[CentOS] quick question on RAID 1

Sun Aug 14 09:52:59 UTC 2005
Ulrik S. Kofod <usk at cybersite.dk>

Lamar Owen sagde:
> On Saturday 13 August 2005 07:55, Ulrik S. Kofod wrote:
>> I have 3 disks that I want to setup in a raid 1, my questions is should I
>> setup the 3rd disk as a spare-disk or should I mirror all 3 disks all the
>> time?
> I believe you can do either, but setting it up as a spare works ok, too.  Nice
> when one drive of a mirrored RAID1 starts throwing errors to throw the spare
> on, let the sync happen, then hot fail the drive that's throwing errors.

I just thought the it would be safer to mirror all 3 from the start as if one fails
the system will rely on only one disk until the spare is synced.

> For RAID 1 the number of drives is not limited to 2, contrary to common
> belief.  You can have three or more; just understand that a RAID 1 set with 4
> 250GB drives (for example) will present itself as a single 250GB RAID

I know I can add as many drives as there is room for in the box to a RAID1 and that
the available disk space will be equal to the size of smallest disk (if they are not
the same size). I just can’t see why I should add one as a spare, when I can mirror
it, and then no sync is needed when one fails (until I replace it).

Space is not an issue, the drives has more than enough space, so I would rather go
for a RAID1 than a RAID5, where I know the system will work on just a single drive.
Performance is also a minor concern as the box is pretty much oversized for the job.

>> Will it perform *much* better to only mirror two and have the 3rd as a
>> spare. Is it not safer to mirror all 3 drives all the time ?
> The md layer will have to perform three writes instead of two; the drive
> interface technology will impact the performance of a triple write more than
> anything else, with straight IDE taking probably a 33% performance hit
> (having 66% of the writes-per-second of the two-drive RAID), with
> well-designed SCSI setups (multiple host adapter channels) potentially taking
> a small fraction of that hit.

I was thinking of putting only one disk on each controller (the MB controller and 2
PCI controllers).  Safety is my main concern, and if one controller fails it will
only take down 1 disk. Is that overkill? Would it be better to have just one per
channel i.e. two on the MB controlller and 1 on a PCI controller?