[LSF/MM TOPIC] Sparseness in storage

[LSF/MM TOPIC] Sparseness in storage

[Douglas Gilbert]

There are a lot of zeros out there. Efficient use of sparseness
involves techniques to detect large quantities of zeros in
advance rather than just reading them all. And on the write side
there are standard techniques to append zeros to a file without
actually writing them.

Seems a damn shame to read a terabyte of zeros and then write them
to another device or file. Carrying the idea further: if we know
random data has no meaning *** and we are asked to copy it,
why not "write" zeros to the output file?

Over the last few years various commands have been added to the
SCSI and ATA command sets to better handle sparseness (and
trim/unmap/write_same can be viewed in this light). File systems
are improving their sparseness handling as well, with Linux
playing "catch up" to NTFS in this regard (e.g. the new
FALLOC_FL_PUNCH_HOLE flag in fallocate() ).

So I am proposing a discussion of the:
   - existing SCSI commands to support sparseness
   - existing ATA commands to support sparseness
   - suggestions for more sparseness support to be
     added to the SCSI and ATA command sets
   - user space tools that support sparseness
   - file system support for sparseness

Perhaps the latter point should involve the file system track as
well.

Doug Gilbert
20100202


*** For example: after ATA CRYPTO SCRAMBLE EXT command (which
     is one of the "sanitize device" commands and is fast) the
     data read will be random and meaningless. If the disk does
     "read zero after trim" why not follow the scramble with a
     trim/unmap of the whole disk?

Re: [LSF/MM TOPIC] Sparseness in storage

[Christoph Hellwig]

On Wed, Feb 02, 2011 at 12:07:23PM -0500, Douglas Gilbert wrote:
> are improving their sparseness handling as well, with Linux
> playing "catch up" to NTFS in this regard (e.g. the new
> FALLOC_FL_PUNCH_HOLE flag in fallocate() ).

FYI: XFS has supported hole punching for more than 15 years, although
the first years obviously were't on Linux.

Re: [LSF/MM TOPIC] Sparseness in storage

[Douglas Gilbert]

On 11-02-02 09:23 PM, Christoph Hellwig wrote:
> On Wed, Feb 02, 2011 at 12:07:23PM -0500, Douglas Gilbert wrote:
>> are improving their sparseness handling as well, with Linux
>> playing "catch up" to NTFS in this regard (e.g. the new
>> FALLOC_FL_PUNCH_HOLE flag in fallocate() ).
>
> FYI: XFS has supported hole punching for more than 15 years, although
> the first years obviously were't on Linux.

I was looking at the Windows API yesterday and you need
to check each volume for sparse support. So it is file
system dependent there as well: VFAT doesn't support
sparseness and NTFS (version ?) does.

 From the perspective of user space tools, POSIX support
for sparseness would be helpful and FALLOC_FL_PUNCH_HOLE
looks like a step in that direction.

Doug Gilbert

RE: [LSF/MM TOPIC] Sparseness in storage

[Shyam_Iyer]

> -----Original Message-----
> From: linux-scsi-owner@vger.kernel.org [mailto:linux-scsi-
> owner@vger.kernel.org] On Behalf Of Douglas Gilbert
> Sent: Wednesday, February 02, 2011 12:07 PM
> To: linux-scsi
> Subject: [LSF/MM TOPIC] Sparseness in storage
> 
> There are a lot of zeros out there. Efficient use of sparseness
> involves techniques to detect large quantities of zeros in
> advance rather than just reading them all. And on the write side
> there are standard techniques to append zeros to a file without
> actually writing them.
> 
> Seems a damn shame to read a terabyte of zeros and then write them
> to another device or file. Carrying the idea further: if we know
> random data has no meaning *** and we are asked to copy it,
> why not "write" zeros to the output file? 
> Over the last few years various commands have been added to the
> SCSI and ATA command sets to better handle sparseness (and
> trim/unmap/write_same can be viewed in this light). File systems
> are improving their sparseness handling as well, with Linux
> playing "catch up" to NTFS in this regard (e.g. the new
> FALLOC_FL_PUNCH_HOLE flag in fallocate() ).
> 
> So I am proposing a discussion of the:
>    - existing SCSI commands to support sparseness
>    - existing ATA commands to support sparseness
>    - suggestions for more sparseness support to be
>      added to the SCSI and ATA command sets
>    - user space tools that support sparseness
>    - file system support for sparseness
> 

On the last point I was talking to Ted on the same topic in Plumbers more so in the context of thin provisioned LUNs and he suggested creating profiles ...

The zeroes out there also tend to change I/O patterns which are quite tied to low-level chunk size details in storage SANs.  I had like to think that the hole size could be linked to the chunk sizes exported by the Storage SANs and so it makes a lot of sense to create profiles.


> Perhaps the latter point should involve the file system track as
> well.
> 
> Doug Gilbert
> 20100202
> 
> 
> *** For example: after ATA CRYPTO SCRAMBLE EXT command (which
>      is one of the "sanitize device" commands and is fast) the
>      data read will be random and meaningless. If the disk does
>      "read zero after trim" why not follow the scramble with a
>      trim/unmap of the whole disk?
> --
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Re: [LSF/MM TOPIC] Sparseness in storage

[Richard Sharpe]

On Wed, Feb 2, 2011 at 12:07 PM, Douglas Gilbert <dgilbert@interlog.com> wrote:
> There are a lot of zeros out there. Efficient use of sparseness
> involves techniques to detect large quantities of zeros in
> advance rather than just reading them all. And on the write side
> there are standard techniques to append zeros to a file without
> actually writing them.
>
> Seems a damn shame to read a terabyte of zeros and then write them
> to another device or file. Carrying the idea further: if we know
> random data has no meaning *** and we are asked to copy it,
> why not "write" zeros to the output file?
>
> Over the last few years various commands have been added to the
> SCSI and ATA command sets to better handle sparseness (and
> trim/unmap/write_same can be viewed in this light). File systems
> are improving their sparseness handling as well, with Linux
> playing "catch up" to NTFS in this regard (e.g. the new
> FALLOC_FL_PUNCH_HOLE flag in fallocate() ).
>
> So I am proposing a discussion of the:
>  - existing SCSI commands to support sparseness
>  - existing ATA commands to support sparseness
>  - suggestions for more sparseness support to be
>    added to the SCSI and ATA command sets
>  - user space tools that support sparseness
>  - file system support for sparseness
>
> Perhaps the latter point should involve the file system track as
> well.

Why not just acknowledge that arrays have some sort of allocation map
and ask for a T10/SCSI command to return it.

-- 
Regards,
Richard Sharpe
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