Re: sync guest calls made async on host - SQLite performance

[Anthony Liguori <anthony@codemonkey.ws>]

Matthew Tippett wrote:
> Thanks Duncan for reproducing the behavior outside myself and Phoronix.
>
> I dug deeper into the actual syscalls being made by sqlite.  The 
> salient part of the behaviour is small sequential writes followed by a 
> fdatasync (effectively a metadata-free fsync).
>
> As Dustin indicates,
>
>    if scsi is used, you incur the cost of virtualization,
>    if virtio is used, your guests fsyncs incur less cost.
>
> So back to the question to the kvm team.  It appears that with the 
> stock KVM setup customers who need higher data integrity (through 
> fsync) should steer away from virtio for the moment.
>
> Is that assessment correct?

No, it's an absurd assessment.

You have additional layers of caching happening because you're running a 
guest from a filesystem on the host.

A benchmark running under a guest that happens do be faster than the 
host does not indicate anything.  It could be that the benchmark is 
poorly written.

What operation, specifically, do you think is not behaving properly 
under kvm?  ext4 (karmic's default filesystem) does not enable barriers 
by default so it's unlikely this is anything barrier related.

> Regards,
>
> Matthew

Regards,

Anthony Liguori