Re: [Xen-devel] new knob to tweak caching mode for backends

[Jim Fehlig <jfehlig@xxxxxxxx>]

Olaf Hering wrote:
> Currently libxl (and xend) has no knob to control cache mode of backend
> driver for block devices. libvirt and qemu have:
> cache=off|none|directsync|writeback|unsafe|writethrough.
>
> The xen qdisk driver in qemu defaults to "writeback". If the diskspec in
> domUâcfg has 'direct-io-safe' then qdisk will default to directsync AIO.
>
> I think these defaults are fine as they provide some sort of data
> integrity.
>
>
> But there is one issue: all the flushing thats going on during guest
> triggered writes does slows down the guest. There should be a knob to
> skip the regular flushes on the host.  If the given backing file will
> contain throw-away data its up the the admin to make this decision.
>   

ACK to the idea of admin control of cache mode.  As you say, admins
already enjoy this capability with libvirt and qemu.

Regards,
Jim


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