Re: how to understand allocsp

[Eric Sandeen <sandeen@xxxxxxxxxxx>]

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