[Gluster-devel] Rebalance data migration and corruption

[Soumya Koduri]

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 at redhat.com>
>>> To: "Sakshi Bansal" <sabansal at redhat.com>, "Susant Palai"
>>> <spalai at redhat.com>
>>> Cc: "Gluster Devel" <gluster-devel at gluster.org>, "Nithya
>>> Balachandran" <nbalacha at redhat.com>, "Shyamsundar
>>> Ranganathan" <srangana at redhat.com>
>>> 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.
>>>