Re: Enable asynchronous commits by default patch revoked?

[Ric Wheeler <rwheeler@redhat.com>]

Andreas Dilger wrote:
> On Aug 24, 2009  09:34 -0400, Theodore Ts'o wrote:
>   
>> On Mon, Aug 24, 2009 at 10:33:10AM +0200, Christian Fischer wrote:
>>     
>>> I try to figure out reasonable mount options for ext4.
>>>
>>> I've seen a "Enable asynchronous commits by default" patch from Sun, 21 Sep 
>>> 2008.
>>>
>>> Why is it revoked?
>>>       
>> It patch was never merged because the ayschronous commits feature
>> disabled all write barriers, so under heavy workloads a power failure
>> could cause data loss.
>>
>> No one has gotten around to looking at this closely; I think adding a
>> strategically placed blkdev_issue_flush() will allow us to safely
>> enable this feature, but it needs careful study.
>>     
>
> I don't think that was the issue, but rather that we wanted to have
> per-block checksums in order to handle the case were some block in
> transaction A is causing a transaction checksum failure, yet transaction
> B has already committed and begun checkpointing.
>
> One option discussed was to add a lightweight 16-bit checksum (e.g. TCP
> checksum) to the high bits of the t_flags of the block tag.  The checksum
> doesn't have to be very strong since the whole-transaction checksum will
> be the primary point of validation.
>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>
>   

I still don't trust the logic. Seems like a very complex (and really 
non-intuitive - async and commit, really?) thing to support for a 
marginal performance impact. Any blkdev_issue_flush() call would dwarf 
the advantage of the async bit of the commit.

If we are looking to better support workloads that suffer from 
journalling, I suspect that we have more natural ways to do that...

ric