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Re: [dm-devel] Barriers still not passing on simple dm devices...




On Tue, 31 Mar 2009, Jens Axboe wrote:

> On Mon, Mar 30 2009, Mikulas Patocka wrote:
> > On Thu, 26 Mar 2009, Jens Axboe wrote:
> > 
> > > On Wed, Mar 25 2009, Mikulas Patocka wrote:
> > >
> > > > > > So I think there should be flag (this device does/doesn't support data 
> > > > > > consistency) that the journaled filesystems can use to mark the disk dirty 
> > > > > > for fsck. And if you implement this flag, you can accept barriers always 
> > > > > > to all kind of devices regardless of whether they support consistency. You 
> > > > > > can then get rid of that -EOPNOTSUPP and simplify filesystem code because 
> > > > > > they'd no longer need two commit paths and a clumsy way to restart 
> > > > > > -EOPNOTSUPPed requests.
> > > > > 
> > > > > And my point is that this case isn't interesting, because most setups
> > > > > don't guarantee proper ordering.
> > > > 
> > > > If the ordering isn't guaranteed, the filesystem should know about it, and 
> > > > mark the partition for fsck. That's why I'm suggesting to use a flag for 
> > > > that. That flag could be also propagated up through md and dm.
> > > 
> > > We can do that, not a problem. The problem is that ordering is almost
> > > never preserved, SCSI does not use ordered tags because it hasn't
> > > verified that its error path doesn't reorder by mistake. So right now
> > > you can basically use 'false' as that flag.
> > 
> > There are three ordering guarantees:
> > 
> > 1. - nothing (for devices with write cache without cache control)
> > 
> > 2. - non-cached ordering: the sequence [submit req a, end req a, submit 
> > req b, end req b] will make the ordering. It is guaranteed that when the 
> > request ends successfully, it is on medium. This is what all the 
> > filesystems, md and dm assume about disks. This consistency model was used 
> > long way before barriers came in.
> > 
> > 3. - barrier ordering: ordering is done with barriers, [submit req a, end 
> > req a, submit req b, end req b] won't guarantee ordering of a and b, a 
> > barrier must be inserted.
> 
> Plus the barrier also allows [submit req a, submit req b] and still
> count on ordering if either one of them is a barrier. It doesn't have to
> be sync, like the (2).
> 
> > --- so you can make a two bitflags that differentiate these models. In 
> > current kernel, model (1) and (2) cannot be differentiated in any way. (3) 
> > can be differentiated only after a trial write and it won't guarantee that 
> > (3) will be valid further.
> 
> But what's the point? Basically no devices are naturally ordered by
> default. Either you need cache flushes, or you need to tell the device
> not to reorder on a per-command basis.
> 
> > > > The reasoning: "write barriers aren't supported => the device doesn't 
> > > > guarantee consistency" isn't valid.
> > > 
> > > It's valid in the sense that it's the only RELIABLE primitive we have.
> > > Are you really suggestion that we just assume any device is fully
> > > ordered, unless proven otherwise?
> > 
> > If someone implements "write barrier's aren't supported => run fsck", then 
> > a lot of systems start fscking needlessly (for example those using md or 
> > dm without write cache) and become inoperational for long time because of 
> > that. So no one can really implement this logic and filesystems don't run 
> > fsck at all when operated over a device that doesn't support ordering. So 
> > you get data corruption if you get crash on those devices.
> 
> Nobody is suggesting that, it's just not a feasible approach. But you

I am saying that the filesystem should run fsck if journaled filesystem is 
mounted on an unsafe device and crash happens.

> have to warn if you don't know whether it provides the ordering
> guarantee you expect to provide consistency and integrity.

The warning of missing barriers (or other actions) should be printed only 
if write cache is enabled. But there's no way how a filesystem on the top 
of several dm or md layers can find out if the disk is running with hdparm 
-w 0 or hdparm -w 1.

> > The barrier can be cancelled with -EOPNOTSUPP at any time. Andi Kleen 
> > submitted a patch that implements failing barriers for device mapper and 
> > he says that md-raid1 does the same thing.
> 
> You are right, if a device is reconfigured beneath you it may very well
> begin to return -EOPNOTSUPP much later. I didn't take that into account,
> I was considering only "plain" devices.
> 
> > Filesystems handle these randomly failed barriers but the downside is that 
> > they must not submit any request concurrently with the barrier. Also, that 
> > -EOPNOTSUPP restarting code is really crap, the request cannot be 
> > restarted from bi_end_io, so bi_end_io needs to handle to another thread 
> > for retry without barrier.
> 
> It can, but it requires you to operate at the request level. So for file
> systems that is problematic, it wont work of course. It would not be
> THAT hard to provide a helper to reissue the request. Not that pretty,
> but...

And it makes barriers useless for ordering.

The filesystem can't do [submit req a], [submit barrier req b], [submit 
req c] and assume that the requests will be ordered. If [b] fails with 
-EOPNOTSUPP, [a] and [c] could be already reordered and data corruption 
has already happened. Even if you catch [b]'s error and resubmit it as 
non-barrier request, it's too late.

So, as a result of this complication, all the existing filesystems send 
just one barrier request and do not try to overlay it with any other write 
requests.

So I'm wondering why Linux developers designed a barrier interface with 
complex specification, complex implementation and the interface is useless 
to provide any request ordering and it's no better than q->issue_flush_fn 
method or whatever was there beffore. Obviously, the whole barrier thing 
was designed by a person who never used it in a filesystem.
 
> > See this patch: http://lkml.org/lkml/2008/12/4/433 (and associated thread)
> > The patch is silly but it just shows what is really hapenning and what the 
> > filesystem must be prepared to deal with.
> 
> It's not that silly, we should add special barrier failing to the
> CONFIG_FAIL stuff. You'd definitely want to exercise that in the file
> system.
> 
> > > Things would also be much easier, if writes never failed.
> > >
> > > -- 
> > > Jens Axboe
> > 
> > I definitelly agree that it shouldn't fail. So remove that -EOPNOTSUPP 
> > error code at all, make barriers always pass to all kinds of devices and 
> > inform the caller via queue flags that the device doesn't support 
> > ordering.
> 
> Not a queue flag. Make it succeed to get rid of the whole retry
> business, but flag the bio with the information anyway.

That's a possibility too.

Mikulas

> -- 
> Jens Axboe
> 


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