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Re: [Bug-tar] Use posix_fadvise to improve archive creation performance

From: Carlo Alberto Ferraris
Subject: Re: [Bug-tar] Use posix_fadvise to improve archive creation performance
Date: Wed, 5 Apr 2017 10:16:51 +0900

Just as a comment about why in my patch I use len explicitly instead of 0: it’s to workaround a bug in linux versions <2.6.6.

> In kernels before 2.6.6, if len was specified as 0, then this was interpreted 
> literally as "zero bytes", rather than as meaning “all bytes through to the 
> end of the file”. 

Since 0 means is supposed to mean “until the end of file”, passing explicitly the length of the file (that we already have) should be semantically the same.


On Apr 4, 2017, at 10:14 PM, Mark <address@hidden> wrote:

On Mon, April 3, 2017 03:17, Paul Eggert wrote:
I've lost context. I prefer not having this depend on an environment

Can't the filesystem in question be fixed to have decent performance in
the typical case where applications access files sequentially? It's not
like 'tar' is a special case. I'd hate to have to modify lots of
programs just to work around a lame filesystem.

I think you're confusing two things:
- Carlo's patch
- The suggestion to allow the user to tell tar to use POSIX_FADV_NOREUSE
and/or POSIX_FADV_DONTNEED. In certain scenarios one, both or neither of
those could perform best. [On Linux POSIX_FADV_NOREUSE is currently a

Let's ignore the second point for now.

Carlo's patch at
 posix_fadvise(fd, offset, len, POSIX_FADV_SEQUENTIAL);
 posix_fadvise(fd, offset, len, POSIX_FADV_WILLNEED);
to give a hint to the OS/filesystem about how the file will be accessed.
There shouldn't be any down-side to doing that. On Linux for example,
POSIX_FADV_SEQUENTIAL causes the filesystem read-ahead amount to be

I'm not qualified to say whether the patch should be committed as-is, but
the principle is sound. [I might choose a different name for the
prefetch() function though.]

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