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Re: [Bug-tar] GNU TAR or STAR -- speed comparision

From: Jan-Benedict Glaw
Subject: Re: [Bug-tar] GNU TAR or STAR -- speed comparision
Date: Thu, 18 Oct 2007 20:54:26 +0200
User-agent: Mutt/1.5.13 (2006-08-11)

On Thu, 2007-10-18 20:24:33 +0200, Joerg Schilling <address@hidden> wrote:
> Jan-Benedict Glaw <address@hidden> wrote:
> > > If you like to compare this, you would need to either call star as "tar"
> > > or tell star to be as insecure as GNU tar is.
> > > 
> > > So test again with "star -no-fsync ..."
> >
> > Is `-no-fsync' the only difference?  How much more secure does this
> > make star over tar?  My guestimation tells me that there's only really
> > a difference in case of a system crash during/right after tarball
> > extraction.
> I have seen unfixable (via fsck) filesystemn corruption on Linux after a
> power outage a short time after extracting a bigger filesystem tree.

Yeah, old story.  We've seen SIGSEGV'ing fscks, too. (Though I've
never seen that with Linux + ext[23].)

However, I think it's not tar's task to ensure that all data is
written to a stable medium. Tar has to ensure that, after exit()ing,
all files are ready to be accessed and match the expectation.

> If you like to evaluate the exit code of any tar, this only makes
> sense if the tar implementation calls fsync() before close(). If it
> does not call fsync(), a zero exit code does not grant anything.

It does grant me that if I start any filesystem operation (or mmap()
or whatever), this'll succeed. But I don't think it's tar's business
to mimic a database server or anything else that must be
"transactionally" safe.  If you want that, maybe a data-journaling
or sync-mounted filesystem is what you actually want to deploy.

> Star (called under the name "star") implements other deviations from the
> behavior of the classical UNIX tar, e.g. different defaults for the unlink()
> setup in extract mode.

Hopefully it doesn't try to unlink() directories :->


      Jan-Benedict Glaw      address@hidden              +49-172-7608481
Signature of:             God put me on earth to accomplish a certain number of
the second  :            things. Right now I am so far behind I will never die.

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