Re: [Libguestfs] [PATCH 1/2] Preallocate output file

On Wed, Oct 23, 2013 at 03:38:30AM +0100, Pádraig Brady wrote: [...] By the way, Eric Sandeen solved the problem. It's a genuine misfeature in ext4 called auto_da_alloc which causes a flush on close if the file has been truncated (ftruncate or O_TRUNC) and the file size is zero bytes. I added these patches which work around the issue: http://git.annexia.org/?p=pxzcat.git;a=commitdiff;h=68640d56b2ea96401a135... http://git.annexia.org/?p=pxzcat.git;a=commitdiff;h=05f0de58de6cbcbdc40f5... >> /* Tell the kernel we won't read the output file. */ >> posix_fadvise (fd, 0, 0, POSIX_FADV_RANDOM|POSIX_FADV_DONTNEED); > > I know this isn't related to your patch, > but I don't think you can set a flag like this. > These are rather operations that need to be done > while writing I think. > > For ref I made the above call after writing various chunks to disk in: > https://github.com/pixelb/dvd-vr/blob/master/dvd-vr.c#L322 > and also dd: > http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commit;h=5f31155 This does explain why the posix_fadvise call has no effect. How about POSIX_FADV_RANDOM? I would have thought that should be done before the write operations ...