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Old 12-18-2010, 08:58 PM
Chuck Munro
 
Default Disk array format for CentOS virtual host

Hello List Members (and Holiday greetings!)

This rambles a bit ... my apologies in advance.

I am in the process of building a large CentOS-based VM host machine
which will replace several individual boxes. I've done the usual
hardware research and ended up with a SuperMicro motherboard, dual
Xeons, lots of ECC RAM, solid power supply, and tons of cooling.

When it comes to the best disk array layout for 7 guest instances of
CentOS (mostly 64-bit) I run into the classical argument of disk I/O
performance versus array robustness. I'm using Linux software RAID so I
don't get locked into a vendor's controller, and because I've got CPU
cycles to burn.

The intent is to use CentOS-6 with KVM after climbing the usual
experimental learning curve. I've got years of CentOS experience, but
none with KVM.

So, underneath all of this I could use RAID-10/RAID-6/RAID-60. Which
one is better for the following work mix:

- 4 web servers, not heavily loaded (yet), mostly CGI.
- 2 independent Zimbra email servers, which seem to spend most of
their time rejecting spam and malware.
- A primarily-inbound FTP server with bandwidth throttling.
- A batch ImageMagick video process which kicks in a couple of
times/day for a few minutes.
- A secondary/backup SMB server, not heavily used.
- A very lightly loaded DDNS server.

Two spindles will be used in RAID-1 for the host OS filesystems.

I have 10 large drives for the guest machines, but because they're 2
TByte I want to ensure reasonable fault tolerance, meaning the ability
to handle at least two drive failures. This really means that an array
rebuild to replace *one* drive stands a good chance of succeeding.

At first glance this would seem to be a system which will be doing more
reads than writes, so RAID-6 or 60 might be a reasonable fit. RAID-10
offers much better write performance according to what I've been
reading, but is less fault tolerant. RAID-60 is a bit of an unknown to
me with regard to performance.

In case you're wondering, I don't consider RAID to be a backup, so all
of this will be automatically backed up to duplicate separate machines,
each having tons of RAID-6 storage. Each individual guest OS will do
its own rsync thing.

Please forgive my lack of VM experience, but an additional question ...

I'm thinking of creating a separate RAID-xx MD partition for each guest.
Should I hand each over as a 'device' to KVM or is it better to place
the guest filesystems in a big file on each separate MD partition? Or,
is it better to create one huge RAID-xx partition and just use big
filesystems-in-a-file. Either way would probably take the same amount
of time (long!) to rebuild an array, if I understand this correctly.

The collective wisdom of this list is most welcome.

Thanks muchly,
Chuck
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