linux-ext4 - Re: [PATCH 1/6] fs: add hole punching to fallocate

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Message-ID: <20101109215357.GI4936@quack.suse.cz>
Date:	Tue, 9 Nov 2010 22:53:57 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ted Ts'o <tytso@....edu>
Cc:	Dave Chinner <david@...morbit.com>, Josef Bacik <josef@...hat.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	xfs@....sgi.com, joel.becker@...cle.com, cmm@...ibm.com,
	cluster-devel@...hat.com
Subject: Re: [PATCH 1/6] fs: add hole punching to fallocate

On Tue 09-11-10 16:41:47, Ted Ts'o wrote:
> On Tue, Nov 09, 2010 at 03:42:42PM +1100, Dave Chinner wrote:
> > Implementation is up to the filesystem. However, XFS does (b)
> > because:
> > 
> > 	1) it was extremely simple to implement (one of the
> > 	   advantages of having an exceedingly complex allocation
> > 	   interface to begin with :P)
> > 	2) conversion is atomic, fast and reliable
> > 	3) it is independent of the underlying storage; and
> > 	4) reads of unwritten extents operate at memory speed,
> > 	   not disk speed.
> 
> Yeah, I was thinking that using a device-style TRIM might be better
> since future attempts to write to it won't require a separate seek to
> modify the extent tree.  But yeah, there are a bunch of advantages of
> simply mutating the extent tree.
> 
> While we're on the subject of changes to fallocate, what do people
> think of FALLOC_FL_EXPOSE_OLD_DATA, which requires either root
> privileges or (if capabilities are in use) CAP_DAC_OVERRIDE &&
> CAP_MAC_OVERRIDE && CAP_SYS_ADMIN.  This would allow a trusted process
> to fallocate blocks with the extent already marked initialized.  I've
> had two requests for such functionality for ext4 already.  
> 
> (Take for example a trusted cluster filesystem backend that checks the
> object checksum before returning any data to the user; and if the
> check fails the cluster file system will try to use some other replica
> stored on some other server.)
  Hum, could you elaborate a bit? I fail to see how above fallocate() flag
could be used to help solving this problem... Just curious...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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