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

[Robin Hill]

On Mon May 01, 2006 at 03:06:33PM -0500, Dean Wilson wrote:

> I'm considering increasing the number of drives I'm using.  When I do
> that, I would like to go to a redundant system for reliability. 
> Unfortunately, I've never used raid before, and would like some input.
> 
> Because of the size required for HD TV as well as space required for
> mail and web server files, I'm aiming for ~1TB of usable storage
> space, which would be prohibitively expensive to double the disks. 
> That said, what are my best options?
> 
> I've noticed that mwadm supports Raid 0, 1, 4, 5, 6
> 
> Raid 0 has no redundancy, so it isn't preferred
> Raid 1is redundant, but requires too many disks (correct?)
> Raid 4 ?
> Raid 5 has redundancy (correct?), but I've heard that it writes very
> slowly.  Would a Raid5 system be too slow for recording 2 HD + 1 SD
> recording, while doing playback to up to two frontends at the same
> time?
> Raid 6 ?
> 
RAID 4 is similar to 5 except that it uses a dedicated disk for the
parity information rather than striping it across all the other disks.
RAID 6 is basically RAID 5 but with two separate parity sets allowing
for failure of two drives (but with 2 drives worth of storage space
lost).

The overhead in RAID 5 (it needs to calculate parity information on
writes) will be pretty negligible on any modern system (except possibly
the EPIAs) so I wouldn't worry about that.

> As I understand it from these discussions, Raid5 requires 5 drives, 4
> of which will be used for storage?  (So 5 250GB drives would sum to
> 1TB of useable HD space?)
> 
Approximately, yes.  It'll be a bit short of a TB because of the way the
storage is calculated (using standard kB insstead of kiB).

> Also, (assuming all raid configurations require more than 4 drives to
> affordably hit my 1TB target with redundancy) I have a motherboard
> that has 4 SATA ports, but with two different controllers, I'll still
> need an SATA card for the 5th+ drive(s) (unless mdadm can mix IDE with
> SATA?  Is this just asking for trouble?)  Will multiple controllers
> adversely affect the raid array?  (If so, is there a controller card
> anyone would recommend with 5+ SATA ports?)
> 
> Thanks for your help!
> 
There's no problem with using multiple controllers, or multiple types of
controller (SATA, PATA or even SCSI) though you'll be limited by the
speed of the slowest drive.

HTH,
        Robin

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