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Date: Fri, 19 Nov 2010 11:16:00 -0500
From: "Ted Ts'o" <tytso@mit.edu>
To: Christoph Hellwig <hch@infradead.org>
Cc: Mark Lord <kernel@teksavvy.com>, Steven Whitehouse <swhiteho@redhat.com>,
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Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate
 super_operation
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On Fri, Nov 19, 2010 at 10:50:03AM -0500, Christoph Hellwig wrote:
> 
> It's a huge issue for virtualization, where naive TRIM implementations
> can expose data deleted in one VM to others.  It's also a huge issues
> for RAIDs as mentioned by you.

Fair enough, I'll buy that.  If you are sharing an SSD using
virtualization across two VM's with different trust bounaries,
non-deterministic TRIM could very well be an issue, depending on how
it the "non-deterministic" bit was implemented.

(I can think of one PCIe-attached flash implementation I know of where
the trim is simply not persistent across a power failure, such that if
Alice trims a block, it will return 0, but if no one rewrites the
block before a power failure, then reading that some block will return
Alice's original data, and not data belonging to Bob.  This would be
an example of a non-deterministic TRIM that would be problematic for
RAID, but not from a security perspective.  OTOH, the trim command is
blazingly fast on this implementation, since there is absolutely no
flash write or erase operation associated with the TRIM, and so mount
-o discard makes a lot of sense for this particular storage device.)

						- Ted
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