On 2018-03-05 10:28, Christoph Hellwig wrote:
On Sat, Mar 03, 2018 at 06:59:26AM +0000, Duncan wrote:
Indeed. Preallocation with COW doesn't make the sense it does on an
overwrite-in-place filesystem.
It makes a whole lot of sense, it just is a little harder to implement.
There is no reason not to preallocate specific space, or if you aren't
forced to be fully log structured by the medium, specific blocks to
COW into. It just isn't quite as trivial as for a rewrite in place
file system to implement.
Yes, there's generally no reason not to pre-allocate space, but given
how BTRFS implements pre-allocation, it doesn't make sense to do so
pretty much at all for anything but NOCOW files, as it doesn't even
guarantee that you'll be able to write however much data you
pre-allocated space for (and it doesn't matter whether you use fallocate
or just write out a run of zeroes, either way does not work in a manner
consistent with how other filesystems do).
There's been discussion before about this, arising from the (completely
illogical given how fallocate is expected to behave) behavior that you
can fallocate more than half the free space on a BTRFS volume but will
then fail writes with -ENOSPC part way through actually writing data to
the pre-allocated space you just reserved (and that it can fail for
other reasons too with -ENOSPC).
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