Re: [PATCH] 2.4.x write barriers (updated for ext3)

[James Bottomley <James.Bottomley@SteelEye.com>]

sct@redhat.com said:
> Generally, that may be true but it's irrelevant.  Internally, the fs
> may keep transactions as independent, but as soon as IO is scheduled,
> those transactions become serialised.  Given that pure sequential IO
> is so much more efficient than random IO, we usually expect
> performance to be improved, not degraded, by such serialisation. 

This is the part I'm struggling with.  Even without error handling and certain 
other changes that would have to be made to give guaranteed integrity to the 
tag ordering, Chris' patch is a very reasonable experimental model of how an 
optimal system for implementing write barriers via ordered tags would work; 
yet when he benchmarks, he sees a performance decrease.

I can dismiss his results as being due to firmware problems with his drives 
making them behave non-optimally for ordered tags, but I really would like to 
see evidence that someone somewhere acutally sees a performance boost with 
Chris' patch.

Have there been any published comparisons of a write barrier implementation 
verses something like the McKusick soft update idea, or even just 
multi-threaded back end completion of the transactions?

James