Re: [RFC][PATCH 0/5] Fiemap, an extent mapping ioctl

[Joel Becker <Joel.Becker@oracle.com>]

On Thu, May 29, 2008 at 11:33:09AM -0400, jim owens wrote:
> Christoph Hellwig wrote:
>>
>> What use is there geeting the extent count for a range?  I'd rather
>> do it only per-file like the xfs ioctl.
>
> I'll answer that from practical experience.  Our api equivalents:
>
>  	__u64	fm_start;	 /* logical offset (inclusive) at
> 				  * which to start mapping (in) */
> 	__u64	fm_length;	 /* logical length of mapping which
> 				  * userspace cares about (in) */
> 	__u32	fm_extent_count; /* size of fm_extents array (in) */
> 	__u32	fm_mapped_extents; /* number of extents that were
> 				    * mapped (out) */
>
> ... note it has no flags field and no separate ioctl_extent_count.
>
> "fm_extent_count" is
>    IN  == max_extents to return.
>    OUT == number of extents remaining in-range after fm_mapped_extents
>
> Pass in fm_extent_count==0 and you get OUT number of extent entries
> within your fm_start + fm_length range.  Which you can use to set
> your malloc size because the FS can have massive extent counts :(

	See, that's a very natural API.  I'd be much happier with that -
"extent_count == 0" is consistent with no extent structures passed in.
It also fits with many other API that do it the same way (eg, snprintf).

Joel

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Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127