linux-ext4 - Re: [Ext4 punch hole 1/5] Ext4 Punch Hole Support: Convert Blocks to Uninit Exts

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Date:	Thu, 3 Mar 2011 09:27:27 -0500
From:	Theodore Tso <tytso@....EDU>
To:	Dave Chinner <david@...morbit.com>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Allison Henderson <achender@...ux.vnet.ibm.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [Ext4 punch hole 1/5] Ext4 Punch Hole Support: Convert Blocks to Uninit Exts

On Mar 2, 2011, at 3:23 PM, Dave Chinner wrote:

> 
> XFS_IOC_ZERO_RANGE converts a range to unwritten extents, not
> uninitialised extents. An uninitialised extent is one that is
> allocated but had not data written to it (i.e. contains stale data),
> while an unwritten/preallocated extent is guaranteed to contain
> zeros.  This may be just a terminology issue, but we should try to
> use the same jargon across all filesystems...

What is the difference from the user's perspective?  If the read from
from an uninitialized extent, what do they get back?   And if the read
from an unwritten extent, what do they get back?   In ext4, in both
cases we would return all zero's.

For XFS, why is it important to maintain the distinction between these
two concept?

Just trying to understand,

-- Ted

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