Re: 1x 3ware controllers vs. 2x 3ware controllers

On Sat, 31 Jul 2004, Jon Lewis wrote:

quoted

Hardware RAID5 or software RAID5?  My experience has been that even the
higher end hardware RAID5 cards I've dealt with give nowhere near the
performace you get with software RAID5.  HW RAID5 is a good way to make
an IO intensive system turn into a slug.

It doesn't matter. On small writes you need to do a lot of reads to get
the parity correct. If you do a lot of sequencial writes then RAID5 is ok,
if you do a lot of random writes, then RAID5 is bad.

HW 3ware raid5 is ok if you just do "cat /dev/null > /dev/sda", you'll get
nice write speeds, but for real life filesystem writes it needs to read a
lot to write the correct parity and this is quite slow. The only thing
software RAID5 solves is that it can use system memory to cache a lot more
than the HW 3ware RAID5 card can, so it can sometimes avoid to read from
the drives before writing parity.

This doesn't change the fact that if you need to do a lot of random writes
and you need to do it quickly, avoid RAID5:ing large number of drives.

So why are scsi raid-5 systems so much better?