[mythtv-users] drive configuration dilemmna
mythtv at rtr.ca
Tue Mar 29 11:51:15 UTC 2011
On 11-03-29 12:59 AM, sonofzev at iinet.net.au wrote:
> I have 3 tuners configured to record up to 2 streams off the same multiplex...
> Although the most I can perceive being used in our house is about 4 streams with
> up to 2 of those being HD.. (1080i MPEG in Australia)... Surprisingly my 4 disc
> RAID 5 does seem to handle about that amount of recording (highly tuned xfs
> filesystems that defrag nightly).. Given that the newer larger drives seem to
> fail more frequently, I am considering converting the current RAID 5 to RAID 6
> and adding 2 additional drives
Well, despite the slowdown that comes with RAID6 vs a single drive,
it should still be fast enough by a factor of 8X-10X here.
By comparison, a single drive (WD Green) 2TB unit here can record 10 streams
simultaneously, with two commflag jobs, and a silky smooth playback happening,
all at the same time. And it's probably not even maxed out with that load.
A problem I have with RAID, is that it multiplies the odds of a failure
causing data loss. Eg. 6 drives = 6X the likelihood of a failure that
affects all data. Sure, perhaps the array will tolerate a single drive
failure (which is kind of the whole point), but then you're running on the
edge for a day or three until that drive is replaced and the RAID resync'd.
I know of several people who "lost everything" during such an interval.
With Mythtv, "storage groups" give a compelling alternative to RAID,
putting each recording entirely onto a single drive from the pool.
So when a drive or two dies, one loses only the data from that drive,
not from everything.
But RAID remains probably the best way to have a single massive storage pool
that different apps can share, and that's a powerful motivation for folks.
Enterprise RAIDs generally use 3-4 drive RAID1 for reliability,
but that's not going to give the capacity that something like Mythtv wants.
Whether it's RAID, or Storage Groups, or any other setup, one must still
have a complete, separate, backup of all data. RAID is not a backup,
though a second RAID could be used for backup.
I prefer using individual 2TB (at present) drives for backups,
keeping them unattached between uses to minimize any possibility
of corruption or electrical failure. I use a pop-up "toaster" style
of external SATA drive bay for this purpose, with a fan added to it
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