Re: RAID Suggestion for webserver
> the "spare" is distributed over all of the disks...in the case of 4
> one of the disks can be thought of as a spare, which leaves you with 3 *
> 40 = 120 GB.
> depending on the failure type, this is your outcome:
Um... the 3ware card has the ability to mark 1 drive as a "spare" that is
outside the normal RAID5 array. The spare is not normally used, and is
idle. When a disk in the RAID 5 array (one of the three disks) fails, then
the spare HD is powered up and is used to replace the failed HD's data.
Thus the actual usable space in this case is only from 3 HDs/
> lose 1 drive - you're ok, 'cus you can reconstruct it from the mirror
> lose 2 drives - depends on which drives...if it's the identical one on
> mirrored side, you're screwed. if it's the opposing one, you can
> reconstruct from the mirror (all the while your whole system is down
> unless the striped setup can use the left-over drive that's not
Right... so you can lose 1 drive with no problem but the 2nd drive you
lose is based on luck... you have a 25% chance (1/3) that the next failed
drive will be the one that cause complete data loss.
> lose 1 drive - you're ok, 'cus you can reconstruct it from the other
> lose 2 drives - you're screwed.
In this case the 2nd drive you lose is okay, because the array starts with
3 disks, 0 failed, and 1 spare disk. Then if 1 disk fails, you have 3
disks, 1 failed, and 0 spare disk. As someone mentioned in another post,
the time the array can lose all data is during the reconstruction of the
spare disk. If the spare disk is not brought fully online yet and another
disk fails, there is full data loss.