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

[Robin Hill]

On Tue May 02, 2006 at 02:07:00PM -0400, Steven Adeff wrote:

> 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.  It's probably a win for
continuous small reads & writes (e.g. databases) since you could read
from one miror while writing to the other - this'll only work for small
writes though as you wouldn't want the mirrors to be out of sync for
more than a fraction of a second.  For writing a number of large files
simultaeously (with redundancy) I'd say RAID 5 would be your best bet.

Cheers,
        Robin

-- 
     ___        
    ( ' }     |       Robin Hill        <myth at robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |
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