Re: [RFC v3 11/13] vfs: add 3 new ioctl interfaces

[Zhi Yong Wu]

On Tue, Oct 16, 2012 at 11:17 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Wed, Oct 10, 2012 at 06:07:33PM +0800, zwu.kernel@xxxxxxxxx wrote:
>> From: Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx>
>>
>>   FS_IOC_GET_HEAT_INFO: return a struct containing the various
>> metrics collected in btrfs_freq_data structs, and also return a
>
> I think you mean hot_freq_data :P
Yeah, sorry.
>
>> calculated data temperature based on those metrics. Optionally, retrieve
>> the temperature from the hot data hash list instead of recalculating it.
>
> To get the heat info for a specific file you have to know what file
> you want to get that info for, right?  I can see the usefulness of
Yes.
> asking for the heat data on a specific file, but how do you find the
> hot files in the first place? i.e. the big question the user
> interface needs to answer is "what files are hot?".
We only tell the user what the files' temperatures are, not what files are hot.
Their temperatures are in the output of debugfs.
>
> Once userspace knows what the hottest files are, it can open them
If the user need to know this type of info, it is easy for us to
provide it. But i don't know what way the user hope to get it via.
> and query the data via the above ioctl, but expecting userspace to
> iterate millions of inodes in a filesystem to find hot files is very
> inefficient.
>
> FWIW, if you were to return file handles to the hottest files, then
> the application could open and query them without even needing to
> know the path name to them. This woul dbe exceedingly useful for
> defragmentation programs, especially as that is the way xfs_fsr
> already operates on candidate files.(*)
ah.
>
> IOWs, sometimes the pathname is irrelevant to the operations that
> applications want to perform - all they care about having an
> efficient method of finding the inode they want and getting a file
> descriptor that points to the file. Given the heat map info fits
> right in to the sort of operations defrag and data mover tools
> already do, it kind of makes sense to optimise the interface towards
> those uses....
>
> (*) i.e. finds them via bulkstat which returns handle information
> along with all the other inode data, then opens the file by handle
> to do the defrag work....
OK.
>
>>   FS_IOC_GET_HEAT_OPTS: return an integer representing the current
>> state of hot data tracking and migration:
>>
>> 0 = do nothing
>> 1 = track frequency of access
>>
>>   FS_IOC_SET_HEAT_OPTS: change the state of hot data tracking and
>> migration, as described above.
>
> I can't see how this is a manageable interface. It is not
> persistent, so after every filesystem mount you'd have to set the
> flag on all your inodes again. Hence, for the moment, I'd suggest
> that dropping per-inode tracking control until all the core issues
> are sorted out....
OK.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
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-- 
Regards,

Zhi Yong Wu
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