Max number of lock on 1 file

[Andrew Beal]

Hi,

appart from the nflocks kernel paramater is there any other restrictions on how many locks you can have on one particular file at any time?

Thanks in advance,

Andrew

[Steven E. Protter · Accepted solution]

Probably not.

There are lots of kinds of locks.

Your database app can lock a row in a table and there really is only one lock on the file.

nflocks as I understand it(probably not so well) is the total number of locks on the ENTIRE system.

I don't think you can really lock an individual file more than once at the same time.

SEP

[Sanjiv Sharma_1]

There should be only one lock on a file at a time.

[A. Clay Stephenson]

With one exception the answer is no. The system can refuse to lock a portion of a file that would cause a system deadlock.

From the lockf man page:
A potential for deadlock occurs if a process controlling a locked resource is put to sleep by accessing the locked resource of another process.

You should also note that the same process might hold locks on many regions of the same file so that the total number of locks might be unexpectedly large.

The way to answer your sort of question, in general, is to consult the man pages (lockf and fcntl) in this case and look at all the reasons that a system call might fail and how errno is set under those conditions.

[A. Clay Stephenson]

The other thing to note is that (assuming nflocks is big enough), one could have individual locks on every byte of a file -- either a single process or many processes. I'm not saying that is very smart but it is certainly possible. Lockf() permits regions as small as 1 byte.