new knob to tweak caching mode for backends

[Olaf Hering <olaf@aepfle.de>]

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
domU…cfg 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.

The use case I have in mind are guest installations, which trigger alot
of disk io. We see a speedup from 22min down to 10min if qemu is forced
to use cache=unsafe. 

I wonder if and how such a knob should be exposed in domU.cfg:disk=[]
To me it looks like a bool similar to direct-io-safe is enough. If the
flag is set in xenstore, xen qdisk sets BDRV_O_NO_FLUSH to skip flushes.


Olaf

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