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 ...
BTW dd has this as an option, and dvd-vr is a specialized tool
often handling large files, so in both these cases FADV_DONTNEED
is appropriate. If pxzcat will always be used for virt images then
that does seem appropriate there too, though if used for general
files then probably not.
cheers,
Pádraig.
Thanks, Rich.
--
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