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

On Thu, Mar 03, 2016 at 10:09:24AM -0800, Christoph Hellwig wrote:

On Thu, Mar 03, 2016 at 01:01:11PM -0500, Martin K. Petersen wrote:

quoted

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.

Any particular reason why we can't just implement those two fallocate
flags for block devices?

--D