[Xen-users] Physical Disk VBD vs File VBD Performance

[Matias <m0topilot@xxxxxxxxx>]

Hi Everyone, disk I/O performance question here.

I have system that uses RAID5 for the the dom0 and I have to domUs
running.  One dom1 uses a file based VBD and dom2 uses another RAID0
array *with one disk*.  I know you say "wtf, one disk???", but I'll
explain later.

When doing 2GB write tests with dd command (see below) on the dom0
I get about 64MB/s to the disk.  Now when I write the same dd command
out on dom2 (physical VBD, RAID0) I get about 30MB/s to the disk.  I then
asked myself if this made sense.  Correct me if I'm wrong, when writing
to RAID5 the data gets striped up and your write performance increases.
Now if I write to the dom2 it has to write the whole data to one spindle.

`dd if=/dev/zero of=file_2gb_1024k_blocks bs=1024k count=2000`

So for shucks n grins, I tested the dom1 (file VBD, RAID5/dom0)
write performance and nearly had a stroke when I saw that it reached about
54MB/s write speed.  I understand that writing on dom1 is faster because of
the RAID5 on dom0.

Granted I'm only writing one file to the disk, but is it better in overall
performance, considering multiple concurrent file reads/writes that dom2 will
experience, to use the physical VBD?  Will the file VBD degrade in performance
earlier than the physical VBD under the same heavy load earlier?

I'm using dom1 as a web server and dom2 as a database server.

About why I'm using one spindle on for the RAID0 array is that I can only
afford 1 drive right now to get started and Dell has assured me that I can
add two more drives to form a RAID5 array - ultimately, it is my goal.

dom0 = Dual Proc, Dual core Woodcrest,6GB (total), RHEL 4.4, RAID5/3 ultra 320 
10K rpm.
dom1 = Centos 5, file VBD, 2GB.
dom2 = Centos 5, physical VBD RAID0/1 ultra 320 10K rpm, 2GB.

Thanks for your time and knowledge,
Matt

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