On Tue, 19 Mar 2013, Chris Mason wrote:
> Quoting Alexandre Oliva (2013-03-19 01:20:10)
> > On Mar 18, 2013, Chris Mason <[email protected]> wrote:
> >
> > > A few questions. Does leveldb use O_DIRECT and mmap together?
> >
> > No, it doesn't use O_DIRECT at all. Its I/O interface is very
> > simplified: it just opens each new file (database chunks limited to 2MB)
> > with O_CREAT|O_RDWR|O_TRUNC, and then uses ftruncate, mmap, msync,
> > munmap and fdatasync. It doesn't seem to modify data once it's written;
> > it only appends. Reading data back from it uses a completely different
> > class interface, using separate descriptors and using pread only.
> >
> > > (the source of a write being pages that are mmap'd from somewhere
> > > else)
> >
> > AFAICT the source of the memcpy()s that append to the file are
> > malloc()ed memory.
> >
> > > That's the most likely place for this kind of problem. Also, you
> > > mention crc errors. Are those reported by btrfs or are they application
> > > level crcs.
> >
> > These are CRCs leveldb computes and writes out after each db block. No
> > btrfs CRC errors are reported in this process.
>
> Ok, so we have three moving pieces here.
>
> 1) leveldb truncating the files
> 2) leveldb using mmap to write
> 3) btrfs snapshots
>
> My guess is the truncate is creating a orphan item that is being
> processed inside the snapshot.
>
> Is it possible to create a smaller leveldb unit test that we might use
> to exercise all of this?
There is a set of unit tests in the leveldb source tree that ought to do
the trick:
git clone https://code.google.com/p/leveldb/
sage
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