[mythtv-users] Hard Drives that Actually Work?

[Brad Templeton]

On Wed, Dec 15, 2004 at 11:04:11PM +0000, Phill Edwards wrote:
> I think it's good to question whether RAID is the best solution for a home 
> setup. Sometimes there's a simpler more useable solution that gives you 
> more of what you need for most scenarios.


RAID-5:
	Uses only one extra disk to provide backup for up to 4
	Can be faster on reading the data due to striping
	Tends to be slower on writing.
	    (Neither of these speed issues affect video much which
	    doesn't need that much speed.)
	Does require at least 3 disks, not all systems can do that.
	Can be hard to set up

Mirroring:
	Requires a doubling of capacity
	Read speed is the same
	Write could be slower but if done properly (two buses) isn't much.

    Both the above forms:
	Will recover data you wrote just moments ago.
	Can keep the system up in the middle of a drive failure
	May support hot-swap -- replace a drive in a running system.
	Give you nothing in the face of software error or operator error,
	    which are actually the most common sources of trouble.

Remote mirroring (traditional backup)

	Only backs it up when it's a backup-interval old (typically a day)
	Often allows recovery for files that were deleted or damaged by
	    software error or operator error, or changing your mind.
	For compressed data, requires a doubling of capacity.
	For non-compressed data, can need only half the capacity since
	    many arguments against compression do not apply.
	You go down if you have an error on your primary disk.  If
	    you lose your backup disks, no issue.

	Most importantly, is in a different computer, and can be in
	    a different building.   Protects you against physical
	    catastrophe such as fire


It's hard to say what is best.  But for video (MythTV) I would think
RAID is not needed since you don't need 100% uptime and hot-swap on
either end.

Indeed, for video there is an argument that backup is barely needed
at all, because it's not your data, and you can, with varying efforts,
go and get it again.   A large percentage of shows in your archive
will appear on TV some time again in the future, or can be bought on
DVD or rented fromm netflix.   Not only is the cost of the netflix
subscription much less than the cost of backup, you only pay in the
event of losing your archive.   If that is a 10% chance, real "expected
value" cost is vastly lower than the cost of maintaining a backup.


Ideal strategy -- find a way to tag videos in archive based on whether
they are "readily replacable" (ie. join netflix), "replacable with work"
or "irreplacable."   Back up the latter, and possibly some of the middle
kind.