Hi Lukáš Ping on the question below... Cheers, Michael On 04/30/2014 09:47 PM, Michael Kerrisk (man-pages) wrote: > Lukáš, > > On 04/30/2014 04:54 PM, Lukáš Czerner wrote: >> 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. > > Thanks. > >>> 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. > > Okay -- see my comment below.. > >>>> + >>>> +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 >>>> + > > So for these two cases, XFS and ext4, should we perhaps better say > > XFS, for extent based files (since Linux 3.14) > ext4, for extent based files (since Linux 3.14) > ? > > Cheers, > > Michael > > >>>> .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. >>>> >>> >>> >>> >> > > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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
