linux-kernel - Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of blocks

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Date:	Thu, 3 Mar 2016 10:09:24 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	shane.seymour@....com, Bruce Fields <bfields@...ldses.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Jeff Layton <jlayton@...chiereds.net>
Subject: Re: [PATCH 2/2] block: create ioctl to discard-or-zeroout a range of
 blocks

On Thu, Mar 03, 2016 at 01:01:11PM -0500, Martin K. Petersen wrote:
> That's not entirely true. Writing the blocks may cause them to be
> allocated on the storage device (depending on which flags we feed it in
> WRITE SAME).
> 
> The filesystems people were wanted the following semantics:
> 
>  - deallocate, don't care about contents for future reads (discard)
>  - deallocate, guarantee zeroes on future reads (zeroout)
>  - (re)allocate, guarantee zeroes on future reads (zeroout)
> 
> Maybe we just need a better naming scheme...

In filesystem terms we have two and three:

 - FALLOC_FL_PUNCH_HOLE assures zeroes are returned, but space is
   deallocated as much as possible
 - FALLOC_FL_ZERO_RANGE assures zeroes are returned, AND blocks are
   actually allocated

Returning stale blocks in a file system is a nasty security risk, so
we don't do that, and so shouldn't storage that offers any kind
of multi tenancy, and if it's just VMs using multiple partitions on it.