Re: [Xen-users] What is the "fastest" way to access disk storage from DomU?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I've read some threads about storage speed here but didn't really got
> a clue on what's the "best" or fastest way to set it up.
>
> At the moment all the virtual disks are configured as
>
>   disk = [ "phy:/dev/vg_xen1/<LV>,xvda1,w", ... ]
>
> The volumes residing on the SAN storage are configured via EVMS and
> I've 200MB/s writing speed from Dom0 (mesured with dd if=/dev/zero
> of=/mnt/file) and "only" around 150MB/s when doing the same from DomU.

When you do the tests of writing speed from dom0, are you writing to the 
domU's filesystem LV?  Otherwise you're not testing like-for-like since 
you're using a different part of the storage.  I'm not sure if this makes a 
difference in your case, but different parts of a physical disk can have 
surprisingly big differences in bandwidth (outer edge of the disk moves 
faster, so better bandwidth).

I'm not too familiar with EVMS, maybe there's some bottleneck there I'm not 
familiar with and therefore missing.  Does EVMS do cluster volume management?  
I guess it does, as you're using it on a SAN ;-)

> Is this expected speed loss or is there any other way to give the DomU
> access to the devices?

You can only give domUs direct access to whole PCI devices at the moment, so 
unless you gave each a separate SAN adaptor, you can't really give them any 
more direct access.

There's some work on SCSI passthrough being done by various people, so maybe 
at some point that'll let you pass individual LUNs through from the SAN.

For really high performance SAN access from domUs, the solution will 
eventually (one fine day, in the future) to use SAN adaptors with 
virtualization support that can natively give shared direct access to 
multiple domUs.  We're not quite there yet though!

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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