Messages in this thread |  | | Date | Mon, 12 Nov 2001 14:36:08 -0800 | From | Andrew Morton <> | Subject | Re: File System Performance |
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Linus Torvalds wrote: > > On Mon, 12 Nov 2001, Lionel Bouton wrote: > > > > Seems not the case with gnu tar : write isn't even called once on the fd > > returned by open("/dev/null",...). In fact a "grep write" on the strace > > output is empty in the "tar cf /dev/null" case. Every file in the tar-ed > > tree is stat-ed but no-one is read-ed. > > Wow. What a sleazy optimization - it can't be anything but a special case. > > How do they do it anyway? By matching on the name Or by knowing what the > minor/major numbers of /dev/null are supposed to be on that particular > operating system?
/* Detect if outputting to "/dev/null". */ { static char const dev_null[] = "/dev/null"; struct stat dev_null_stat;
dev_null_output = (strcmp (archive_name_array[0], dev_null) == 0 || (! _isrmt (archive) && stat (dev_null, &dev_null_stat) == 0 && S_ISCHR (archive_stat.st_mode) && archive_stat.st_rdev == dev_null_stat.st_rdev)); }
> And what's the _point_ of the optimization? I've never heard of a "tar > benchmark"..
Ask Al. He understands those GNU folks.
It's actually a bug. I may _want_ an ISREG /dev/null...
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