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

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.


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