[Beowulf] Surviving a double disk failure

David Mathog mathog at caltech.edu
Fri Apr 10 13:15:54 PDT 2009

Billy Crook <billycrook at gmail.com> wrote:

> As a very,
> very, general rule, you might put no more than 8TB in a raid5, and no
> more than 16TB in a raid6, including what's used for parity, and
> assuming magnetic, enterprise/raid drives.  YMMV, Test all new drives,
> keep good backups, etc...

Thankfully I don't have to do this myself, not having data anywhere near
that size to cope with, but it seems to me that backing up a nearly full
16TB RAID is likely to be a painful, expensive, exercise. 

Going with tape first...

The fastest tape drives that I know of are Ultrium 4's at 120 MB/s.  In
theory that could copy 1GB every 8.3 seconds, 1TB every 8300 seconds (
AKA 138 minutes, or a bit over 2 hours), and for that 16 TB data set,
something over 32 hours.  Except that there is no tape with that
capacity, Max listed is still 800 GB, so it would take 20 tapes.  And
really obtaining a sustained 120MB/s from the RAID to the tape is likely
extremely challenging.  In any case, it looks like this calls for a tape
robot of some sort, with many drives in it.  Not cheap.  On the plus
side, transporting a box of 20 tape cartridges to "far away" is not
particularly difficult, and they are fairly impervious to abuse during

The other obvious option is to replicate the RAID.  Now if the duplicate
RAID is on site, connected by a 1000baseT network, one could obtain a
very similar transfer rate - and a full backup would take just as long
as for the single tape drive (neglecting rewind and cartridge change
times).  This at the expense of still losing all the data in some sort
of sitewide disaster.  I can imagine, and suspect somebody has this
already, implementing, a specialized disk->disk connect, such that one
would plug Raid A into Raid B, and all N disks in A could copy
themselves in parallel onto all N disks in B at full speed.  Assuming
1TB disks and a sustained 75Mb/sec read from A and write to B, the whole
copy would be done in about 222 minutes.  Not exactly the blink of an
eye, but a heck of a lot better than 32 hours.   Placing the backup RAID
physically offsite would improve the odds of the data surviving, but
reduce the bandwidth available, and moving the copied RAID physically
offsite after each backup is a recipe for short disk lives.

Since all of the obvious options are so slow, I expect most sites are
doing incremental backups.  Which is fine, until the day comes when one
has to restore the entire data array from two year's worth of
incremental backups.  Or maybe folks  carry the tape incremental backups
to the offsite backup RAID and apply them there?

Is there an easier/faster/cheaper way to do all of this?


David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech

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