On Sat, 31 Jul 2004, Jon Lewis wrote: > 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. -- Mikael Abrahamsson email: swmike@xxxxxxxxx - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
