Re: [PATCH v2] fallocate.2: Document FALLOC_FL_ZERO_RANGE

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> To: Lukas Czerner <lczerner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>> Cc: mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
>> 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-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
>>> ---
>>> 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/
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