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

[Robin Hill]

On Mon May 01, 2006 at 05:00:22PM -0500, Dean Wilson wrote:

> Thanks for your help!
> 
> > 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.
> 
> Is this good/bad/indifferent for Myth?  Can I specify which disk gets
> the parity info?  (If so, since I'm likely to have to have a single
> PATA drive anyway (with the others being SATA), would it benefit me to
> specify this drive as the parity disk?
> 
The parity drive is generally considered to be the bottleneck in a RAID
4 array, so having it on the (albeit only slightly) slower PATA
interface would probably not be a good idea.  I'd recommend RAID 5
rather than RAID 4 - it's slightly quicker in writing because of
spreading out the parity information, and will be far better
tested/optimised as it's used by many more people than RAID 4 will be.

> > 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).
> 
> Is this necessary?  As I understand it, as long as I don't have a huge
> array of disks, the likelyhood of more than two disks dying at the
> same time are low.  So as long as I replace the disk ASAP, I should be
> fine with only one parity disk, yes?
> 
Yes - RAID 6 is hardly ever used.  Anyone really worried about failure
goes for either mirrored RAID 5 or a RAID 5 array of mirrors.  I've
never had 2 drives fail (totally) at the same time, either at home or at
work.

> Speaking of replacing disks, how difficult is it to replace a bad disk
> with mdadm?  (And how will I know if a disk has gone bad?  Will it
> limp along, or force me to replace it immediately?)
> 
It's very easy to replace the disk - once it fails it'll be taken out of
the array automatically, so you just need to replace the physical drive
then hot-add the new drive to the array.  It'll then manage the rebuild
for you (though there'll be a performance impact during this time).  As
for knowing when it's failed - mdadm doesn't automatically notify you
(though I think you can run it in a monitoring mode to do this) but
there's plenty of tools you can install to monitor it.  Oh, and with a
failed drive the system will continue working normally, just without any
redundancy on the data.

Cheers,
        Robin

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