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    SubjectRe: vfs: Add MS_FLUSHONFSYNC mount flag
    Fernando Luis Vázquez Cao wrote:
    > On Thu, 2009-02-12 at 15:30 -0600, Eric Sandeen wrote:
    >> Jan Kara wrote:
    >>> On Thu 12-02-09 11:13:37, Eric Sandeen wrote:
    >> ...
    >>
    >>>> Also that way if you have 8 partitions on a battery-backed blockdev, you
    >>>> can tune it once, instead of needing to mount all 8 filesystems with the
    >>>> new option.
    >>> Yes, but OTOH we should give sysadmin a possibility to enable / disable
    >>> it on just some partitions. I don't see a reasonable use for that but people
    >>> tend to do strange things ;) and here isn't probably a strong reason to not
    >>> allow them.
    >>>
    >>> Honza
    >> But nobody has asked for that, have they? So why offer it up a this point?
    >>
    >> They could use LD_PRELOAD to make fsync a no-op if they really don't
    >> care for it, I guess... though that's not easily per-fs either.
    >>
    >> But do we really want to go out of our way to enable people to
    >> short-circuit data integrity paths and then file bugs when their files
    >> go missing? :)
    >
    > Well, it is just a matter of using safe defaults. IMHO, a scenario where
    > the administrator wants to optimize writes to a certain partition and
    > _explicitly_ clears MS_FLUSHONFSYNC on that superblock is not completely
    > unreasonable.

    One case is "this device is safe with a write cache and flush is not
    necessary for data consistency" - that's the per-bdev setting.

    The other case is "I don't really care and I just want to go faster" -
    that's the per-mount setting.

    I'd be much more likely to support the first case, as it's needed for
    maximum performance w/o sacrificing correctness, when properly used.

    The latter case is really only for cutting corners and giving people
    more rope than they need to hang themselves.

    >> (I guess the blockdev tunable is similarly dangerous, but it more
    >> clearly meets the explicit need (writecache-safe devices))
    >
    > If distributions use sane defaults and we document the mount option or
    > bdev tunable properly I guess it might make sense to allow system
    > administrators to shoot themselves in the foot.
    >
    > (By the way, in this patch-set a patch for mount(8) is included.)

    ... one of the advantages of making it a bdev tunable is that you don't
    have to mess with mount(8) :)

    -Eric
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