is a bad idea. You should have 2^n data disks, but you have 2^1 +
 1 = 3 data disks. As parity information are calculated in the power of
 two and blocks are written in the power of two, you probably have read
 operations, when you only want to write.

quoted

filesystem.  Typical file size is about 2GB-5GB. I usually get around
50MB/sec transfer speed when writting files to the array. Is this
typcial or is it below normal?  A friend has a 20 disk RAID6 using the
same filesystem and chunk size and gets around 150MB/sec. Any input on
this??

I would remove two disks, to get 16 + 2 drives (2^4). Performance
 probably would be limited by CPU speed then. 150MB/s for 18 drives is
 also bad, this is only the performance of two single raid0 drives.

I'd have to agree. My 5 disk raid5 array gets me 200-400MB/s, depending on 
the kernel. I'm using a 512K chunk size, formatted with XFS, with 32 AGs, 
and xfs_info reporting: sunit=128 swidth=512 blks (which should be 
right...), and mounted with: 
noatime,nodiratime,logbufs=8,allocsize=512m,largeio,swalloc

oh, not quite 200MB/s, iozone is showing 112MB/s write, and 300MB/s read. 
I'm pretty sure that has something to do with the writeback stuff though, 
and aught to be improved in 2.6.32+ (I have yet to find a good time to 
upgrade my server). I know I have seen the SAS card, and an initial array 
handle more throughput than that when I was first testing stuff months and 
months ago. It was more like 200-350 write, and 400-550 read.

But yeah, 50MB/s is pretty bad for a raid array. The individual disks in my 
array are all capable of more than that each.  (Yes, I know raid5 will not 
give a linear improvement when adding more drives, but it aught to be a heck 
of a lot better than a decrease in performance)

Cheers,
Bernd
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-- 
Thomas Fjellstrom
tfjellstrom@shaw.ca