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
On Thu, 2006-08-24 at 15:53 -0500, Eric Sandeen wrote:
> Ming Zhang wrote:
> > Hi All
> >
> > Play with xfs_io and feel confused about this allocsp.
> >
> > Starting with a empty file.
> >
>
> 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
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