Re: Tracking actual disk write sources instead of flush thread

[Phillip Susi <psusi@ubuntu.com>]

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On 04/23/2014 07:19 PM, Andreas Dilger wrote:
> I think that adding a pointer or integer per page would meet
> resistance, but I think it is pretty reasonable to track this on a
> per-inode basis. It is fairly uncommon to have multiple threads
> writing to the same file, and I would guess it is vanishingly rare
> that different applications are writing to the same file at one
> time.

Wherever the current counter is should be fine, the question is when
it gets updated.  Rather than updating it on sync it just needs
updated on actual write() / page dirty.

> Storing {current->comm}.{pid} would take 20 bytes of space per
> inode, but would be much more useful than just storing {pid}, since
> a process may be long gone by the time that the blocks are even
> submitted to disk due to delayed allocation and such.
> 
> It would be possible to store a refcounted struct with this info
> pointed to from the inode, since it would only be useful on inodes
> being written, but that has to be balanced against the complexity
> of maintaining that struct and the potential of saving 12 bytes per
> inode (since there would still need to be a pointer in the inode).

That might be a bit overkill.  I believe the current interface just
has a counter of bytes of IO hung off the task, so you can't catch
very short lived processes.  That's probably not too bad.  I don't
think it needs tracked on a per inode basis; just being able to sort
out that this process is doing the writing instead of some other
random process or the flush task that actually ends up submitting the bio.


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