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Re: [dm-devel] Device-mapper cluster locking


On Wed, 7 Apr 2010, Jonathan Brassow wrote:

> I've been working on a cluster locking mechanism to be primarily used by
> device-mapper targets.  The main goals are API simplicity and an ability
> to tell if a resource has been modified remotely while a lock for the
> resource was not held locally.  (IOW, Has the resource I am acquiring the
> lock for changed since the last time I held the lock.)
> The original API (header file below) required 4 locking modes: UNLOCK,
> MONITOR, SHARED, and EXCLUSIVE.  The unfamiliar one, MONITOR, is similar to
> UNLOCK; but it keeps some state associated with the lock so that the next
> time the lock is acquired it can be determined whether the lock was
> acquired EXCLUSIVE by another machine.
> The original implementation did not cache cluster locks.  Cluster locks
> were simply released (or put into a non-conflicting state) when the lock
> was put into the UNLOCK or MONITOR mode.  I now have an implementation
> that always caches cluster locks - releasing them only if needed by another
> machine.  (A user may want to choose the appropriate implementation for
> their workload - in which case, I can probably provide both implementations
> through one API.)

Maybe you can think about autotuning it --- i.e. count how many times 
caching "won" (the lock was taken by the same node) or "lost" (the lock 
was acquired by another node) and keep or release the lock based on the 
ratio of these two counts. Decay the counts over time, so that it adjusts 
on workload change.

How does that dlm protocol work? When a node needs a lock, what happens? 
It sends all the nodes message about the lock? Or is there some master 
node as an arbiter?

> The interesting thing about the new caching approach is
> that I probably do not need this extra "MONITOR" state.  (If a lock that
> is cached in the SHARED state is revoked, then obviously someone is looking
> to alter the resource.  We don't need to have extra state to give us what
> can already be inferred and returned from cached resources.)

Yes, MONITOR and UNLOCK could be joined.

> I've also been re-thinking some of my assumptions about whether we
> /really/ need separate lockspaces and how best to release resources
> associated with each lock (i.e. get rid of a lock and its memory
> because it will not be used again, rather than caching unnecessarily).
> The original API (which is the same between the cached and non-caching
> implementations) only operates by way of lock names.  This means a
> couple of things:
> 1) Memory associated with a lock is allocated at the time the lock is
>    needed instead of at the time the structure/resource it is protecting
>    is allocated/initialized.
> 2) The locks will have to be tracked by the lock implementation.  This
>    means hash tables, lookups, overlapping allocation checks, etc.
> We can avoid these hazards and slow-downs if we separate the allocation
> of a lock from the actual locking action.  We would then have a lock
> life-cycle as follows:
> - lock_ptr = dmcl_alloc_lock(name, property_flags)
> - dmcl_write_lock(lock_ptr)
> - dmcl_unlock(lock_ptr)
> - dmcl_read_lock(lock_ptr)
> - dmcl_unlock(lock_ptr)
> - dmcl_free_lock(lock_ptr)

I think it is good --- way better than passing the character string on 
every call, parsing the string, hashinh it and comparing.

If you do it this way, you speed up lock acquires and releases.

> where 'property flags' is, for example:
> PREALLOC_DLM: Get DLM lock in an unlocked state to prealloc necessary structs

How would it differ from non-PREALLOC_DLM behavior?

> CACHE_RD_LK: Cache DLM lock when unlocking read locks for later acquisitions


> CACHE_WR_LK: Cache DLM lock when unlocking write locks for later acquisitions


> USE_SEMAPHORE: also acquire a semaphore when acquiring cluster lock

Which semaphore? If the user needs a specific semaphore, he can just 
acquire it with down() --- there is no need to overload dm-locking with 
that. Or is there any other reason why it is needed?

> Since the 'name' of the lock - which is used to uniquely identify a lock by
> name cluster-wide - could conflict with the same name used by someone else,
> we could allow locks to be allocated from a new lockspace as well.  So, the
> option of creating your own lockspace would be available in addition to the
> default lockspace.

What is the exact lockspace-lockname relationship? You create
locspace "dm-snap" and lockname will be UUID of the logical volume?

> The code has been written, I just need to arrange it into the right functional
> layout...  Would this new locking API make more sense to people?  Mikulas,
> what would you prefer for cluster snapshots?
>  brassow

I think using alloc/free interface is good.

BTW. also, think about failure handling. If there is a communication 
problem, the lock may fail. What to do? Detach the whole exception store 
and stop touching it? Can unlock fail?


> <Original locking API>
> enum dm_cluster_lock_mode {
>         /*
>          *
>          * Aquire the lock in this mode to monitor if another machine
>          * aquires this lock in the DM_CLUSTER_LOCK_EXCLUSIVE mode.  Later,
>          * when aquiring the lock in DM_CLUSTER_LOCK_EXCLUSIVE or
>          * DM_CLUSTER_LOCK_SHARED mode, dm_cluster_lock will return '1' if
>          * the lock had been aquired DM_CLUSTER_LOCK_EXCLUSIVE.
>          *
>          * This is useful because it gives the programmer a way of knowing if
>          * they need to perform an operation (invalidate cache, read additional
>          * metadata, etc) after aquiring the cluster lock.
>          */
> };
> /**
>  * dm_cluster_lock_init
>  * @uuid: The name given to this lockspace
>  *
>  * Returns: handle pointer on success, ERR_PTR(-EXXX) on failure
>  **/
> void *dm_cluster_lock_init(char *uuid);
> /**
>  * dm_cluster_lock_exit
>  * @h: The handle returned from dm_cluster_lock_init
>  */
> void dm_cluster_lock_exit(void *h);
> /**
>  * dm_cluster_lock
>  * @h      : The handle returned from 'dm_cluster_lock_init'
>  * @lock_nr: The lock number
>  * @mode   : One of DM_CLUSTER_LOCK_* (how to hold the lock)
>  * @callback: If provided, function will be non-blocking and use this
>  *           to notify caller when the lock is aquired.  If not provided,
>  *           this function will block until the lock is aquired.
>  * @callback_data: User context data that will be provided via the callback fn.
>  *
>  * Returns: -EXXX on error or 0 on success for DM_CLUSTER_LOCK_*
>  *         1 is a possible return if EXCLUSIVE/SHARED is the lock action,
>  *         the lock operation is successful, and an exlusive lock was aquired
>  *         by another machine while the lock was held in the
>  *         DM_CLUSTERED_LOCK_MONITOR state.
>  **/
> int dm_cluster_lock(void *h, uint64_t lock_nr, enum dm_cluster_lock_mode mode,
>                     void (*callback)(void *data, int rtn), void *data);
> /*
>  * dm_cluster_lock_by_name
>  * @lock_name: The lock name (up to 128 characters)
>  *
>  * Otherwise, the same as 'dm_cluster_lock'
>  */
> int dm_cluster_lock_by_str(void *h, const char *lock_name,
>                            enum dm_cluster_lock_mode mode,
>                            void (*callback)(void *data, int rtn), void *data);
> </Original locking API>
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