On 02/08/2016 09:13 AM, Shyam wrote:
On 02/06/2016 06:36 PM, Raghavendra Gowdappa wrote:
----- Original Message -----
From: "Raghavendra Gowdappa" <rgowdapp@xxxxxxxxxx>
To: "Sakshi Bansal" <sabansal@xxxxxxxxxx>, "Susant Palai"
<spalai@xxxxxxxxxx>
Cc: "Gluster Devel" <gluster-devel@xxxxxxxxxxx>, "Nithya
Balachandran" <nbalacha@xxxxxxxxxx>, "Shyamsundar
Ranganathan" <srangana@xxxxxxxxxx>
Sent: Friday, February 5, 2016 4:32:40 PM
Subject: Re: Rebalance data migration and corruption
+gluster-devel
Hi Sakshi/Susant,
- There is a data corruption issue in migration code. Rebalance
process,
1. Reads data from src
2. Writes (say w1) it to dst
However, 1 and 2 are not atomic, so another write (say w2) to
same region
can happen between 1. But these two writes can reach dst in the
order
(w2,
w1) resulting in a subtle corruption. This issue is not fixed yet
and can
cause subtle data corruptions. The fix is simple and involves
rebalance
process acquiring a mandatory lock to make 1 and 2 atomic.
We can make use of compound fop framework to make sure we don't suffer a
significant performance hit. Following will be the sequence of
operations
done by rebalance process:
1. issues a compound (mandatory lock, read) operation on src.
2. writes this data to dst.
3. issues unlock of lock acquired in 1.
Please co-ordinate with Anuradha for implementation of this compound
fop.
Following are the issues I see with this approach:
1. features/locks provides mandatory lock functionality only for
posix-locks
(flock and fcntl based locks). So, mandatory locks will be
posix-locks which
will conflict with locks held by application. So, if an application
has held
an fcntl/flock, migration cannot proceed.
What if the file is opened with O_NONBLOCK? Cant rebalance process skip
the file and continue in case if mandatory lock acquisition fails?
We can implement a "special" domain for mandatory internal locks.
These locks will behave similar to posix mandatory locks in that
conflicting fops (like write, read) are blocked/failed if they are
done while a lock is held.
So is the only difference between mandatory internal locks and posix
mandatory locks is that internal locks shall not conflict with other
application locks(advisory/mandatory)?
2. data migration will be less efficient because of an extra unlock
(with
compound lock + read) or extra lock and unlock (for non-compound fop
based
implementation) for every read it does from src.
Can we use delegations here? Rebalance process can acquire a
mandatory-write-delegation (an exclusive lock with a functionality
that delegation is recalled when a write operation happens). In that
case rebalance process, can do something like:
1. Acquire a read delegation for entire file.
2. Migrate the entire file.
3. Remove/unlock/give-back the delegation it has acquired.
If a recall is issued from brick (when a write happens from mount), it
completes the current write to dst (or throws away the read from src)
to maintain atomicity. Before doing next set of (read, src) and
(write, dst) tries to reacquire lock.
With delegations this simplifies the normal path, when a file is
exclusively handled by rebalance. It also improves the case where a
client and rebalance are conflicting on a file, to degrade to mandatory
locks by either parties.
I would prefer we take the delegation route for such needs in the future.
Right. But if there are simultaneous access to the same file from any
other client and rebalance process, delegations shall not be granted or
revoked if granted even though they are operating at different offsets.
So if you rely only on delegations, migration may not proceed if an
application has held a lock or doing any I/Os.
Also ideally rebalance process has to take write delegation as it would
end up writing the data on destination brick which shall affect READ
I/Os, (though of course we can have special checks/hacks for internal
generated fops).
That said, having delegations shall definitely ensure correctness with
respect to exclusive file access.
Thanks,
Soumya
@Soumyak, can something like this be done with delegations?
@Pranith,
Afr does transactions for writing to its subvols. Can you suggest any
optimizations here so that rebalance process can have a transaction
for (read, src) and (write, dst) with minimal performance overhead?
regards,
Raghavendra.
Comments?
regards,
Raghavendra.
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