12x performance drop on md/linux+sw raid1 due to barriers [xfs]

[Leon Woestenberg]

Hello,

On 15 dec 2008, at 23:50, Peter Grandi wrote:

> [ ... ]
>
>>> The purpose of barriers is to guarantee that relevant data is
>>> known to be on persistent storage (kind of hardware 'fsync').
>>>
>
>> [ ... ] Unfortunately in my understanding none of this is
>> reflected by Documentation/block/barrier.txt
>
> But we are talking about XFS and barriers here. That described
> just a (flawed, buggy) mechanism to implement those. Consider
> for example:
>
>  http://www.xfs.org/index.php/XFS_FAQ#Write_barrier_support.
>  http://www.xfs.org/index.php/XFS_FAQ#Q._Should_barriers_be_enabled_with_storage_which_has_a_persistent_write_cache.3F
>
> In any case as to the kernel "barrier" mechanism, its
> description is misleading because it heavily fixates on the
> ordering issue, which is just a consequence, but yet mentions
> the far more important "flush/sync" aspect.
>
> Still, there is a lot of confusion about barrier support and
> what it means at which level, as reflected in several online
> discussions and the different behaviour of different kernel
> versions.
>
The semantics of a barrier are whatever semantics we describe to it.  
So we can continue to be confused about it.

I strongly disagree on the ordering issue being a side effect.

Correct ordering can be proven to be enough to provide transactional  
correctness, enough to ensure that filesystems can not get corrupted  
on power down.

Using barriers to guarantee that (all submitted) write requests  
(before the barrier) made it to the medium are a stronger predicate.

The Linux approach and documentation talks about the first type of  
semantics (which I rather like for them being strong enough and not  
more).

Regards,

Leon