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SubjectRe: Linux 2.6.29
On Wed, Mar 25 2009, Ric Wheeler wrote:
> Jens Axboe wrote:
>> On Wed, Mar 25 2009, Jeff Garzik wrote:
>>> Jens Axboe wrote:
>>>> On Tue, Mar 24 2009, Jeff Garzik wrote:
>>>>> Linus Torvalds wrote:
>>>>>> But I really don't understand filesystem people who think that
>>>>>> "fsck" is the important part, regardless of whether the data is
>>>>>> valid or not. That's just stupid and _obviously_ bogus.
>>>>> I think I can understand that point of view, at least:
>>>>> More customers complain about hours-long fsck times than they do
>>>>> about silent data corruption of non-fsync'd files.
>>>>>> The point is, if you write your metadata earlier (say, every 5
>>>>>> sec) and the real data later (say, every 30 sec), you're
>>>>>> actually MORE LIKELY to see corrupt files than if you try to
>>>>>> write them together.
>>>>>> And if you write your data _first_, you're never going to see
>>>>>> corruption at all.
>>>>> Amen.
>>>>> And, personal filesystem pet peeve: please encourage proper
>>>>> FLUSH CACHE use to give users the data guarantees they deserve.
>>>>> Linux's sync(2) and fsync(2) (and fdatasync, etc.) should poke
>>>>> the block layer to guarantee a media write.
>>>> fsync already does that, at least if you have barriers enabled on your
>>>> drive.
>>> Erm, no, you don't enable barriers on your drive, they are not a
>>> hardware feature. You enable barriers via your filesystem.
>> Thanks for the lesson Jeff, I'm obviously not aware how that stuff
>> works...
>>> 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.
>>> Furthermore, a blatantly obvious place to flush data to media --
>>> fsync(2), fdatasync(2) and sync_file_range(2) -- should cause the
>>> block layer to issue a FLUSH CACHE for __any__ filesystem. But that
>>> doesn't happen either.
>>> So, no, for 95% of Linux users, fsync does _not_ already do that. If
>>> you are lucky enough to use XFS or ext4, you're covered. That's it.
>> The point is that you need to expose this choice somewhere, and that
>> 'somewhere' isn't manually editing fstab and enabling barriers or
>> fsync-for-real. And it should be easier.
>> Another problem is that FLUSH_CACHE sucks. Really. And not just on
>> ext3/ordered, generally. Write a 50 byte file, fsync, flush cache and
>> wit for the world to finish. Pretty hard to teach people to use a nicer
>> fdatasync(), when the majority of the cost now becomes flushing the
>> cache of that 1TB drive you happen to have 8 partitions on. Good luck
>> with that.
> And, as I am sure that you do know, to add insult to injury, FLUSH_CACHE
> is per device (not file system).
> When you issue an fsync() on a disk with multiple partitions, you will
> flush the data for all of its partitions from the write cache....

Exactly, that's what my (vague) 8 partition reference was for :-)
A range flush would be so much more palatable.

Jens Axboe

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