Re: Fatal crash on xen4.2 HVM + qemu-xen dm + NFS

[Alex Bligh <alex@alex.org.uk>]

Ian,

--On 22 January 2013 10:13:58 +0000 Ian Campbell <Ian.Campbell@citrix.com> 
wrote:

> AIUI it is intentional. The safe option (i,e, the one which doesn't risk
> trashing the filesystem) is not to cache (so that writes have really hit
> the disk when the device says so). We want to always use this mode with
> PV devices.

Except it isn't the safe option because O_DIRECT (apparently) is not safe
with NFS or (we think) iSCSI, and in a far nastier way. And fixing that
is more intrusive (see your patch series).

I can't test it at the moment (travelling), but if not using O_DIRECT
fixed things, would you accept something that somehow allowed one to
specify an option of not using O_DIRECT? I need to look at how to get
the disk config line from xen_disk.c if so.

> However the performance impact of not caching the emulated (esp IDE)
> devices is enormous and so the trade off was made to cache them on the
> theory that they would only be used during install and early boot and
> that OSes would switch to PV drivers ASAP. Also AIUI the emulated IDE
> interface doesn't have any way to request that something really hits the
> disk anyway, but I might be misremembering that.

You mean the equivalent of flush/fua? I can't comment, but I believe
the write barrier stuff isn't in 4.2 in xen_disk either.

> Check the archives from around the time this change was made, I'm pretty
> sure there was some discussion of why things are this way.

I tried but couldn't find anything. I could only find the patches
going to qemu-devel and not on the main list, but I may well have missed
them as I'm relying on Google finding stuff in archives at that time.

-- 
Alex Bligh