Re: Thin device provisioning

[Matthew Wilcox <matthew@wil.cx>]

I've spoken with a few Linux filesystem people.  They find it
significantly easier to send a single LBA/length pair at a time.
Modern filesystems try quite hard to keep fragmentation to a minimum, so
they don't expect a performance hit from sending multiple commands.
They're non-blocking writes, and the IO elevators can take care of
sending more important reads first.

On Fri, Aug 08, 2008 at 05:58:15PM -0400, Knight, Frederick wrote:
> Thank you for your input.  Yes, this was discussed.  Most filesystems do
> not create fully contiguous files, and therefore when a file is deleted,
> some number of discontiguous extents must be punched/discarded.  This is
> the reason for the list of LBA/length pairs.
> 
> The alternative is that the filesystem must create 1 unique request to
> the driver for each extent, and the driver must create 1 CDB for each
> extent, and the filesystem must then iterate through the entire file for
> each discontiguous range.  It was felt that this would be a problem for
> filesystems and drivers to do this type of operation, and it was
> preferred to simply supply a list.
> 
> Thanks again for the input,
> 
> 	Fred Knight
> 	SAN Standards Technologist
> 	NetApp
> 
> 
> 
> -----Original Message-----
> From: Matthew Wilcox [mailto:matthew@wil.cx] 
> Sent: Friday, August 08, 2008 9:15 AM
> To: Knight, Frederick
> Cc: David Woodhouse; ricwheeler@gmail.com
> Subject: Thin device provisioning
> 
> * From the T10 Reflector (t10@t10.org), posted by:
> * Matthew Wilcox <matthew@wil.cx>
> *
> Good morning Fred,
> 
> I've been looking at your 08-149r0.pdf with a view to using the 'PUNCH'
> command to implement the Linux 'DISCARD' command.  It's a little
> over-specified for what we need and this causes the implementation to be
> a little more complex than I would like.  The excess capability is the
> ability to do multiple punches in a single command.  Do you really need
> to be able to add/remove lots of ranges atomically, or could you use a
> command specified like this:
> 
> 0       0x9F
> 1       service action
> 2-9     LBA
> 10-13   length
> 14      reserved
> 15      control
> 
> and send one command for each range?
> 
> Apologies if this has already been covered in a T10 discussion; I'm not
> a member and though I've searched the archives, I may have missed a
> discussion.
> 
> --
> Intel are signing my paycheques ... these opinions are still mine "Bill,
> look, we understand that you're interested in selling us this operating
> system, but compare it to ours.  We can't possibly take such a
> retrograde step."
> 
> *
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-- 
Intel are signing my paycheques ... these opinions are still mine
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."