Re: [RFC] relaxed barrier semantics

[Vladislav Bolkhovitin <vst@vlnb.net>]

Jan Kara, on 07/31/2010 04:47 AM wrote:
> On Fri 30-07-10 16:20:25, Christoph Hellwig wrote:
>> On Fri, Jul 30, 2010 at 05:44:08PM +0400, Vladislav Bolkhovitin wrote:
>>> Yes, but why not to make step further and allow to completely eliminate
>>> the waiting/draining using ORDERED requests? Current advanced storage
>>> hardware allows that.
>>
>> There is a few caes where we could do that - the fsync without metadata
>> changes above would be the prime example.  But there's a lot lower
>> hanging fruit until we get to the point where it's worth trying.
>    Umm, I don't understand you. I think that fsync in particular is an
> example where you have to wait and issue cache flush if the drive has
> volatile write cache. Otherwise you cannot promise to the user data will be
> really on disk in case of crash. So no ordering helps you.

Isn't there the second wait for journal update?

>    And if you are speaking about a drive without volatile write caches, then
> fsync without metadata changes is just trivial and you don't need any
> ordering.

A drive can reorder queued SIMPLE requests at any time doesn't matter if 
it has volatile write caches or not. So, if you expect in-order requests 
execution (with journal updates you do?), you need to enforce that order 
either by ORDERED requests or (local) queue draining.

Vlad