On Wed, 30 Apr 2014, Michael Kerrisk (man-pages) wrote: > Date: Wed, 30 Apr 2014 16:48:42 +0200 > From: "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> > To: Lukas Czerner <lczerner@xxxxxxxxxx>, linux-man@xxxxxxxxxxxxxxx > Cc: mtk.manpages@xxxxxxxxx > Subject: Re: [PATCH v2] fallocate.2: Document FALLOC_FL_ZERO_RANGE > > Hi Lukas > > On 04/30/2014 04:09 PM, Lukas Czerner wrote: > > FALLOC_FL_ZERO_RANGE was added in Linux 3.14, > > for zeroing ranges in the allocated space in a file. > > > > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx> > > --- > > v2: Rebase and update the description > > > > man2/fallocate.2 | 46 +++++++++++++++++++++++++++++++++++++++++++++- > > 1 file changed, 45 insertions(+), 1 deletion(-) > > > > diff --git a/man2/fallocate.2 b/man2/fallocate.2 > > index 73c4f12..30f27c3 100644 > > --- a/man2/fallocate.2 > > +++ b/man2/fallocate.2 > > @@ -180,6 +180,48 @@ ext4 (only for extent-based files) > > .\" commit 9eb79482a97152930b113b51dff530aba9e28c8e > > and XFS. > > .\" commit e1d8fb88a64c1f8094b9f6c3b6d2d9e6719c970d > > +.SS Zeroing file space > > +Specifying > > +.BR FALLOC_FL_ZERO_RANGE > > +flag (available since Linux 3.14) in > > +.I mode > > +zeroes space in the byte range starting at > > +.I offset > > +and continuing for > > +.I len > > +bytes. > > +Within the specified range, blocks are preallocated for the regions > > +that span the holes in the file. After a successful call, subsequent > > +reads from this range will return zeroes. > > + > > +Zeroing is done within the file system preferably by converting range into > > +unwritten extents which requires very little IO to be issued mostly for > > +metadata. This means that the range will not be physically zeroed out > > +on the device. > > I just want to confirm my understanding of what's going on here. > FALLOC_FL_ZERO_RANGE is serving at least two purposes: > 1. Future reads from the specified range will return zero. > 2. Blocks (in the form of extents) will be preallocated for the holes > in the range, thus ensuring that the necessary storage space is > allocated for the file. However, those allocated blocks won't actually > be written to. (Metadata trickery will ensure that reads from that > region won't actually touch the blocks; the kernel will simply deliver > back zeroes.) > > Right? Yes, that's exactly right. > > And a question: does FALLOC_FL_ZERO_RANGE work for non-extent-based > filesystems? If yes, how is FALLOC_FL_ZERO_RANGE implemented? Are the > blocks corresponding to a hole actually allocated and explicitly > written with zeroes? As it is now, FALLOC_FL_ZERO_RANGE does not work on non-extent based files. This is also the case for collapse range and the regular preallocation at least in ext4 case. Thanks! -Lukas > > Cheers, > > Michael > > > > + > > +If the > > +.B FALLOC_FL_KEEP_SIZE > > +flag is specified in > > +.IR mode , > > +the behavior of the call is similar, > > +but the file size will not be changed even if > > +.IR offset + len > > +is greater than the file size. This behaviour is the same as when > > +preallocating space with > > +.B FALLOC_FL_KEEP_SIZE > > +specified. > > + > > +Not all filesystems support > > +.BR FALLOC_FL_ZERO_RANGE ; > > +if a filesystem doesn't support the operation, an error is returned. > > +The operation is supported on at least the following filesystems > > +.IP * 3 > > +XFS (since Linux 2.14) > > +.\" commit 376ba313147b4172f3e8cf620b9fb591f3e8cdfa > > +.IP * > > +ext4 (since Linux 3.14) > > +.\" commit b8a8684502a0fc852afa0056c6bb2a9273f6fcc0 > > + > > .SH RETURN VALUE > > On success, > > .BR fallocate () > > @@ -243,7 +285,9 @@ no other flags are permitted with > > .B EINVAL > > .I mode > > is > > -.BR FALLOC_FL_COLLAPSE_RANGE , > > +.BR FALLOC_FL_COLLAPSE_RANGE > > +or > > +.BR FALLOC_FL_ZERO_RANGE, > > but the file referred to by > > .I fd > > is not a regular file. > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
