linux-kernel - Re: Finding hardlinks

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Date:	Fri, 5 Jan 2007 13:12:35 +0000
From:	Pavel Machek <pavel@....cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	matthew@....cx, bhalevy@...asas.com, arjan@...radead.org,
	mikulas@...ax.karlin.mff.cuni.cz, jaharkes@...cmu.edu,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	nfsv4@...f.org
Subject: Re: Finding hardlinks

Hi!

> > > > Some of us have machines designed to cope with cosmic rays, and would be
> > > > unimpressed with a decrease in reliability.
> > > 
> > > With the suggested samefile() interface you'd get a failure with just
> > > about 100% reliability for any application which needs to compare a
> > > more than a few files.  The fact is open files are _very_ expensive,
> > > no wonder they are limited in various ways.
> > > 
> > > What should 'tar' do when it runs out of open files, while searching
> > > for hardlinks?  Should it just give up?  Then the samefile() interface
> > > would be _less_ reliable than the st_ino one by a significant margin.
> > 
> > You need at most two simultenaously open files for examining any
> > number of hardlinks. So yes, you can make it reliable.
> 
> Well, sort of.  Samefile without keeping fds open doesn't have any
> protection against the tree changing underneath between first
> registering a file and later opening it.  The inode number is more

You only need to keep one-file-per-hardlink-group open during final
verification, checking that inode hashing produced reasonable results.

							Pavel
-- 
Thanks for all the (sleeping) penguins.
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