[Gluster-devel] Improvement of eager locking

[Xavier Hernandez]

Hi,

currently eager locking is implemented by checking the open-fd-count 
special xattr for each write. If there's more than one open on the same 
file, eager locking is disabled to avoid starvation.

This works quite well for file writes, but makes eager locking unusable 
for other request types that do not involve an open fd (in fact, this 
method is only for writes on regular files, not reads or directories). 
This may cause a performance problem for other operations, like metadata.

To be able to use eager locking for other purposes, what do you think 
about this proposal:

Instead of implementing open-fd-count on posix xlator, do something 
similar but in locks xlator. The difference will be that locks xlator 
can use the pending locking information to determine if there are other 
processes waiting for a resource. If so, set a flag in the cbk xdata to 
let high level xlators know that they should not use eager locking (this 
can be done only upon request by xdata).

I think this way provides a more precise way to avoid starvation and 
maximize performance at the same time, and it can be used for any 
request even if it doesn't depend on an fd.

Another advantage is that if one file has been opened multiple times but 
all of them from the same glusterfs client, that client could use a 
single inodelk to manage all the accesses, not needing to release the 
lock. Current implementation in posix xlator cannot differentiate from 
opens from the same client or different clients.

What do you think ?

Xavi