[mythtv-users] Storage Questions. LVM, RAID, NFS

James C. Dastrup jc at dastrup.com
Tue May 2 18:43:25 UTC 2006


>> On 5/2/06, Robin Hill <myth at robinhill.me.uk> wrote:
>> > On Tue May 02, 2006 at 10:48:05AM -0400, Steven Adeff wrote:
>> > > Would a 4 disk RAID10 be better than RAID5 for a recordings drive
>> > > considering I may have times where I've got my 3x HD recorders and my
>> > > PVR150, (possibly swaping for a PVR500) plus having at most 2x HD
>> > > capable frontends running?
>> > >
>> > I'm not sure there'll be much difference.  In both cases you're
>> > effectively writing data across two drives (with block striping) so the
>> > only advantage RAID 10 has is there's no need for parity calculation.
>> > RAID 10 should provide a benefit for simultaneous reading and writing
>> > though, due to the mirrored drives.
>>
>> I'd have a 4 disk RAID5 array as well though, so it would be writing
>> across 4 drives (plus the parity information), whereas with RAID10 it
>> would be writing across two drives. This is where I don't quite
>> understand how RAID10 is supposed to be faster than RAID5?
>>
>With a 4 disk RAID 5 you'll be writing 3 blocks plus parity at once,
>whereas with a 4 disk RAID 10 you'll be writing 2 blocks (plus mirrors)
>at once.  In terms of raw I/O the RAID 5 should be faster.  The slight
>complication is that RAID 5 will need to calculate the parity before
>writing - this shouldn't have any impact in a modern system though.  For
>reading the speeds will probably be about the same - both should be able
>to read from all 4 disks at once.
>
>So, IMO, RAID 10 won't give you any advantage.  
>

RAID 10 will be much faster than RAD 5. I have two HD tuners and one
SD tuner and I switched to RAID 10 after too many headaches with RAID 5.
It's just too slow if you have multiple HD tuners. In case you're curious
about what type of RAID 5 hardware was too slow for 2 HD tuners + 1
SD tuner + HD playback, I had a Compaq DL380 with 5i+ RAID controller,
10K Ultra160 drives and 128MB Battery Backed Write Back cache.  And,
yes, it was too slow.  RAID 5 is not popular for it's speed, it's popular
because it's gets the most usable space out of your drives.


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