🚀 go-pugleaf

[Forgive the reply to myself - just to clarify:]

I inadvertently munged two concepts together, I think:  I do understand how
a 2-dimensional array arrangement works where there are m * n data disks
(conceptually, an m by n rectangle) and m + n parity disks (arranged along
two sides of the rectangle, providing parity in 2 dimensions), but think I
have heard mention of some means of adding a second parity disk to a simple
array that makes double-disk failures tolerable, which is what I'm curious
about (since I don't see off-hand how it could be done).

Thanks,  - bill

Bill Todd <billtodd@foo.mv.com> wrote in message
news:7sll6t$l8j$1@pyrite.mv.net...
> But it still takes more disks:  3*n vs. 2*(n+1).  That said, given disk
> prices these days, I wonder how important that is, even in read-mostly (or
> full-stripe-write) environments where RAID-5 has no performance penalty.
>
> I've never investigated the 'two-dimensional parity' approach that
> supposedly uses 2 parity drives in a manner that allows *any two* drives
in
> a RAID set to fail simultaneously without data loss:  I assume it carries
> greater write and rebuild overhead than RAID-5, but would be interested in
a
> pointer to a good discussion.
>
> - bill
>
> Kjell Tore Nannestad <knannest@c2i.net> wrote in message
> news:37EE4665.C2FF28E@c2i.net...
> > I guess it's easier to make a second mirror using RAID 1.
> > .
> >
> >
> > prostorage wrote:
> >
> > > In article <37EB92A6.8579E99A@ultrafast.net.no.sp.am>,
> > > RAIDER@ultrafast.net.no.sp.am says...
> > > >Jim, let it be known that a hot spare drive in a RAID 5 setup does
> > > >NOTHING to protect from a failure of more than one drive at a time.
> What
> > > >it does do, however, it minimizes MTTR (Mean Time To Repair) because
> > > >most raid controllers will automatically initiate a rebuild of a
failed
> > > >drive onto a hot spare. Should another disk fail while the rebuild is
> > > >still in progress - the RAID set and all the data on it will be lost.
> > > >RAIDER
> > > >
> > > >Jim Pappe wrote:
> > > >>
> > > >> with Raid 5 plus hot spare - 2 drive can fail ..
> > > >> www.raidexpert.com
> > > >>
> > >
> > > RAIDER is correct
> > > a second "warm" or "hot" spare would do nothing to save you if 2
> > > drives went down at the same time
> > >
> > > the only way that i know of to live through the failure of 2 drives
> > > simultaneously is to have 2 separate RAID 5's running as the
> > > mirrored drives in a RAID 1 (& we've done this)
> > >
> > >   _____                         .     .
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>