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    SubjectRe: vfs: Add MS_FLUSHONFSYNC mount flag
    Theodore Tso wrote:
    > On Sun, Feb 22, 2009 at 03:15:33PM +0100, Pavel Machek wrote:
    >> Sounds like posix violation to
    >> me... '/sys/fsync_does_not_really_sync'?
    > It's as much a Posix violation as noatime. :-)
    >> Perhaps it is better done at glibc level? Environment variables
    >> already mostly have semantics you want.....
    > Environment variables own't allow us to switch fsync's on and off
    > while the process is running, so that the system can run "safe" while
    > it is on AC mains, but "low power" when on battery.

    Overall, for each filesystem, we really should _default_ to a safe mode
    where fsync(2), fdatasync(2), sync_file_range(2) and transaction commits
    are guaranteed to be committed to stable storage -- i.e. SYNC CACHE /
    FLUSH CACHE, a forced unit access (FUA) bit, or guarantee that the
    underlying storage has disabled write-back caching.

    Correctness should come before performance. Linux has not had
    credibility here, in the ATA+ext[23] space at least, and it is embarrassing.

    Modern SATA and SCSI disks can do tagged command queueing with the FUA
    bit, which does a lot to mitigate the performance loss seen with a less
    capable "FLUSH CACHE + barrier" setup.


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      Last update: 2009-02-23 00:45    [W:0.020 / U:55.844 seconds]
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