Re: [PATCH] ext3: wait on all pending commits in ext3_sync_fs

[Ric Wheeler]

Eric Sandeen wrote:
> Jan Kara wrote:
>
>   
> > > In looking at what we have today, I wonder if we can make things smarter
> > > so that we don't commit empty transactions in any case?
> > >       
> >   Probably it does not make sence to commit such transactions and we might
> > save some time in sync paths if we do so. So yes, I think skipping empty
> > transaction commit might be worthwhile and it shouldn't be hard to do
> > either. But I'd give it serious testing just in case some unexpectedly
> > relies on this behaviour - wouldn't this interfere e.g. with sync
> > transaction batching autotuning code? Untested patch below...
> > 								Honza
> >     
>
>
> Cool, thanks!  This's stop:
>
> # sync
>
> from spinning up disks under idle filesystems too, I think.
>
> I was looking at something similar but was still working out how many
> things to check before deciding if the transaction was in fact empty.  :)
>
> -Eric
>   

Without having dived into the patch in detail, one worry I would have is 
that we still might care to spin up a drive for empty transactions in 
order to invalidate the drive's write cache.

For example, if we have the following sequence:

   (1) user app performs series of writes to file A
   (2) pages dirtied from writes to A are destaged to the disk over time
   (3) user app issues fsync(file A) to make sure that the data will 
survive a power outage

At this point in time, would this change prevent us from spinning up the 
drive and invalidating the disk write cache for that fsync() ?

Regards,

Ric

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