[mythtv-users] Migrating RAID

[Richard Freeman]

Jay R. Ashworth wrote:
> On Wed, Oct 31, 2007 at 07:22:08PM -0700, Yan Seiner wrote:
>> I am bringing up a new server which will have 6x400GB SATA drives for 
>> data.  I plan to allocate all of it to myth.  The OS lives on SCSI drives.
>>
>> Currently I have 2 empty 400 GB drives in the new server and 4 - 400 GB 
>> drives in the old server, with about 800GB of myth stuff I want to 
>> keep.  I plan to use the 4 drives from the old server in the new server.
> 
> Well, you *know* what my answer is going to be here, right?
> 
> SuperDLT 220 and LTO-1 tape drives and tape are quite a bit cheaper
> than you think, until you look.  
> 
> Grab the trial version of BackupEdge, backup to tape.  Change drives;
> build arrays; restore.
> 

Uh, the tapes alone are about as expensive as hard drives of equivalent
size - the drives cost a fortune unless you have a link to somebody
selling one for $100.  And it sounds like he ALREADY has a backup.  I'm
all for reasonable levels of redundancy (that's why I use raid in the
first place), but I don't need to guard my TV shows with the same fervor
that I guard corporate databases at work.  Unless he REALLY needs
regular offsite backups he'd be better off just buying a few more hard
drives to use to transfer the data in one operation and then have a LOT
of extra storage when he is done.

The plan he proposed is reasonable, although reshaping arrays is a bit
slow and perhaps a little risky (it is completely safe in theory, but
probably not as well tested as the other features of software raid).
One thing he can do to save himself a little time (assuming he has a
backup) is to create his initial raid-5 in degraded mode (make it a
4-drive array missing a drive instead of a complete 3-drive array) - so
that he has 1.2TB of space but no redundancy.  He'll be closer to his
destination that way.  If he loses a drive during the migration he will
lose everything (so this is really a good idea only if he already has a
backup).  Either way he needs to reshape his array.

However, all this talk about backups brings up a question.  If a backup
for all this data already exists, why not just build an array with all 6
drives and restore the data from the backup?  Or do we intend to do all
this between recording shows during prime time?  :)