how to understand allocsp

Hi All

Play with xfs_io and feel confused about this allocsp.

Starting with a empty file.

xfs_io> bmap
/t/x: no extents

I assumed this will allocate space from 0 with 1048576 bytes. but seems this is wrong.

xfs_io> allocsp 0 1048576
xfs_io> bmap
/t/x: no extents

Do this from a non-zero start, found file filled with 0. xfs_io> allocsp 1048576 0
xfs_io> bmap
/t/x:

        0: [0..2047]: 2147745792..2147747839

XFS_IOC_ALLOCSP64

Alter storage space associated with a section of the ordinary file specified. The section
~~~~no idea what this "alter" mean here.

is specified by a variable of type xfs_flock64_t, pointed to by the
final argument. The data type xfs_flock64_t contains the following
members: l_whence is 0, 1, or 2 to indicate that the relative offset l_start will be measured from the start of the file, the current position, or the end of the file, respectively. l_start is the offset from the position specified in l_whence. l_len is the size of the section. An l_len value of zero frees up to the end of the file; in this case, the end of file (i.e., file size) is set to the beginning of the section freed. Any data previously written into this section is no longer accessible. If the section specified is beyond the current end of file, the file is grown and filled with zeroes. The l_len field is currently ignored, and should be set to zero.

this "currently ignored" is ONLY when "section is beyond the current end of file" or for any l_start?

Thanks!

Ming

I honestly never keep this all straight without going back to the code, but I'd start by looking at the comments for, and code in, xfs_change_file_space and xfs_alloc_file_space.

/*
* XFS_IOC_RESVSP and XFS_IOC_UNRESVSP will reserve or unreserve
* file space.
* These calls do NOT zero the data space allocated to the file,
* nor do they change the file size.
*
* XFS_IOC_ALLOCSP and XFS_IOC_FREESP will allocate and free file
* space.
* These calls cause the new file data to be zeroed and the file
* size to be changed.
*/

The semantics of all this could be clearer, IMHO.

-Eric

see the code for this XFS_IOC_ALLOCSP

if (startoffset > fsize) {
error = xfs_alloc_file_space(ip, fsize,
startoffset - fsize, 0,

attr_flags);
                        if (error)
                                break;
                }

                va.va_mask = XFS_AT_SIZE;
                va.va_size = startoffset;

                error = xfs_setattr(bdp, &va, attr_flags, credp);

                if (error)
                        return error;

                clrprealloc = 1;
                break;

so if offset is larger than current file size, it will allocate the space and fill with zero. this is exactly what i met with "allocsp 1048576 0".

i would say this this pretty misleading. since "allocsp 0 1048576" can be easily thought as "allocate 1048576 space start from 0" but now seems only an ATTR is set.

Ming