Re: Typical RAID5 transfer speeds

On 19/12/2009 01:05, 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

Sorry, but where did you get that from? p = d1 xor d2 xor d3 has nothing 
to do with powers of two, and I'm sure blocks are written whenever they 
need to be, not in powers of two.

you probably have read operations, 
when you only want to write.

That will depend on how much data you're trying to write. With 3 data 
discs and a 2M chunk size, writes in multiples of 6M won't need reads. 
Writing a 25M file would therefore write 4 stripes and need to read to 
do the last 1M. With 4 data discs, it'd be 8M multiples, and you'd write 
3 stripes and need a read to do the last 1M. No difference.

Cheers,

John.