Re: new knob to tweak caching mode for backends

[Jim Fehlig <jfehlig@suse.com>]

Stefano Stabellini wrote:
> On Tue, 27 May 2014, Jim Fehlig wrote:
>   
>> 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
>>> 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.
>>>   
>>>       
>> ACK to the idea of admin control of cache mode.  As you say, admins
>> already enjoy this capability with libvirt and qemu.
>>     
>
> There is nothing wrong with having advanced options for people that know
> what they are doing and/or they are happy with taking risks and loosing
> data. What's important is not to rely on these "advanced options" to get
> a decent default configuration.
>   

Agreed.  The current default, which is used by qemu too, is the best
choice IMO.

> So I think that as long as we keep the default behaviour sane, we can
> expose whatever advanced options we like.

Cool.  So in libvirt terms, cache not specified or cache='default' would
yield the same behavior as today.

Regards,
Jim


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