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    SubjectRe: [RFC][PATCH 0/3] add FALLOC_FL_NO_HIDE_STALE flag in fallocate
    On 4/17/12 1:43 PM, Ted Ts'o wrote:
    > On Tue, Apr 17, 2012 at 01:59:37PM -0400, Ric Wheeler wrote:
    >>
    >> You could get both security and avoid the run time hit by fully
    >> writing the file or by having a variation that relied on "discard"
    >> (i.e., no need to zero data if we can discard or track it as
    >> unwritten).
    >
    > It's certainly the case that if the device supports persistent
    > discard, something which we definitely *should* do is to send the
    > discard at fallocate time and then mark the space as initialized.
    >
    > Unfortunately, not all devices, and in particular no HDD's for which I
    > aware support persistent discard. And, writing all zero's to the file
    > is in fact what a number of programs for which I am aware (including
    > an enterprise database) are doing, precisely because they tend to
    > write into the fallocated space in a somewhat random order, and the
    > extent conversion costs is in fact quite significant. But writing all
    > zero's to the file before you can use it is quite costly; at the very
    > least it burns disk bandwidth --- one of the main motivations of
    > fallocate was to avoid needing to do a "write all zero pass", and
    > while it does solve the problem for some use cases (such as DVR's),
    > it's not a complete solution.

    Can we please start with profiling the workload causing trouble, see why
    ext4 takes such a hit, and see if anything can be done there to fix
    it surgically, rather than just throwing this big hammer at it?

    In my (admittedly quick, hacky) test, xfs suffed about a 1% perf degradation,
    ext4 about 8%. Until we at least know why ext4 is so much worse, I'll
    signal a strong NAK for this change, for whatever may or may not be worth. :)

    -Eric


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