On Wed, Sep 22, 2021 at 03:49:31PM +1000, Dave Chinner wrote: > On Tue, Sep 21, 2021 at 09:13:54PM -0700, Darrick J. Wong wrote: > > On Wed, Sep 22, 2021 at 01:59:07PM +1000, Dave Chinner wrote: > > > On Tue, Sep 21, 2021 at 07:38:01PM -0700, Darrick J. Wong wrote: > > > > On Tue, Sep 21, 2021 at 07:16:26PM -0700, Dan Williams wrote: > > > > > On Tue, Sep 21, 2021 at 1:32 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > > > > > > > > > > > On Tue, Sep 21, 2021 at 10:44:31AM +1000, Dave Chinner wrote: > > > > > > > I think this wants to be a behavioural modifier for existing > > > > > > > operations rather than an operation unto itself. i.e. similar to how > > > > > > > KEEP_SIZE modifies ALLOC behaviour but doesn't fundamentally alter > > > > > > > the guarantees ALLOC provides userspace. > > > > > > > > > > > > > > In this case, the change of behaviour over ZERO_RANGE is that we > > > > > > > want physical zeros to be written instead of the filesystem > > > > > > > optimising away the physical zeros by manipulating the layout > > > > > > > of the file. > > > > > > > > > > > > Yes. > > > > > > > > > > > > > Then we have and API that looks like: > > > > > > > > > > > > > > ALLOC - allocate space efficiently > > > > > > > ALLOC | INIT - allocate space by writing zeros to it > > > > > > > ZERO - zero data and preallocate space efficiently > > > > > > > ZERO | INIT - zero range by writing zeros to it > > > > > > > > > > > > > > Which seems to cater for all the cases I know of where physically > > > > > > > writing zeros instead of allocating unwritten extents is the > > > > > > > preferred behaviour of fallocate().... > > > > > > > > > > > > Agreed. I'm not sure INIT is really the right name, but I can't come > > > > > > up with a better idea offhand. > > > > > > > > > > FUA? As in, this is a forced-unit-access zeroing all the way to media > > > > > bypassing any mechanisms to emulate zero-filled payloads on future > > > > > reads. > > > > > > Yes, that's the semantic we want, but FUA already defines specific > > > data integrity behaviour in the storage stack w.r.t. volatile > > > caches. > > > > > > Also, FUA is associated with devices - it's low level storage jargon > > > and so is not really appropriate to call a user interface operation > > > FUA where users have no idea what a "unit" or "access" actually > > > means. > > > > > > Hence we should not overload this name with some other operation > > > that does not have (and should not have) explicit data integrity > > > requirements. That will just cause confusion for everyone. > > > > > > > FALLOC_FL_ZERO_EXISTING, because you want to zero the storage that > > > > already exists at that file range? > > > > > > IMO that doesn't work as a behavioural modifier for ALLOC because > > > the ALLOC semantics are explicitly "don't touch existing user > > > data"... > > > > Well since you can't preallocate /and/ zerorange at the same time... > > > > /* For FALLOC_FL_ZERO_RANGE, write zeroes to pre-existing mapped storage. */ > > #define FALLOC_FL_ZERO_EXISTING (0x80) > > Except we also want the newly allocated regions (i.e. where holes > were) in that range being zeroed to have zeroes written to them as > well, yes? Otherwise we end up with a combination of unwritten > extents and physical zeroes, and you can't use > ZERORANGE|EXISTING as a replacement for PUNCH + ALLOC|INIT > > /* > * For preallocation and zeroing operations, force the filesystem to > * write zeroes rather than use unwritten extents to indicate the > * range contains zeroes. > * > * For filesystems that support unwritten extents, this trades off > * slow fallocate performance for faster first write performance as > * unwritten extent conversion on the first write to each block in > * the range is not needed. > * > * Care is required when using FALLOC_FL_ALLOC_INIT_DATA as it will > * be much slower overall for large ranges and/or slow storage > * compared to using unwritten extents. > */ > #define FALLOC_FL_ALLOC_INIT_DATA (1 << 7) I prefer FALLOC_FL_ZEROINIT_DATA here, because in the ZERO|INIT case we're not allocating any new space, merely rewriting existing storage. I also want to expand the description slightly: /* * For preallocation, force the filesystem to write zeroes rather than * use unwritten extents to indicate the range contains zeroes. For * zeroing operations, force the filesystem to write zeroes to existing * written extents. --D > > Cheers, > > Dave. > > -- > Dave Chinner > david@xxxxxxxxxxxxx
