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    SubjectRe: [RFC PATCH v2 15/21] loop: use aio to perform io on the underlying file
    On 04/20/2012 11:20 AM, Jeff Moyer wrote:
    > Dave Kleikamp<dave.kleikamp@oracle.com> writes:
    >
    >> On 04/20/2012 09:48 AM, Maxim V. Patlasov wrote:
    >>> On 03/30/2012 07:43 PM, Dave Kleikamp wrote:
    >>>> From: Zach Brown<zab@zabbo.net>
    >>>>
    >>>> This uses the new kernel aio interface to process loopback IO by
    >>>> submitting concurrent direct aio. Previously loop's IO was serialized
    >>>> by synchronous processing in a thread.
    >>>>
    >>>
    >>> The patch ignores REQ_FLUSH bit of bi_rw. Is it simply overlook?
    >>
    >> Good question. Since the loop device is sending only direct IO requests,
    >> it shouldn't be necessary to explicitly flush page cache, but REQ_FLUSH
    >
    > REQ_FLUSH isn't about the page cache, it's about flushing the volatile
    > disk write cache. You need to handle that.

    I guess O_DIRECT doesn't routinely issue flushes simply because it's too
    expensive? Apps that care about consistent IO and O_DIRECT are expected
    to not have writeback caching enabled? 'cause there's no way they're
    issuing syncs themselves.

    So yeah, I'd agree that the loop code should be reworked a bit so that
    both the filebacked and aio methods call vfs_sync() when they see
    REQ_FLUSH.

    Bleh.

    - z
    (Sorry, no real time to dig into this now. Lots more time in two months!)


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