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