[mythtv-users] OT Hardware RAID 5 non-Recommendations
drees76 at gmail.com
Thu Dec 21 03:15:19 UTC 2006
On 12/20/06, Marco Nelissen <marcone at xs4all.nl> wrote:
> >not even close. I could barely get 1 HD 1 PVR-150 and 1 HD playback
> >with my original RAID5 setup, I since went to a RAID 10 with 4 drives
> >and its finally capable of handling all that along with my SQL
> >database. I'm looking to add two more drives, at which point I think I
> >can go back to RAID5 and still get the speed I need and gain a lot of
> >extra space (which would be *lovely*.
> What on earth was your original RAID array made of? Floppies? My single
> drive mythtv box handles recording multiple HD streams while playing
> back one just fine. Any modern harddrive should be able to handle that.
With raid 5 doing a lot of small writes is often very slow unless you
have a hardware raid controller with a decent amount of onboard
memory. Raid 5 is slow for small writes because each write requires a
read to generate parity data so it's not uncommon to see a Raid 5
array under random small write load to perform worse than a single
Because mythtv writes and flushes data out to disk in small chunks (I
wish you could configure how mythtv flushed data to disk) it can
really kill raid 5 performance.
Raid 10 doesn't suffer from this since you never have to perform any
reads to perform a write so your typical Raid 10 array will write n/2
times as fast a single drive and read n times as fast as a single
drive in the best case scenario (where n is the number of drives in
Raid 5 read performance usually scales up linearly with the number of
disks (best case), write performance for large writes should scale to
n-1 (no parity needs to be read from disk).
Keep in mind that those are best case numbers and other factors
usually come into play so real life performance is usually a decent
amount slower than the theoretical limit.
For example, there seems to be a software Raid 5 performance bug which
limits write performance to single disk speeds even for large writes:
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