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Re: [Qemu-devel] [PATCH for-2.6 v2 0/3] Bug fixes for gluster

From: Ric Wheeler
Subject: Re: [Qemu-devel] [PATCH for-2.6 v2 0/3] Bug fixes for gluster
Date: Wed, 20 Apr 2016 06:40:03 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/20/2016 05:24 AM, Kevin Wolf wrote:
Am 20.04.2016 um 03:56 hat Ric Wheeler geschrieben:
On 04/19/2016 10:09 AM, Jeff Cody wrote:
On Tue, Apr 19, 2016 at 08:18:39AM -0400, Ric Wheeler wrote:
On 04/19/2016 08:07 AM, Jeff Cody wrote:
Bug fixes for gluster; third patch is to prevent
a potential data loss when trying to recover from
a recoverable error (such as ENOSPC).
Hi Jeff,

Just a note, I have been talking to some of the disk drive people
here at LSF (the kernel summit for file and storage people) and got
a non-public confirmation that individual storage devices (s-ata
drives or scsi) can also dump cache state when a synchronize cache
command fails.  Also followed up with Rik van Riel - in the page
cache in general, when we fail to write back dirty pages, they are
simply marked "clean" (which means effectively that they get

Long winded way of saying that I think that this scenario is not
unique to gluster - any failed fsync() to a file (or block device)
might be an indication of permanent data loss.



I think you are right, we likely do need to address how QEMU handles fsync
failures across the board in QEMU at some point (2.7?).  Another point to
consider is that QEMU is cross-platform - so not only do we have different
protocols, and filesystems, but also different underlying host OSes as well.
It is likely, like you said, that there are other non-gluster scenarios where
we have non-recoverable data loss on fsync failure.

With Gluster specifically, if we look at just ENOSPC, does this mean that
even if Gluster retains its cache after fsync failure, we still won't know
that there was no permanent data loss?  If we hit ENOSPC during an fsync, I
presume that means Gluster itself may have encountered ENOSPC from a fsync to
the underlying storage.  In that case, does Gluster just pass the error up
the stack?

I still worry that in many non-gluster situations we will have
permanent data loss here. Specifically, the way the page cache
works, if we fail to write back cached data *at any time*, a future
fsync() will get a failure.
And this is actually what saves the semantic correctness. If you threw
away data, any following fsync() must fail. This is of course
inconvenient because you won't be able to resume a VM that is configured
to stop on errors, and it means some data loss, but it's safe because we
never tell the guest that the data is on disk when it really isn't.

gluster's behaviour (without resync-failed-syncs-after-fsync set) is
different, if I understand correctly. It will throw away the data and
then happily report success on the next fsync() call. And this is what
causes not only data loss, but corruption.

Yes, that makes sense to me - the kernel will remember that it could not write data back from the page cache and the future fsync() will see an error.

[ Hm, or having read what's below... Did I misunderstand and Linux
   returns failure only for a single fsync() and on the next one it
   returns success again? That would be bad. ]

I would need to think through that scenario with the memory management people to see if that could happen.

That failure could be because of a thinly provisioned backing store,
but in the interim, the page cache is free to drop the pages that
had failed. In effect, we end up with data loss in part or in whole
without a way to detect which bits got dropped.

Note that this is not a gluster issue, this is for any file system
on top of thinly provisioned storage (i.e., we would see this with
xfs on thin storage or ext4 on thin storage).  In effect, if gluster
has written the data back to xfs and that is on top of a thinly
provisioned target, the kernel might drop that data before you can
try an fsync again. Even if you retry the fsync(), the pages are
marked clean so they will not be pushed back to storage on that
second fsync().
I'm wondering... Marking the page clean means that it can be evicted
from the cache, right? Which happens whenever something more useful can
be done with the memory, i.e. possibly at any time. Does this mean that
two consecutive reads of the same block can return different data even
though no process has written to the file in between?

This we should tease out with a careful review of the behavior, but I think that might be able to happen.


Time 0: File has pattern A at offset 0. Any reads at this point see pattern A

Time 1: Write pattern B to offset 0. Reads now see pattern B.

Time 2: Run out of space on the backing store (before the data has been written back)

Time 3: Do an fsync() *OR* have the page cache fail to write back that page

Time 4: Under memory pressure, the page which was marked clean, is dropped

Time 5: Read offset 0 again - do we now see pattern A again? Or an IO error?

Also, O_DIRECT bypasses the problem, right? In that already the write
request would fail there, not only the fsync(). We recommend that for
production environments anyway.

O_DIRECT bypasses the page cache, but that data is allowed to be held in a volatile write cache (say in a disk's write cache) until the target device sees an fsync().

The safest (and horribly slow way) to be 100% safe is to write O_DIRECT|O_SYNC which bypasses the page cache and sends effectively a cache flush after each IO.

Most applications use fsync() after O_DIRECT at more strategic times though I assume (or don't know about this behavior).

Same issue with link loss - if we lose connection to a storage
target, it is likely to take time to detect that, more time to
reconnect. In the interim, any page cache data is very likely to get
dropped under memory pressure.

In both of these cases, fsync() failure is effectively a signal of a
high chance of data that has been already lost. A retry will not
save the day.

At LSF/MM today, we discussed an option that would allow the page
cache to hang on to data - for re-tryable errors only for example -
so that this would not happen. The impact of this is also
potentially huge (page cache/physical memory could be exhausted
while waiting for an admin to fix the issue) so it would have to be
a non-default option.
Is memory pressure the most common case, though?

I think it really depends on the type of storage device we have under us.

The odd effect that I see is that calling fsync() could actually make
data less safe than it was if the call fails. With the kernel marking
the pages clean on failure, instead of evicting "really clean" pages, we
can now evict "dirty, but failed writeout" pages even without any real
memory pressure, just because they can't be distinguished any more. Or
maybe they aren't even evicted, but the admin fixes the problem and we
could now write them to the disk if only they were still marked dirty
and wouldn't be ignored in the writeout.

fsync() is just the messenger that something bad happened - it is always better to know that we lost data since the last fsync() call rather than not know, correct?

Keep in mind that data will have this issue any time memory pressure (or other algorithms) cause data to be written back from the page cache, even if the application has not used an fsync().

Even if the admin "fixes" the issues (adds more storage, kicks a fibre channel switch, re-inserts a disk), IO might have been dropped forever from the page cache.

I'm sure there are solutions that are more intelligent than the extremes
of "mark clean on error" and "keep failed pages indefinitely" and that
cover a large part of use cases where qemu wants to resume a VM after a
failure (for local files perhaps most commonly resuming after ENOSPC).

Even just evicting pages immediately on a failure would probably be an
improvement because reads would then be consistent. And keeping the data
around until we *really* need memory might solve the problem for all
practical purposes. If we do eventually need the memory and throw away
data, fsync() consistently returning an error after throwing away data
is still safe, but we have a much better behaviour in the average case.

I think that we will need some discussions with the kernel memory
management team (and some storage kernel people) to see what seems
reasonable here.
It's a good discussion to have, but for the network protocols (like with
gluster) we tend to use the native libraries and don't even go through
the kernel page cache. So I think we shouldn't stop discussing the
semantics of these protocols and APIs while talking about the kernel
page cache.

Network protocols are also where error like "network is down" become
more relevant, so if anything, we want to have better error recovery
than on local files there.

I agree that with gluster we can try various schemes pretty easily when the error appears because of something internal to gluster (like a network error to a remote gluster server) but we cannot shield applications from data loss when we are just the messenger for an error on the storage servers local storage stack.

This is an important discussion to work through though - not just for qemu, I think it has a lot of value for everyone.



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