On 4/1/19 10:02 AM, Pranith Kumar Karampuri wrote:
On Sun, Mar 31, 2019 at 11:29 PM Soumya Koduri <skoduri@xxxxxxxxxx
<mailto:skoduri@xxxxxxxxxx>> wrote:
On 3/29/19 11:55 PM, Xavi Hernandez wrote:
> Hi all,
>
> there is one potential problem with posix locks when used in a
> replicated or dispersed volume.
>
> Some background:
>
> Posix locks allow any process to lock a region of a file multiple
times,
> but a single unlock on a given region will release all previous
locks.
> Locked regions can be different for each lock request and they can
> overlap. The resulting lock will cover the union of all locked
regions.
> A single unlock (the region doesn't necessarily need to match any
of the
> ranges used for locking) will create a "hole" in the currently
locked
> region, independently of how many times a lock request covered
that region.
>
> For this reason, the locks xlator simply combines the locked regions
> that are requested, but it doesn't track each individual lock range.
>
> Under normal circumstances this works fine. But there are some cases
> where this behavior is not sufficient. For example, suppose we
have a
> replica 3 volume with quorum = 2. Given the special nature of posix
> locks, AFR sends the lock request sequentially to each one of the
> bricks, to avoid that conflicting lock requests from other
clients could
> require to unlock an already locked region on the client that has
not
> got enough successful locks (i.e. quorum). An unlock here not
only would
> cancel the current lock request. It would also cancel any previously
> acquired lock.
>
I may not have fully understood, please correct me. AFAIU, lk xlator
merges locks only if both the lk-owner and the client opaque matches.
In the case which you have mentioned above, considering clientA
acquired
locks on majority of quorum (say nodeA and nodeB) and clientB on nodeC
alone- clientB now has to unlock/cancel the lock it acquired on nodeC.
You are saying the it could pose a problem if there were already
successful locks taken by clientB for the same region which would get
unlocked by this particular unlock request..right?
Assuming the previous locks acquired by clientB are shared (otherwise
clientA wouldn't have got granted lock for the same region on nodeA &
nodeB), they would still hold true on nodeA & nodeB as the unlock
request was sent to only nodeC. Since the majority of quorum nodes
still
hold the locks by clientB, this isn't serious issue IMO.
I haven't looked into heal part but would like to understand if this is
really an issue in normal scenarios as well.
This is how I understood the code. Consider the following case:
Nodes A, B, C have locks with start and end offsets: 5-15 from mount-1
and lock-range 2-3 from mount-2.
If mount-1 requests nonblocking lock with lock-range 1-7 and in parallel
lets say mount-2 issued unlock of 2-3 as well.
nodeA got unlock from mount-2 with range 2-3 then lock from mount-1 with
range 1-7, so the lock is granted and merged to give 1-15
nodeB got lock from mount-1 with range 1-7 before unlock of 2-3 which
leads to EAGAIN which will trigger unlocks on granted lock in mount-1
which will end up doing unlock of 1-7 on nodeA leading to lock-range
8-15 instead of the original 5-15 on nodeA. Whereas nodeB and nodeC will
have range 5-15.
Let me know if my understanding is wrong.
Both of us mentioned the same points. So in the example you gave ,
mount-1 lost its previous lock on nodeA but majority of the quorum
(nodeB and nodeC) still have the previous lock (range: 5-15) intact. So
this shouldn't ideally lead to any issues as other conflicting locks are
blocked or failed by majority of the nodes (provided there are no brick
dis/re-connects).
Wrt to brick disconnects/re-connects, if we can get in general lock
healing (not getting into implementation details atm) support, that
should take care of correcting lock range on nodeA as well right?
That said I am not suggesting that we should stick to existing behavior,
just trying to get clarification to check if we can avoid any
overhead/side-effects with maintaining multiple locks.
Thanks,
Soumya
Thanks,
Soumya
> However, when something goes wrong (a brick dies during a lock
request,
> or there's a network partition or some other weird situation), it
could
> happen that even using sequential locking, only one brick
succeeds the
> lock request. In this case, AFR should cancel the previous lock
(and it
> does), but this also cancels any previously acquired lock on that
> region, which is not good.
>
> A similar thing can happen if we try to recover (heal) posix
locks that
> were active after a brick has been disconnected (for any reason) and
> then reconnected.
>
> To fix all these situations we need to change the way posix locks
are
> managed by locks xlator. One possibility would be to embed the lock
> request inside an inode transaction using inodelk. Since inodelks
do not
> suffer this problem, the follwing posix lock could be sent safely.
> However this implies an additional network request, which could
cause
> some performance impact. Eager-locking could minimize the impact
in some
> cases. However this approach won't work for lock recovery after a
> disconnect.
>
> Another possibility is to send a special partial posix lock request
> which won't be immediately merged with already existing locks once
> granted. An additional confirmation request of the partial posix
lock
> will be required to fully grant the current lock and merge it
with the
> existing ones. This requires a new network request, which will add
> latency, and makes everything more complex since there would be more
> combinations of states in which something could fail.
>
> So I think one possible solution would be the following:
>
> 1. Keep each posix lock as an independent object in locks xlator.
This
> will make it possible to "invalidate" any already granted lock
without
> affecting already established locks.
>
> 2. Additionally, we'll keep a sorted list of non-overlapping
segments of
> locked regions. And we'll count, for each region, how many locks are
> referencing it. One lock can reference multiple segments, and each
> segment can be referenced by multiple locks.
>
> 3. An additional lock request that overlaps with an existing
segment,
> can cause this segment to be split to satisfy the non-overlapping
property.
>
> 4. When an unlock request is received, all segments intersecting
with
> the region are eliminated (it may require some segment splits on the
> edges), and the unlocked region is subtracted from each lock
associated
> to the segment. If a lock gets an empty region, it's removed.
>
> 5. We'll create a special "remove lock" request that doesn't
unlock a
> region but removes an already granted lock. This will decrease the
> number of references to each of the segments this lock was
covering. If
> some segment reaches 0, it's removed. Otherwise it remains there.
This
> special request will only be used internally to cancel already
acquired
> locks that cannot be fully granted due to quorum issues or any other
> problem.
>
> In some weird cases, the list of segments can be huge (many locks
> overlapping only on a single byte, so each segment represents
only one
> byte). We can try to find some smarter structure that minimizes this
> problem or limit the number of segments (for example returning
ENOLCK
> when there are too many).
>
> What do you think ?
>
> Xavi
>
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--
Pranith
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