[Techtalk] Software RAID

[Conor Daly]

On Tue, Aug 20, 2002 at 03:39:37PM -0700 or so it is rumoured hereabouts, 
Kai MacTane thought:
> At 8/20/02 03:19 PM , Dave North wrote:
> >Kai:
> > > I am currently doing nothing at all with either hardware or software RAID.
> > > But I might do so at some point in the near-to-distant future
> >
> >I suspect there are quite a number of people thinking about it, and the
> >minimum requirement is just two disks, each with a roughly similar
> >partition. And maybe 'no fear' of building a kernel, if need be.
> 
> I'd actually be most likely to do a RAID 0+5 (I'm pretty sure that's 
> right)... the one where two volumes hold the data, and a third keeps parity 
> data that allows all the data to be reconstructed if any of the three 
> drives should crash or die. (Thus turning 3 drives of X KB each into a 
> total of only 2X usable K of storage, but with peace of mind, even in the 
> absence of any other backup solution.)
 
AIUI, what you're talking about there is simply RAID 3 (multiple disks
striped + 1 parity disk).  The parity disk is the bottleneck for this
setup since every write requires a write to the parity disk.  For a
largely static dataset (eg. MP3 collection) this works well since the
parity disk is rarely used.  For a /home, RAID 5 is that much better since
the parity data is spread over all disks in the set.  Writing speed is not
limited by a single disk.

> I'm generally considering this for my MP3 server, which has roughly 100 GB 
> of stuff on it -- and none of it backed up, because I have no place to back 
> all this stuff up *to*. It'd take something like 150 CDs! I don't even want 
> to think about trying to back it all up to tape.
> 
> So, yeah... I'd need three drives instead of two, but otherwise, you are 
> right, it wouldn't take much. Except time. And having three drives to 
> spare, *aside* from the ones that currently have all the data on them.

AFAICT you can do it with only two extra drives (Maybe you could borrow a
temp drive?).  It goes something like:

1. Create a RAID 0 striped array on two blank disks.
2. Copy data onto RAID 0 set.
3. Sacrifice one of the original data disks to be the parity disk.
4. Edit /etc/raidtab (or whatever it's called) to specify a RAID 3
   and to include disk 3 as the parity disk.
5. Have the RAID 3 set "rebuild" its parity disk.

That gets you your 3 disk RAID with only the cost of the third disk, if
you get to borrow a temporary one that is...

Conor
-- 
Conor Daly <conor.daly at oceanfree.net>

Domestic Sysadmin :-)
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