> I'm looking for advice on the best possible raid configuration
> for my requirements and hardware availability.
>
> I have 4 9 gig SCSI drives. The machine is basically going
> to be a centralized fileserver for web servers which surround
> it and mount /home via nfs.
>
> These drives are only going to be used for /home.
>
> I would love the ability to add drive space to the raid
> partition at a future date.
>
> Redundancy is more important then speed, although I'd
> like to get the best of both worlds.
How important are the web services this machine will support?
If they are mission-critical (for instance, if this is the principal front-end for a
web-based business), you should look at server mirroring, because RAID only protects
you against disk failure, and there may be plenty of other SPOFs in a PC (SCSI
controller, CPU, PSU etc.).
If not, how much downtime can you afford?
The best reliability (and good read performance, worse write performance, which is
probably what you want if this partition contains a whole load of user web sites) on
a single machine will probably be achieved with a RAID-1 setup, where each side of
the mirror has its own controller. You could use a single controller, but two will
give you better performance, offer more bandwidth, and protect against the failure of
a controller (assuming your SCSI driver can handle that occurrence).
Unfortunately, it used to be the case that you could only combine RAID-1 with
RAID-0/linear as mirroring on top of striping, not the other way round. I'm not sure
if this is still the case. If so, you will have to stripe two pairs of disks and then
mirror the pairs with RAID-1. This will make it difficult to expand the array,
because, although you could add extra disks to a linear array (I think, but I don't
know that much about RAID-0/linear), the RAID-1 array of the linear arrays could not
cope with the expanded size of its component arrays. So if you are going to go this
way, you might as well do RAID-1 on RAID-0, which will give better performance than
RAID-linear.
If it is now possible to do RAID-linear on RAID-1, you could mirror two pairs of
disks with RAID-1, and then join them with RAID-linear (still giving 18G). RAID-0
would probably be faster than RAID-linear, but RAID-linear would give you the option
to add space dynamically later. You could simply buy another pair of disks, create a
RAID-1 array with them, and then join them to the existing linear array. With this
setup you even have a chance of surviving the simultaneous failure of 2 disks, as
long as the disks are not both on the same RAID-1 array.
If you want to squeeze more out of your 4 disks, you should use RAID-5. This offers
perfectly satisfactory redundancy for most uses, but offers no options to dynamically
increase the size of the array (so to add disks, you would simply have to backup the
array, add the disks, recreate a larger array, and then restore the backed-up data to
the new array).
If your server will be remote and hard to access, you could do a 3-disk RAID-5 array,
with the 4th disk as a hot spare. But this would only give you the same disk space
(18G) as the RAID-1 solution. In normal operation, you would get better performance
out of the RAID-1 options, but this RAID-5 option would be quicker to return to
normal performance in the event of a failure.
If the server is easily accessible or you can accept the risk and lower performance
of running in degraded mode for a while, then you would be better off doing a 4-disk
RAID-5 array. This will give good enough redundancy/reliability and performance for
most uses, and give you 27G of storage.
All in all, if it's possible and you are not worried about wasting disk space, the
RAID-linear on RAID-1 sounds the best bet for you. If not, you might as well go for a
4-disk RAID-5 array.
While you are at it, if you want to make your life easier, you should get hot-swap
cartridges to put the disks in. Otherwise, you may have to turn off the server to
replace a failed disk, even though it was capable of continuing to run. This also
makes testing the system for failures much easier. And if you get cartridges with
fans in them, you will increase the expected life of your disks.
And remember that storage is only one element of redundancy. It would not be prudent
to focus exclusively on this and ignore the PSUs, fans, controllers, CPUs....
> I like the idea of
> the autostart partition label mentioned in the RAID5.HOWTO.
> Is this also available in raid 1.
Yes, autostart can be used with RAID-1, or even RAID-0, as well as RAID-5 (but
remember your kernel cannot live on a striped array). However, I can't figure out how
it would work with a RAID-1/0 combination. My guess is that you could only autostart
the base arrays, not the arrays which built on them.
Hope I've got the above right. If not, I'm sure more knowledgeable RAID experts will
set you straight.
Cheers,
Bruno Prior [EMAIL PROTECTED]
