Re: [Xen-devel] request of blkfront may disorder after migration without waiting in suspend phase of migration

[Daniel Stodden <daniel.stodden@xxxxxxxxxx>]

On Wed, 2011-07-13 at 05:09 -0400, Ian Campbell wrote: 
> On Tue, 2011-07-12 at 19:23 +0100, Konrad Rzeszutek Wilk wrote:
> > On Sun, Jul 10, 2011 at 12:36:06PM +0800, alice wan wrote:
> > > Hi konrad,
> > > 
> > > I found the mainline linux 3.0-rc6 xen-blkfront driver doesn't implement
> > > suspend func. In my opinion, the blkfront should wait till responses of 
> > > all
> > > the pending requests come back, then domu can be suspended,
> > 
> > I think you mean blkback. It should be the job of blkback to account for
> > all I/Os and make sure they have completed before migrating?
> 
> blkfront has a resume hook which (AIUI) is intended to recover things
> after a migration and this seemingly does not require any work on the
> suspend path (or at least was presumably thought not to when it was
> written...).

>From a pre-copy migration perspective, adding a suspend hook to synch on
pending I/O is not ideal. It's just going to add unsolicited memory
footprint, with fairly arbitrary amounts of additional memory getting
dirtied ('arbitrary' depending on what else is allowed to keep running
in the guest in that state).

I guess that's why post-migration recovery was preferred, and I still
don't see why that's to blame.

Daniel

> Ian.
> 
> > 
> > I did have a patch for the 2.6.32 to deal with this but I never was able
> > to reproduce this. Are you able to reproduce this? Can you give me
> > step by step instructions on how to do it?
> > 
> > > otherwise,  after the migration, vm of target host will request from 
> > > target
> > > host, meanwhile, the blkback of source host maybe do the pending requests
> > > until they's done.
> > > 
> > > if migrate the vm with high io pressure,  the requests of this vm may
> > > disorder.
> > > 
> > > And i saw the gpl windows pv driver, blkfront will change its state and 
> > > wait
> > > for blkback closing, after blkback closing, the pending requests 
> > > definitely
> > > are done.
> > > 
> > > More, i did some test with migration,  while vm was running dd task,  xm
> > > migrate -l .  before it suspend, gdb tapdisk2.  after migration, new vm
> > > started. then quit gdb, the pending request were written back to disk 
> > > after
> > > new requests.
> > > 
> > > above mentioned is just my opinion, i need your advice and confirmation or
> > > deny
> > > 
> > > any help is appreciated.
> > > 
> > > 
> > > Regards
> > > 
> > > wanjia
> > 
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> > 
> > 
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> 
> 
> 
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