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

[Ric Wheeler <rwheeler@redhat.com>]

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