storage area network abt raid

[Alex Gerulaitis]

[pradeep kagitala] “in one interview they asked me Configure RAID 5 to Raid 1+0, Raid 0+1
is it possible”

Yes.

Alex (DV411)

[Eric Hansen]

[Matt Geier] “In the world of Video Editing and Real Time RAID performance, stripping tends to be very bad. It’s a lot of Large I/O’s that need to go back and forth quickly on the disk, and stripping in most cases adds a lot of overhead that tends to slow down the disk performance. “

Hey Matt. could you go into more detail here? i thought the opposite is true in terms of I/O performance on a RAID-0 stripe. are you saying that if 10 editors were hitting a RAID-5, they would get more performance than if they were hitting a RAID-0?

thanks

e

Eric Hansen – http://www.erichansen.tv

[Chris Gordon]

First, RAID5 and RAID6 use striping and are “striping with parity” and “striping with double parity” respectively (see Wikipedia’s article on RAID for more discussion). Striping effectively spreads the IO across multiple disks giving you overall increased performance. I’m not sure what was meant by “In the world of Video Editing and Real Time RAID performance, stripping tends to be very bad” — without it, you’d be limited to a single disk.

So let’s assume a situation where you have 10 disks to work with.

RAID0, a simple stripe across all 10 disks with no redundancy. All 10 disks are used to read and write. This gives you the absolute best performance for a given set of disks, but you’re SOL when just one of those disks dies.

RAID5, striping with parity. This effectively gives you 9 disks for reads and writes with 1 used for the parity (it’s more complicated than this since the parity is distributed across the disks, but good enough for discussion). This is good for video work since video is typically dominated by simple, large, sequential reads and writes. With the reads you get 9 disks to perform the large reads. The number of disks to absorb the writes outweighs the impact of having to perform the parity calculations and do the additional write of the parity data. Now for the problems with RAID5 (or RAID6):
– When a disk dies, ALL of the other disks are impacted as they ALL must be read from in order to rebuild everything on a new disk. In addition to the hit from the rebuild IO work, any reads from the disk during the degraded time require parity calculations to give you the complete data. You should make sure your array can handle the your IO demand during a rebuild/failure situation.
– Since the rebuild so heavily exercises the remaining disks in the array, you can see additional failures occur at this time.
– Updates to files, especially when the update is smaller than the strip size, are typically slow. To do the update, you have to read all of the rest of the stripe, perform a new parity calculation, then write the updated data and new parity. Video work typically doesn’t see a great deal of updates, so you probably won’t encounter this.
– Small IOs may not get the benefit of hitting all 9 disks and their write performance may be adversely impacted by the parity calcs and writes.

RAID10, striping across mirror sets. With this you mirror pairs of disks and then build a stripe across them. You effectively have 5 disks for read and write. For simple, large, sequential reads and writes, 5 is less than 9, so you don’t get as much throughput as in the RAID5 case. At the same time, a disk failure only requires one disk to be read from with the rest of the array is unaffected (and there are no parity calcs to do a read during the degraded period, nor parity calcs to rebuild the replacement disk). Additionally small random IOs or updates do not feel any penalties from the parity calcs as there are none.

Now assuming a RAID5 of 10 disks, if I build a RAID10 set of 18 disks I would get at least as good performance as the 10 disk RAID5 and not suffer any of the problems with parity RAIDs. Of course this would cost a little bit more. Of course an 18 disks RAID0 would beat them all in performance (but may result in a resume updating event when just one disk dies).

Hope that helps.