[PLUG] RAID5 suggestions

Wil Cooley wcooley at nakedape.cc
Thu Oct 6 00:52:14 UTC 2005


On Wed, 2005-10-05 at 14:38 -0700, Scott Van Hoosen wrote:
> I'm building a file server with 6 300GB SATA drives, using software
> RAID5. This will probably be running Fedora Core 4. I want most of the
> drive space available for file storage (/home/shares). Thing is, I know
> you can't boot off a software RAID5 partition, just single or mirrored.
> How would you folks set up your drives with redudancy and best use of
> drive space?

I wouldn't use RAID5 is what I'd do.  While I've read plenty about
RAID5, there are certain aspects that remain elusive to me, so I tend to
prefer RAID1 and RAID0, which are simple and clear.

I'd peel off about 100MB of a couple drives for RAID1 /boot (or, with
300GB drives, do on all 6 and have a 6-way mirror).  Then I'd take out
1/2 or 1/4 of your RAM from each drive for swap space and set them all
at the same priority (the kernel with balance the swapping between them,
in a RAID0-like fashion, except if you lose some of them it will use
what's available, unlike a real RAID0, which would prevent you from
using any of it).

Partition the rest of the space on each drive as one partition and then
you could do several things.  Exactly how or what depend on the
application, but you could:
  o Use RAID5 on all 6 partitions
  o Make 3 RAID1 volumes and stripe (RAID0) across them--this is the
best performance, although the available space is less than RAID5.  This
is called RAID0+1 or 1+0; I've never actually been clear which.  You can
also make two RAID0 stripes and then mirror them with RAID1.
  o Make 3 RAID1 pairs and use them as you see fit; for example, the
first pair could be for your regular system stuff, then you could do
RAID0 with the remaining 2 RAID1 volumes.  Or you could do RAID5 with
the remaining 4 volumes.  You could also keep them partially independent
of each other if you're dealing with database applications and a DBA who
like to control what data goes on what spindles.

At any rate, with whatever scheme you choose, use LVM to allocate your
actual file systems.  In the latter case of two RAID1 volumes and a
picky DBA, for example, you can specifically allocate certain logical
volumes to certain physical volumes.  With FC4, you can even resize
ext2/3 filesystems on-line (while mounted), so it's even easier
(although I wish they'd included e2fsadm, which is easier than using
lvextend+ext2online, partly because I find myself continuously
forgetting the name of the latter command).  The LVM HOWTO gives you a
pretty good introduction.

Wil
-- 
Wil Cooley                                 wcooley at nakedape.cc
Naked Ape Consulting                        http://nakedape.cc
* * * * Linux, UNIX, Networking and Security Solutions * * * *
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