On Sat, Oct 26, 2013 at 08:06:45PM +0300, Lasse Collin wrote:
> > - For reasons I don't understand, both regular xzcat and pxzcat cause
> > the output file to be flushed to disk after the program exits. This
> > causes any program which consumes the output of the file to slow down.
>
> I have no idea. I see you committed something that seems to be related
> to this after your email. With a quick reading I don't understand it
> well, it seems to be working around some issue with ftruncate() with
> ext4.
That's right. It turned out to be a misfeature in ext4: if you
truncate a file from a non-zero size down to a zero size, ext4 flags
the file and flushes it on close. ("Truncate" includes both O_TRUNC
and ftruncate). This can be disabled with the noauto_da_alloc mount
option, but of course that is not the default.
As you already saw I have added this tedious workaround (several
iterations because I wanted it to work correctly if the output is a
block device as well as a regular file):
http://git.annexia.org/?p=pxzcat.git;a=commitdiff;h=68640d56b2ea96401a1355ab56603b0837058d21
http://git.annexia.org/?p=pxzcat.git;a=commitdiff;h=05f0de58de6cbcbdc40f5a661d406b3fbe5a9060
http://git.annexia.org/?p=pxzcat.git;a=commitdiff;h=8b88d6a25dccb0e086f3d2b1d3cab99d722912aa
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines. Supports shell scripting,
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