[CentOS] KVM Host Disk Performance

Sun Apr 3 18:03:53 UTC 2011
Dawid Horacio Golebiewski <dawid.golebiewski at tu-harburg.de>

Cannot give you any useful input  but I am certainly interested in any
progress you make, so keep communicating :D


-----Original Message-----
From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On Behalf
Of Chuck Munro
Sent: Sonntag, 3. April 2011 19:47
To: CentOS Maiing List
Subject: [CentOS] KVM Host Disk Performance

Hello all,

I'm having quite an interesting time getting up to speed with KVM/QEMU and
the various ways of creating virtual Guest VMs.  But disk I/O performance
remains a bit of a question mark for me.  I'm looking for suggestions and
opinions ....

This new machine has tons of disk space, lots of CPU cores and loads of RAM,
so those are not issues.

I currently have several software RAID-6 md's, each of which is about
700 GBytes.  The md's are created as partitionable arrays, and I assign the
resulting "raw" disk devices to each CentOS Guest OS, which then partitions
the space according to its needs.

Unlike using the more common "filesystem-in-a-file" method, the use of
partitionable MD devices usually results in wasted disk space, but my aim is
to provide the best performance possible with robust RAID.

Is there a better-performing way of assigning disk space to Guest VMs? 
What about 'qcow2'?  I understand its copy-on-write offers several
advantages, but does it perform better than using partitionable md arrays or
does the underlying filesystem overhead get in the way?

Perhaps the native overhead of RAID-6 dual parity makes the whole question
moot.  Would the use of RAID-10 with hot spares be paranoid enough?  I'm
using 2 TByte drives which inherently have much greater chances of data
errors, so I must keep data reliability in mind.

I don't want to start a religious war over RAID and Guest VM disk
methodology, but I have the luxury of trying various disk configurations
until I get a reasonable balance between performance and reliability with
these large drives.

So many questions ... so much to learn.  Lots of Googling around is quite
useful but can lead to information overload :-)

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