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

On 04/03/2009 04:11 AM, Jens Axboe wrote:
On Thu, Apr 02 2009, Mikulas Patocka wrote:
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.

Right, you can't possibly now that. Hence we have to print the warning.

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,
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

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.

That's not quite true, it was done in conjunction with file system
people. At a certain level, we are restricted by what the hardware can
actually do. It's certainly possible to make sure your storage stack can
support barriers and be safe in that regard, but it's certainly also
true that reconfiguring devices may void that guarantee. So it's not
perfect, but it's the best we can do. The worst part is that it's
virtually impossible to inform of such limitations.

If we get rid of -EOPNOTSUPP and just warn in such cases, then you
should never see -EOPNOTSUPP in the above sequence. You may not actually
be safe, hence we print a warning. It'll also make the whole thing a lot
less complex.

And to wrap up with the history of barriers, there was NOTHING before.
->issue_flush_fn is a later addition to just force a flush for fsync()
and friends, the original implementation was just a data bio/bh with
barrier semantics, providing no reordering before and after the data

Nobody was interested in barriers when they were done. Nobody. The fact
that it's taken 6 years or so to actually emerge as a hot topic for data
consistency should make that quite obvious. So the original
implementation was basically a joint effort with Chris on the reiser
side and EMC as the hw vendor and me doing the block implementation.

And I will restate that back at EMC we tested the original barriers (with reiserfs mostly, a bit on ext3 and ext2) and saw significant reduction in file system integrity issues after power loss.

The vantage point I had at EMC while testing and deploying the original barrier work done by Jens and Chris was pretty unique - full ability to do root cause failures of any component when really needed, a huge installed base which could send information home on a regular basis about crashes/fsck instances/etc and the ability (with customer permission) to dial into any box and diagnose issues remotely. Not to mention access to drive vendors to pressure them to make the flushes more robust. The application was also able to validate that all acknowledged writes were consistent.

Barriers do work as we have them, but as others have mentioned, it is not a "free" win - fsync will actually move your data safely out to persistent storage for a huge percentage of real users (including every ATA/S-ATA and SAS drive I was able to test). The file systems I monitored in production use without barriers were much less reliable.

As others have noted, some storage does not need barriers or flushed (high end arrays, drives with no volatile write cache) and some need it but stink (low cost USB flash sticks?) so warning is a good thing to do...


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