LVM + RAID follow up

[warlord at MIT.EDU (Derek Atkins)]

"Rich Braun" <richb at pioneer.ci.net> writes:

> need a couple hundred gigs of storage.  Note also that to run RAID5
> you need separate controllers for each drive.  A standard
> motherboard contains two PATA controllers, you need to make at least
> a third available if you want good performance.  (SATA is probably
> the solution, now that the drives are nearly as cheap as PATA.)

SATA is only barely more pricey than PATA.  With 300G-400G drives
(which is getting close to the minimum $/GB curve) the price
differential between PATA and SATA is relatively small.  You can get
a PCI SATA driver with... 4 SATA ports?  Or 8?  Add that to the Mobo
and you can probably get 6-8 SATA drives.  With 8 400GB drives using
RAID-5 + 1 hot spare you can combined them into 2.4TB of storage.

My plan was to use this space for MythTV Backend Storage and as an
online backup server for my desktops, laptops, and critical data
backups (e.g. web server content).

I'd like it all in a single file system instead of multiple partitions
because I don't know a priori how much space I want to the different
uses.

>>  But I also don't know how
>> to combine the RAID vs. LVM vs. partitions in order to have
>> a protected /boot partition in addition to a very very large /
>
> I don't bother with putting /boot on RAID, haven't found a reliable way to do
> it yet.  But it doesn't change very often so you can simply rsync it to your
> other drive whenever you do a kernel update.

I use RAID-1 on all my servers now.  It's a cheap
get-out-of-disk-failure-free card.  I don't use LVM on those servers;
I just don't see the point.  It seems to add complexity to what I view
as little gain.  I have two MD devices, /boot and /.  I've got grub
installed on both drives and I've got grub.conf configured to boot off
both drives.  What this means is that I can pull out either drive and
the machine still boots!

I've got a script that will rebuild the raid array (including copying
the partition table from the old drive to the new drive, setting up
grub, and rebuilding the raid).  It works great and takes all the
guesswork out of a job that I rarely perform.  Indeed, I've never
(yet) had to perform this task on a live server; only in my testing.

> The distros, at least SuSE, will set up LVM-over-RAID for you out of the box,
> just follow the directions.  Make sure to leave some unallocated space in the
> LVM.  (My advice--set up a small /boot partion, 200M to 500M, and make a
> second partition /dev/md0 take up the rest of the space on the smaller of the
> two drives.  Make these two partitions the same size on the larger drive.  If
> the two drives are the same, proceed; if there is remaining space on the
> larger one, you can set up some scratch storage.  Then decide what logical

I guess this was part of my question (and confusion)..  Do I want LVM
over RAID or RAID over LVM?  Or do I want LVM over RAID over LVM?
Also, if I want to do a RAID5 for / but RAID-1 for /boot, how do I
want to lay that out?  With RAID-5 do all my drives/partitions need to
be the same size like they do with RAID-1?

And then what's my upgrade path if I decide to swap out to larger
drives?  Let's say that 3-5 years from now I decide to swap out my
400G drives with 1TB drives -- what would be the right process to do
that such that my main raid partition size increases?  (Then I can
resize my ext3 FS).

I don't trust reiserfs for a server -- it's not designed to handle
catastrophic failures like a power outage.  Yeah, you could setup a
UPS with auto-shutdown, but why take the extra chance with a fragile
filesystem?

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available

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