Open Stack

On 11/30/2015 12:42 PM, Joshua Harlow wrote:
> Hi all,
> 
> I just wanted to bring up an issue, possible solution and get feedback 
> on it from folks because it seems to be an on-going problem that shows 
> up not when an application is initially deployed but as on-going 
> operation and running of that application proceeds (ie after running for 
> a period of time).
> 
> The jist of the problem is the following:
> 
> A <<pick your favorite openstack project>> has a need to ensure that no 
> application on the same machine can manipulate a given resource on that 
> same machine, so it uses the lock file pattern (acquire a *local* lock 
> file for that resource, manipulate that resource, release that lock 
> file) to do actions on that resource in a safe manner (note this does 
> not ensure safety outside of that machine, lock files are *not* 
> distributed locks).
> 
> The api that we expose from oslo is typically accessed via the following:
> 
>    oslo_concurrency.lockutils.synchronized(name, lock_file_prefix=None, 
> external=False, lock_path=None, semaphores=None, delay=0.01)
> 
> or via its underlying library (that I extracted from oslo.concurrency 
> and have improved to add more usefulness) @ 
> http://fasteners.readthedocs.org/
> 
> The issue though for <<your favorite openstack project>> is that each of 
> these projects now typically has a large amount of lock files that exist 
> or have existed and no easy way to determine when those lock files can 
> be deleted (afaik no? periodic task exists in said projects to clean up 
> lock files, or to delete them when they are no longer in use...) so what 
> happens is bugs like https://bugs.launchpad.net/cinder/+bug/1432387 
> appear and there is no a simple solution to clean lock files up (since 
> oslo.concurrency is really not the right layer to know when a lock can 
> or can not be deleted, only the application knows that...)
> 
> So then we get a few creative solutions like the following:
> 
> - https://review.openstack.org/#/c/241663/
> - https://review.openstack.org/#/c/239678/
> - (and others?)
> 
> So I wanted to ask the question, how are people involved in <<your 
> favorite openstack project>> cleaning up these files (are they at all?)
> 
> Another idea that I have been proposing also is to use offset locks.
> 
> This would allow for not creating X lock files, but create a *single* 
> lock file per project and use offsets into it as the way to lock. For 
> example nova could/would create a 1MB (or larger/smaller) *empty* file 
> for locks, that would allow for 1,048,576 locks to be used at the same 
> time, which honestly should be way more than enough, and then there 
> would not need to be any lock cleanup at all... Is there any reason this 
> wasn't initially done back way when this lock file code was created? 
> (https://github.com/harlowja/fasteners/pull/10 adds this functionality 
> to the underlying library if people want to look it over)

I think the main reason was that even with a million locks available,
you'd have to find a way to hash the lock names to offsets in the file,
and a million isn't a very large collision space for that.  Having two
differently named locks that hashed to the same offset would lead to
incredibly confusing bugs.

We could switch to requiring the projects to provide the offsets instead
of hashing a string value, but that's just pushing the collision problem
off onto every project that uses us.

So that's the problem as I understand it, but where does that leave us
for solutions?  First, there's
https://github.com/openstack/oslo.concurrency/blob/master/oslo_concurrency/lockutils.py#L151
which allows consumers to delete lock files when they're done with them.
 Of course, in that case the onus is on the caller to make sure the lock
couldn't possibly be in use anymore.

Second, is this actually a problem?  Modern filesystems have absurdly
large limits on the number of files in a directory, so it's highly
unlikely we would ever exhaust that, and we're creating all zero byte
files so there shouldn't be a significant space impact either.  In the
past I believe our recommendation has been to simply create a cleanup
job that runs on boot, before any of the OpenStack services start, that
deletes all of the lock files.  At that point you know it's safe to
delete them, and it prevents your lock file directory from growing forever.

I know we've had this discussion in the past, but I don't think anyone
has ever told me that having lock files hang around was a functional
problem for them.  It seems to be largely cosmetic complaints about not
cleaning up the old files (which, as you noted, Oslo can't really solve
because we have no idea when consumers are finished with locks) and
given the amount of trouble we've had with interprocess locking in the
past I've never felt that a cosmetic issue was sufficient reason to
reopen that can of worms.  I'll just note again that every time we've
started messing with this stuff we run into a bunch of sticky problems
and edge cases, so it would take a pretty compelling argument to
convince me that we should do so again.

Of course, if someone wants to take another stab at changing this stuff
again I guess more power to them, but to my knowledge we've finally had
our interprocess locking in a good state for a while now so I'm not in
favor of messing with it.  That's my two cents -- don't spend it all in
one place. ;-)

> 
> In general would like to hear peoples thoughts/ideas/complaints/other,
> 
> -Josh
> 
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