Re: [PATCH] Use direct I/O for loop devices

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Aug 18, 2022 at 09:02:11AM +0200, Jan Beulich wrote:
> On 17.08.2022 22:46, Demi Marie Obenour wrote:
> > This is a huge performance improvement for two reasons:
> > 
> > 1. It uses the filesystem’s asynchronous I/O support, rather than using
> >    synchronous I/O.
> > 2. It bypasses the page cache, removing a redundant layer of caching and
> >    associated overhead.
> > ---
> >  tools/hotplug/Linux/block | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/hotplug/Linux/block b/tools/hotplug/Linux/block
> > index 
> > 2691b56951c9b82094471a141b9e0bed04abb929..75785f3a5422c4a5f962a4cd4f6acae5080d036d
> >  100644
> > --- a/tools/hotplug/Linux/block
> > +++ b/tools/hotplug/Linux/block
> > @@ -330,7 +330,7 @@ mount it read-write in a guest domain."
> >          else
> >            roflag=''
> >          fi
> > -        do_or_die losetup $roflag "$loopdev" "$file"
> > +        do_or_die losetup --direct-io=on $roflag "$loopdev" "$file"
> 
> I guess you want to first check (maybe in tools/configure) that losetup
> actually supports that option. The old-ish one I'm looking at doesn't,
> according to its --help output at least.

What version are you referring to?

In Qubes OS the current plan is to use a block “script” written in C, to
improve performance and (when combined with kernel patches) eliminate
race conditions.  This code could be made a wrapper for the C version.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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