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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

    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.


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      Last update: 2009-03-25 21:29    [W:0.047 / U:3.832 seconds]
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