On Wed, Dec 14, 2016 at 2:54 PM, Xavier Hernandez <xhernandez@xxxxxxxxxx> wrote:
On 12/14/2016 10:17 AM, Pranith Kumar Karampuri wrote:
On Wed, Dec 14, 2016 at 1:48 PM, Xavier Hernandez <xhernandez@xxxxxxxxxx
<mailto:xhernandez@xxxxxxxxxx>> wrote:
There's another issue with the patch that Ashish sent.
The original problem is that a setattr on a symbolic link gets
transformed to a regular file while the fop is being executed. Even
if we apply the Ashish' patch to avoid the assert, the setattr fop
will still succeed and incorrectly change the attributes of a
gluster special file that shouldn't change.
I think that's a bigger problem that needs to be addressed globally.
I'm sure this is not an easy solution, but probably the best way
would be to have distinct inodes for the gluster link files and the
original file. This way most of these problems should be solved.
Is there any reason why there is a difference in type of the file on
hashed/cached subvols? We can have the same type of file on both dht
subvolumes? That will prevent unlink of regular file and recreate with
the actual type of the file?
I think the problem is not only the type of the inode. There are more things involved. If we allow operations intended for regular files to succeed on the dht link file itself, the operation won't be visible and may affect future actions.
How it's prevented that the setattr modifies an already created link file ? or at least, are these changes propagated to the real file later and the link is restored to the original state ? if so, how dht detects all this without any locks ? if it's able to detect that, why does it send the setattr request anyway ?
I think the assert messages are coming at the time of marking the file as '---------T' file. DHT makes sure the actual fop happens on the cached subvolume. But this linkto file will be present in hashed subvolume indicating it is a linkto file (i.e. 'T' file and there will be an extended attribute telling where the actual file is in an extended attrbute). With EC in picture this marking of linkto file by doing a setattr as '---------T' file is asserting.
Xavi
On 12/14/2016 09:02 AM, Xavier Hernandez wrote:
On 12/14/2016 06:10 AM, Raghavendra Gowdappa wrote:
----- Original Message -----
From: "Pranith Kumar Karampuri" <pkarampu@xxxxxxxxxx
<mailto:pkarampu@xxxxxxxxxx>>
To: "Ashish Pandey" <aspandey@xxxxxxxxxx
<mailto:aspandey@xxxxxxxxxx>>
Cc: "Gluster Devel" <gluster-devel@xxxxxxxxxxx
<mailto:gluster-devel@gluster.org >>, "Shyam Ranganathan"
<srangana@xxxxxxxxxx <mailto:srangana@xxxxxxxxxx>>,
"Nithya Balachandran"
<nbalacha@xxxxxxxxxx <mailto:nbalacha@xxxxxxxxxx>>,
"Xavier Hernandez" <xhernandez@xxxxxxxxxx
<mailto:xhernandez@xxxxxxxxxx>>,
"Raghavendra Gowdappa" <rgowdapp@xxxxxxxxxx
<mailto:rgowdapp@xxxxxxxxxx>>
Sent: Tuesday, December 13, 2016 9:29:46 PM
Subject: Re: 1402538 : Assertion failure during
rebalance of symbolic
links
On Tue, Dec 13, 2016 at 2:45 PM, Ashish Pandey
<aspandey@xxxxxxxxxx <mailto:aspandey@xxxxxxxxxx>>
wrote:
Hi All,
We have been seeing an issue where re balancing
symbolic links leads
to an
assertion failure in EC volume.
The root cause of this is that while migrating
symbolic links to
other sub
volume, it creates a link file (with attributes
.........T) .
This file is a regular file.
Now, during migration a setattr comes to this link
and because of
possible
race, posix_stat return stats of this "T" file.
In ec_manager_seattr, we receive callbacks and check
the type of
entry. If
it is a regular file we try to get size and if it is
not there, we
raise an
assert.
So, basically we are checking a size of the link
(which will not have
size) which has been returned as regular file and we
are ending up when
this condition
becomes TRUE.
Now, this looks like a problem with re balance and
difficult to fix at
this point (as per the discussion).
We have an alternative to fix it in EC but that will
be more like a
hack
than an actual fix. We should not modify EC
to deal with an individual issue which is in other
translator.
I am afraid, dht doesn't have a better way of handling this.
While DHT
maintains abstraction (of a symbolic link) to layers above,
the layers
below it cannot be shielded from seeing the details like a
linkto file
etc.
That's ok, and I think it's the right thing to do. From the point of
view of EC, it's irrelevant how the file is seen by upper layers. It
only cares about the files below it.
If the concern really is that the file is changing its type
in a span
of single fop, we can probably explore the option of locking
(or other
synchronization mechanisms) to prevent migration taking
place, while a
fop is in progress.
That's the real problem. Some operations receive an inode
referencing a
symbolic link on input but the iatt structures from the callback
reference a regular file. It's even worse because it's an
asynchronous
race so some of the bricks may return a regular file and some
may return
a symbolic link. If there are more than redundancy bricks
returning a
different type, the most probably result will be an I/O error
caused by
inconsistent answers.
Ashish wrote a patch to check the type of the inode at the input
instead
of relying on the answers. While this could avoid the assertion
issued
by ec, it doesn't solve the race, leaving room for the I/O errors I
mentioned earlier.
But, I assume there will be performance penalties for that too.
Yes. I don't see any other way to really solve this problem. A
lock is
needed.
In ec we already have a problem that will need an additional lock on
rmdir, unlink and rename to avoid some races. This change will
also need
support from locks xlator to avoid granting locks on deleted
inodes. If
dht is using one of these operations to replace the symbolic
link by the
gluster link file, I think this change could solve the I/O
errors, but
I'm not sure we could completely solve the problem.
I'm not sure how dht does the transform from a symbolic link to a
gluster link file, but if it involves more than one fop from the
point
of view of ec, there's nothing that ec can do to solve the
problem. If
another client accesses the file, ec can return any intermediate
state.
DHT should take some lock to do all operations atomically and avoid
problems on other clients.
I think that the mid-term approach to completely solve the problem
without a performance impact should be to implement some kind of
transaction mechanism that will reuse lock requests. This would
allow,
among other things, that multiple atomic operations could be
performed
by different xlators but sharing the locks instead of requiring each
xlator to take an inodelk on its own.
Xavi
Now the question is how to proceed with this? Any
suggestions?
Raghavendra/Nithya,
Could one of you explain the difficulties in
fixing this
issue in
DHT so that Xavi will also be caught up with why we
should add this
change
in EC in the short term.
Details on this bug can be found here -
https://bugzilla.redhat.com/show_bug.cgi?id=1402538
<https://bugzilla.redhat.com/show_bug.cgi?id=1402538 >
----
Ashish
--
Pranith
--
Pranith
--
Pranith
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