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.
