Re: Linux 2.6.29

Ric Wheeler
2009-03-26T00:32:51Z

Linus Torvalds wrote:
> On Wed, 25 Mar 2009, Ric Wheeler wrote:
>
>> One concern with doing this above the file system is that you are not in the
>> context of a transaction so you have no clean promises about what is on disk
>> and persistent when. Flushing the cache is primitive at best, but the way
>> barriers work today is designed to give the transactions some pretty critical
>> ordering semantics for journalling file systems at least.
>>
>> I don't see how you could use this approach to make a really robust, failure
>> proof storage system, but it might appear to work most of the time for most
>> people :-)
>>
>
> You just do a write barrier after doing all the filesystem writing, and
> you return with the guarantee that all the writes the filesystem did are
> actually on disk.
>
>
In this case, you have not gained anything - same number of barrier
operations/cache flushes and looser semantics for the transactions?
> No gray areas. No questions. No "might appear to work".
>
> Sure, there might be other writes that got flushed _too_, but nobody
> cares. If you have a crash later on, that's always true - you don't get
> crashes at nice well-defined points.
>
> Linus
>
This is pretty much how write barriers work today - you carry down other
transactions (even for other partitions on the same disk) with you...

ric