[Gllug] [OT] Re: cheap disk source

[Kostas Georgiou]

On Wed, Jul 25, 2007 at 10:25:36AM +0100, John Hearns wrote:

> James Roberts wrote:
> >
> > 
> > John,
> > 
> > may I ask: what are your uptime views on RAID6 no hot spare -vs- RAID5 + 
> > hot spare?
> 
> I do not have views on uptime I'm afraid.
> 
> What RAID 6 is intended for is to cope with double disk failure.
> If your RAID 5 set loses a disk it will start to rebuild onto a hot 
> spare (assuming there is one). During that time window if you lose 
> another disk, or there are bad blocks on the spare, then you can lose data.

It's even worst than that actually have a look at
http://storagefoo.blogspot.com/2006/05/raid-dp-vs-raid-10-protection.html
for example. The 10^14 bit error rate means that every time you rebuild
your ~11TB raid5 array you'll be corrupting a bit (or 1 in 10 times in
your 1TB array). No idea how accurate the 10^14 figure is though. I have
around 70TB and I haven't seen any corruption yet but then again the
data is transient and we can easilly afford to loose them so I doubt that
we'll notice if it happens.

> Raid 6 has a performance penalty wrt Raid 5
> 
> Of course, if you have the requirement and the budget (ie twice the 
> number of disks!) there is RAID 51 - mirrored RAID 5 sets.
> You pays your money and you takes your choice.

Or read http://www.baarf.com/ and go with raid10 or raid01 if you care
about performance :)

Cheers,
Kostas
-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug