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Re: [Xen-users] tap:aio Performance

On Wed, Feb 14, 2007 at 12:26:32PM +1300, Peter wrote:
> Originally we had tried 3.0.4-0 with loop back mounted file systems.
> For some reason the dom0 crashed (after running for a day or so).  It
> did this a couple of times on one host server for us, and again once on
> another server.
> We have just tried 3.0.4-1 with tap:aio file systems on the domUs. 
> After that we've gone a couple of days with no kernel crash.  Good so far.

You don't say whether the underlying file you are pointing to is sparse
or pre-allocated (non sparse) ?  In the sparse case it is expected that
performance is terrible - because every write requires the undering FS
to allocate some more blocks - which in turn causes a journal sync. If
you use non-sparse then all the blocks are pre-allocated so you don't get
the journal bottleneck.

> However it seems that performance is a lot slower.
> e.g. on a domU:
> :/$ time sudo du -s
> 2020684 .
> real    9m25.646s
> user    0m0.044s
> sys     0m0.144s
> On a laptop with a puny 5400 rpm drive: $ time sudo du -s
> 86923472        .
> real    0m18.376s
> user    0m0.532s
> sys     0m10.737s
> And things like bonnie seem to make no real progress.
> And on startup things 'seem' slower.

Slower than what?  I'd certainly expect tap:aio: to be slower than file:
because file: is not actually flushing your data to disk - it hangs around
in memory and is flushed by the host kernel VM as needed. Since this isn't
remotely safe for your data, its not even worth comparing tap:aio with file:.

> Has anyone else experienced slower disk IO with 3.0.4/blktap?

Yes, when using sparse files ontop of a journalled fs.

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