Re: fallocate() man page - darft 2

[Amit K. Arora]

Hi Michael,

On Mon, Jul 30, 2007 at 09:44:10PM +0200, Michael Kerrisk wrote:
> Amit, David,
> 
> I've edited the previous version of the page, adding David's license, and
> integrating Amit's comments.  I've also added a few new FIXMES.  ("FIXME
> Amit" again.)

Ok, Thanks!
 
> Could you please review the changes, and the FIXMEs.

Please find my comments below..
 
> Cheers,
> 
> Michael

--
Regards,
Amit Arora
 
> .\" Copyright (c) 2007 Silicon Graphics, Inc. All Rights Reserved
> .\" Written by Dave Chinner <dgc@xxxxxxx>
> .\" May be distributed as per GNU General Public License version 2.
> .\"
> .TH FALLOCATE 2 2007-07-20 "Linux" "Linux Programmer's Manual"
> .SH NAME
> fallocate \- manipulate file space
> .SH SYNOPSIS
> .nf
> .\" FIXME . eventually this #include will probably be something
> .\" different when support is added in glibc.
> .B #include <linux/falloc.h>
> .PP
> .BI "long fallocate(int " fd ", int " mode ", loff_t " offset \
> ", loff_t " len ");
> .\" FIXME . check later what feature text macros are  required in
> .\" glibc
> .SH DESCRIPTION
> .BR fallocate ()
> allows the caller to directly manipulate the allocated disk space
> for the file referred to by
> .I fd
> for the byte range starting at
> .I offset
> and continuing for
> .I len
> bytes.
> .\" FIXME Amit: in other words the affected byte range
> .\" is the bytes from (offset) to (offset + len - 1), right?

<Amit>
Yes, you are right.
</Amit> 

> The
> .I mode
> argument determines the operation to be performed on the given range.
> Currently only one flag is supported for
> .IR mode :
> .TP
> .B FALLOC_FL_KEEP_SIZE
> This flag allocates and initializes to zero the disk space
> within the range specified by
> .I offset
> and
> .IR len .
> After a successful call, subsequent writes into this range
> are guaranteed not to fail because of lack of disk space.
> Preallocating zeroed blocks beyond the end of the file
> is useful for optimizing append workloads.
> Preallocating blocks does not change
> the file size (as reported by
> .BR stat (2))
> even if it is less than
> .\" FIXME Amit: "offset + len" is written here.  But should it be
> .\" "offset + len - 1" ?

<Amit>
Good point. This text was directly taken from the man page of
posix_fallocate and is also there on the posix specifications at:
http://www.opengroup.org/onlinepubs/009695399/functions/posix_fallocate.html

The current posix_fallocate() implementation and also the fallocate()
implementation in ext4 are based on above documentation, wherein EOF is
compared with "offset + len" and not with "offset + len - 1".

I am not sure if this is right or wrong. But, this is as per posix
specifications. ;)
</Amit>

> .IR offset + len .
> .\"
> .\" Note from Amit Arora:
> .\" There were few more flags which were discussed, but none of
> .\" them have been finalized upon. Here are these flags:
> .\" FA_FL_DEALLOC, FA_FL_DEL_DATA, FA_FL_ERR_FREE, FA_FL_NO_MTIME,
> .\" FA_FL_NO_CTIME
> .\" All of the above flags were debated upon and we can not say
> .\" if any/which one of these flags will make it to the later kernels.
> .PP
> If
> .B FALLOC_FL_KEEP_SIZE
> flag is not specified in
> .IR mode ,
> the default behavior is almost same as when this flag is specified.
> The only difference is that on success,
> the file size will be changed if
> .\" FIXME Amit: "offset + len" is written here.  But should it be
> .\" "offset + len - 1" ?

<Amit>
Please see my previous comment.
</Amit>

> .IR offset + len
> is greater than the file size.
> This default behavior closely resembles the behavior of the
> .BR posix_fallocate (3)
> library function,
> and is intended as a method of optimally implementing that function.
> .PP
> Because allocation is done in block size chunks,
> .BR fallocate ()
> may allocate a larger range than that which was specified.
> .SH RETURN VALUE
> .BR fallocate ()
> returns zero on success, or an error number on failure.
> Note that
> .\" FIXME . the library wrapper function will do the right
> .\" thing, returning -1 on error and setting errno.
> .I errno
> is not set.
> .SH ERRORS
> .TP
> .B EBADF
> .I fd
> is not a valid file descriptor, or is not opened for writing.
> .TP
> .B EFBIG
> .IR offset + len
> exceeds the maximum file size.
> .TP
> .B EINVAL
> .I offset
> was less than 0, or
> .I len
> was less than or equal to 0.
> .TP
> .B ENODEV
> .I fd
> does not refer to a regular file or a directory.
> (If
> .I fd
> is a pipe or FIFO, a different error results.)
> .TP
> .B ENOSPC
> There is not enough space left on the device containing the file
> referred to by
> .IR fd .
> .TP
> .B ESPIPE
> .I fd
> refers to a pipe or FIFO.
> .TP
> .B ENOSYS
> The file system containing the file system referred to by

<Amit>
There is a typo above. We have "file system" repeated twice in above
sentence. Second one should be "file".
</Amit>

> .I fd
> does not support this operation.
> .TP
> .B EINTR
> A signal was caught during execution.
> .TP
> .B EIO
> An I/O error occurred while reading from or writing to a file system.
> .TP
> .B EOPNOTSUPP
> The
> .I mode
> is not supported by the file system containing the file referred to by
> .IR fd .
> .SH VERSIONS
> .BR fallocate ()
> .\" FIXME . To confirm that this syscall does actually get released
> .\" with 2.6.23.
> is available on Linux since kernel 2.6.23.
> .SH CONFORMING
> .BR fallocate ()
> is Linux specific.
> .SH SEE ALSO
> .BR ftruncate (2),
> .BR posix_fallocate (3),
> .BR posix_fadvise (3)
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