Re: [PATCH] ext4: Don't call sb_issue_discard() in ext4_free_blocks()

[Ted Ts'o <tytso@mit.edu>]

On Tue, Nov 09, 2010 at 10:00:37AM -0800, Jiaying Zhang wrote:
> I would like to spend some time to see whether we can add
> sb_issue_discard() somewhere else for non-journaled mode.
> It is a useful feature to be included for both modes.

It certainly can be done, but we'll have to do the trim from a kernel
thread or workqueue context.  (Of course, we need to make sure that
the blocks don't get reused until the trim happens --- or, if we want
to use those blocks, that we take them off the to-be-trimmed list
before we reuse them.)

I am a bit concerned about just adding a new thread, though.
Especially if it's per filesystem, since on a system with a very high
spindle/ext4 file system count, this could get a bit crazy.  It's a
bit better in 2.6.36 now that with concurrency managed workqueues,
there's only one workqueue thread per file system, instead of one
workqueue thread per file system per core (so on a system with 50
spindles and 32 cores, there would be 1600 workqueue threads!).

						- Ted