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SubjectRe: Linux 2.6.29
Jens Axboe wrote:
> On Wed, Mar 25 2009, Jeff Garzik wrote:
>> Stating "fsync already does that" borders on false, because that assumes
>> (a) the user has a fs that supports barriers
>> (b) the user is actually aware of a 'barriers' mount option and what it
>> means
>> (c) the user has turned on an option normally defaulted to off.
>>
>> Or in other words, it pretty much never happens.
>
> That is true, except if you use xfs/ext4. And this discussion is fine,
> as was the one a few months back that got ext4 to enable barriers by
> default. If I had submitted patches to do that back in 2001/2 when the
> barrier stuff was written, I would have been shot for introducing such a
> slow down. After people found out that it just wasn't something silly,
> then you have a way to enable it.
>
> I'd still wager that most people would rather have a 'good enough
> fsync' on their desktops than incur the penalty of barriers or write
> through caching. I know I do.

That's a strawman argument: The choice is not between "good enough
fsync" and full use of barriers / write-through caching, at all.

It is clearly possible to implement an fsync(2) that causes FLUSH CACHE
to be issued, without adding full barrier support to a filesystem. It
is likely doable to avoid touching per-filesystem code at all, if we
issue the flush from a generic fsync(2) code path in the kernel.

Thus, you have a "third way": fsync(2) gives the guarantee it is
supposed to, but you do not take the full performance hit of
barriers-all-the-time.

Remember, fsync(2) means that the user _expects_ a performance hit.

And they took the extra step to call fsync(2) because they want a
guarantee, not a lie.

Jeff





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