Re: automatic freeing of space on ENOSPC

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Jun 29, 2021 at 12:47:11PM -0400, Theodore Ts'o wrote:
> On Mon, Jun 28, 2021 at 03:49:08PM -0400, J. Bruce Fields wrote:
> > Is there anything analogous to a "shrinker", but for disk space?  So,
> > some hook that a filesystem could call to say "I'm running out of space,
> > could you please free something?", before giving up and returning
> > ENOSPC?
> 
> In addition to the issues raised by Neil, Amir, Dave, and others, the
> other challenge with the file system calling a "please try to free
> something before I return ENOSPC" is that this would almost certainly
> require blocking a system call while some userspace daemon tried to
> free up some space --- or were you thinking that the nfsd kernel code
> would be tracking all of the silly-rename files so it could release
> space really quickly on demand?

Something like that, yep.

> Even if this is only a kernel callback, I'd be concerned about
> potential locking hierarchy problems if we are calling out from block
> allocation subsystem to nfsd, only to have nfsd call back in to
> request unlinking a silly-renamed file.
> 
> So the suggestion that we not wait until we're down to 0 blocks free,
> but when we hit some threshold (say, free space dips below N minutes
> worth of worst or average case block allocations), trigger code which
> deletes silly-renamed files, is probably the best way to go.  In which
> case, a callback is not what is needed; and if N is large enough, this
> could done via a pure user-space-only solution.

Makes sense, thanks!

--b.