[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [Qemu-devel] [PATCH v2] xen_disk: support "direct-io-safe" backend option





--On 28 June 2013 11:44:41 +0100 Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx> wrote:

Looks useful. Are you planning to do this for both emulated and pv
disks?

Emulated disks don't have the same problem because they don't try to
use O_DIRECT on pages shared with the guest via the Xen grant table
mechanism.

I should have been more specific. The original thread maintained
emulated disks always had O_DIRECT turned off, despite the fact
the rationale for using O_DIRECT for PV disks was that not using
O_DIRECT in some circumstances might be unsafe, because it was
the only way to get any decent performance out of them. I think
we ran the 'no O_DIRECT might be unsafe' argument to ground, but
if the rationale for Stefano's patch is not just speed but additional
safety (for instance against the host dying and losing the page
cache for file systems that have barriers switched off), then
there is an argument to use it for emulated disks too.

But as Stefano says:

--On 28 June 2013 11:56:29 +0100 Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> wrote:

This is PV only, at least for the moment: emulated disks always use
writeback caching.
From the performance point of view, making this change for IDE disks is
not very important (because IDE is slow anyway).

... perhaps 'who cares'.

--
Alex Bligh

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.