Re: number of global spares?

On Sat, 2005-08-27 at 08:56 +1000, Neil Brown wrote:

On Friday August 26, strombrg@dcs.nac.uci.edu wrote:

quoted

I've been working on a RAID setup with dual RAID controllers and
three expansion boxes - 48 disks in all, including data, parity and
global spares.

If there are 48 drives, why do your drive-numbers go up to 59?
Confusing but not important.

Agreed.  The hardware likes to do that.

Presumably these are 360G drives (or there abouts) and you are hoping
to use about 42 for data and the remaining 6 for redundancy.

Around 380G.

I feel this a bit tight but could be workable.

If you were using Linux-soft-raid, I would probably suggest 3 16-drive raid6
arrays, possibly making 1 a 15 drive raid6 so there is one global spare.
However I gather you are using hardware RAID - do the controllers
support RAID6 ??

I'd feel better about RAID 6, but the hardware doesn't support it.

quoted

Please be sure to use a fixed-pitch font when viewing the tables found
below.  BTW, if people weren't so terrified of HTML, I could just make a
nice HTML table for easy reading without silly font requirements...

You mean some mail readers use variable-width-fonts to display
text/plain?  How broken!

Heh.

quoted

Does anyone have any comments on:

1) The sanity of these 10 disk RAID 5's?

It depends on the drives.
If you are using you-only-get-what-you-pay-for-IDE-drives, then I
would say it is insane.
If you are using you-pay-for-the-quality SCSI drives, then you should
be fairly safe.

They're SATA's.

quoted

2) The degree of loss of reliability incurred by moving 3 disks from
global spare to data?

That depends a bit on your warranty arrangements on the drives.  If
it's next-day-replacement (Really, truly) then it is probably OK.  If
it is 'send us the bad drive and we'll see what we can do', then I
would suggest thinking again.

This'll be the next thing I ask the vendor.

quoted

3) The degree of loss of reliability incurred by moving 2 disks from
global spare to data?

Should be safe enough.

However I don't understand why you have 4 9+1 arrays, and 1 X+1 for
varying X.  Maybe 9+1 is the largest the controller will give you, so
you have to go to 5 arrays.
In that case, 4 8+1 arrays, 1 9+1 array, and 2 global spares would
seem a more sensible arrangement.

I'm going to pitch this (or maybe 3 8+1's) to the owner of the equipment
and the vendor.

It was the vendor that suggested the 9+1's.