[Libvir] Re; virDomainBlockStats

Gareth Bult gareth at encryptec.net
Sun Jan 27 20:22:06 UTC 2008


I will have to experiment to see what happens by forcing failures and measuring the damage ..

fyi; I'm pretty much 100% sure aio does not work on "gluster" .. indeed it seems this may be my "main" problem with aio as now I've moved some testing kit onto local partitions, I am getting some better results. That said, I'm getting of the order of within 5% native speed over gluster, which seems to be approaching that of local partitions .. 
(using "file:" .. AND all filesystems were created as sparse ..)
[not really an issue as we *need* a cluster fs for migration]

It sounds like I might have to hack the loopback driver and force a regular sync out of it .. however, testing first .. 

Also (!) our "accidental" policy of storing critical data (DB's etc) outside of the DomU's on raw network filesystems has probably been rather fortunate ...


Note; my hack to libvirt to work with file: seems to be 100% across my particular platform ... 


----- Original Message -----
step 3.: "Daniel P. Berrange" <berrange at redhat.com>
To: "Gareth Bult" <gareth at encryptec.net>
Cc: "libvir-list" <libvir-list at redhat.com>
Sent: 27 January 2008 17:03:09 o'clock (GMT) Europe/London
Subject: Re: [Libvir] Re; virDomainBlockStats

On Sun, Jan 27, 2008 at 02:53:46PM +0000, Gareth Bult wrote:
> Ok, question;
> Does TAP:AIO work on networked filesystems .. my testing says not.

It does on Fedora / RHEL at least - it should work on any FS that
supports direct IO & async IO.

> XEN without a good underlying cluster filesystem is to say the 
> least "limited". If you can't use AIO in this sort of environment, 
> it also becomes limited .. and if libvirt requires this, it means 
> libvirt can't be used on large-scale roll-outs on (certain?) 
> network filesystems.

We need to fix libvirt so that stats work with file: based devices
too. We just need to figure out the logic required in order to
generate the correct device ID.

> Interestingly (looking at the Xen list re; performance problems) 
> I'm getting > 65Mb/sec on my DomU's .vs. 70Mb/sec on my host 
> nodes Dom0 ... and it's proving to be reliable atm .. 

You ought to be able to get within 5% of host performance when 
using either phy: (for real block devs) or tap:aio:. NB make
sure you do *not* use sparse files - fully allocate the file
if you want good performance. Sparse file have terrible performance
characteristics as blocks as allocated on-demand which causes
metadata syncs of the underlying host FS, which serializes all
I/O operations.

> Be interesting to know why Xen only documents "file" given 
> the critical nature / apparent flaws in the driver.

Xen documentation leeaves alot to be desired :-(

> Is there "no" way of forcing a loopback interface to flush itself?

Unfortunately not - its inherant in the impl of loop devices.

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