[CentOS] distributed storage/home-made cloud recommendations

Sun Feb 5 22:42:20 UTC 2012
Boris Epstein <borepstein at gmail.com>

On Sun, Feb 5, 2012 at 5:31 PM, Ross Walker <rswwalker at gmail.com> wrote:

> On Feb 5, 2012, at 10:32 AM, Phil Schaffner <Philip.R.Schaffner at NASA.gov>
> wrote:
> > Boris Epstein wrote on 02/04/2012 11:57 AM:
> >> What is RAID0+1?
> >
> > Nested RAID.  Paraphrasing http://en.wikipedia.org/wiki/RAID :
> >
> > For a RAID 0+1, drives are first combined into multiple level 0 RAIDs
> > that are themselves treated as single drives to be combined into a
> > single RAID 1.
> Probably the worse setup, a failure on both sides of a mirror means total
> loss and with the # of disks on each side of this setup the chance of this
> is much greater, recovery from a failure is a lot longer cause the whole
> stripe needs to re-mirror. While performance of reads is equal to 1+0 the
> writes are equal to a single mirror cause both sides need to complete
> before the next operation can run or only one write operation on the array
> at a time.
> Much better RAID level is 1+0 which is a series of mirrors striped
> together. While a failure on both sides of any one mirror is total for the
> array there is only 1 disk on either side so the odds are less, recovery
> from failure is faster as well cause only one disk needs to be re-mirrored.
> Performance of reads and writes are equal because each mirror can perform
> writes independant of the others, or # of write operations equal to the
> number of mirrors.
> -Ross
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What you are saying seems to make sense actually. I wonder how much a RAID6
with a few spares would make sense. If we are talking a large number of
disks then RAID 6 + 2 spares means overpaying only for 5 disks. Not a lot
if the total number of them is, say, 20.