Re: rewrite very slow

[David Chinner <dgc@sgi.com>]

On Fri, Feb 15, 2008 at 06:36:06PM -0600, Eric Sandeen wrote:
> DS wrote:
> > Hello,
> > 
> > I need some help to tunning my storage.
> 
> ...
> > TIME 43 sekund
> 
> 
> What kernel?  when I test on my 2.6.23.9-85.fc8 and 2.6.22.5 boxes, I
> see 2 and 7 seconds for rewrite, respectively.
> 
> but granted, on ext3 I get 0 seconds for every run.
> 
> Also the difference appears to be O_TRUNC (which the perl script does);
> if I code it in c:
> 
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> 
> void main(void)
> {
>         int i;
>         int fd;
>         char file[4];
> 
>         for (i = 0; i < 100; i++) {
>                 sprintf(file, "%d.txt", i);
>                 fd = open(file, O_CREAT|O_RDWR|O_TRUNC, 0644);
>                 write(fd, "aaaaaaaaaaaaaaaaaaa\n");
>                 close(fd);
>         }
> }
> 
> rewrite is a bit slower w/ O_TRUNC in place, plenty fast w/o it.  Not
> sure about the xfs/ext3 difference... this is probably a side-effect of
> flushes xfs put into place on truncate (IIRC?)

Yup - after a truncate we use flush-on-close semantics if the file
is closed before pdflush does writeback. yes, it has a measurable
impact on silly microbenchmarks like this, but nobody even noticed
it when we introduced this code 2-3 years ago...

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group