[Stgt-devel] Data deduplication, stgt backing store & SSC

Fri Aug 3 05:07:44 CEST 2007

On Fri, 2007-08-03 at 12:44 +1000, Mark Harvey wrote:
> Re: deduplication
> 
> Just so we are both on the same page...
> 
> My understanding of the term deduplication is where the is some sort
> of finger print for each block of data. The block of data is only
> stored if its finger print is unique. Non-unique finger prints blocks
> are then 'referenced' back to the original finger-print block of data.

yes. exactly. just different solution has different pros and cons. fix
block, variable block, algorithm, ...

i do not expect a easy OSS de-dup can have here but at least our
infrasture should enable this possibility.

> 
> i.e. Very similar to Single Instance Store but at a block layer.
> Single Instance Store (SIS) is more used at an application layer (e.g.
> MS Exchange does SIS on each email)

yes

> 
> May be adding some sort of field (fit md5sum ??) in the header so
> indexing virtual media will be quicker ?
> 
> 64bytes for each block of data seems a little extravagant at this point in time.
> Although unlike disk, tape blocks are typically 64k, 128k or larger.

> 
> Regards
> Mark
> 
> On 8/3/07, Ming Zhang <blackmagic02881 at gmail.com> wrote:
> > On Fri, 2007-08-03 at 11:07 +1000, Mark Harvey wrote:
> > > On 8/3/07, Ming Zhang <blackmagic02881 at gmail.com> wrote:
> > > > On Fri, 2007-08-03 at 10:53 +1000, Mark Harvey wrote:
> > > > > On 8/3/07, Ming Zhang <blackmagic02881 at gmail.com> wrote:
> > > > > > On Thu, 2007-08-02 at 14:58 -0400, Pete Wyckoff wrote:
> > > > > > > markh794 at gmail.com wrote on Tue, 31 Jul 2007 19:57 +1000:
> > > > > > > > For Variable block SSC device, the block size written needs to be
> > > > > > > > tracked.
> > > > > > > [..]
> > > > > > > > My current thoughts of a solution:
> > > > > > > > ========================
> > > > > > > > A block header describes each block written -> Analogy to the 'tar'
> > > > > > > > format where a header is written, followed by the 'data' followed by
> > > > > > > > another header, followed by more data...repeat...until blank header...
> > > > > > > [..]
> > > > > > > > However the current implementation for iSCSI -> bs_sync uses a
> > > > > > > > pread64()/pwrite64() and writes data based on information stored in
> > > > > > > > scsi_cmd -
> > > > > > > >  pwrite64(fd, cmd->uaddr, cmd->len, cmd->offset)
> > > > > > > >  pread64(fd, cmd->uaddr, cmd->len, cmd->offset)
> > > > > > > >
> > > > > > > >
> > > > > > > > Would it be OK to add a 'blk_header' structure to struct scsi_cmd and if
> > > > > > > > blk_header is set, write this blk header as well ?
> > > > > > > > I will attempt to put the above idea into code and submit for comment...
> > > > > > >
> > > > > > > I think rather than trying to add stuff to existing backing stores
> > > > > > > that you should consider writing your own.  You need to store both
> > > > > > > "metadata" (block descriptors) and data, and none of the BSes are
> > > > > > > set up for that.
> > > > > > >
> > > > > > > The complexity of trying to glue in the blk_header so that all BSes
> > > > > > > know how to tack that on top seems big.  Then you have to tell them
> > > > > > > to read the header, and consider fields in that when determining how
> > > > > > > much further data to read.  It gets messy fast.
> > > > > >
> > > > > > also considering tape can have compression and encryption, then each
> > > > > > block is variable size even in fixed size mode. so support variable size
> > > > > > is a must eventually.
> > > > >
> > > > > Yep. The SSC metadata (header for each tape block) contains a
> > > > > structure with a 'block type' , size of original data & size of data
> > > > > stored.
> > > > >
> > > > > All accounted for. Just no code for compression or encryption - as yet..
> > > > >
> > > > > enum {
> > > > >         BLK_NOOP,
> > > > >         BLK_UNCOMPRESS_DATA,
> > > > >         BLK_COMPRESSED_DATA,
> > > > >         BLK_ENCRYPTED_DATA,
> > > > >         BLK_FILEMARK,
> > > > >         BLK_BOT,
> > > > >         BLK_EOD,
> > > > > };
> > > >
> > > > are these bits? can i have multiple bits set like compressed before
> > > > encryption? also leave room for data deduplication for example storing a
> > > > token. ;)
> > > >
> > >
> > > Arr.. no.
> > >
> > > Good catch.
> > >
> > > I'll re-do it so they are bits.
> >
> > thx. bit might now be good enough as well. ;)
> >
> > >
> > > Re: Data deduplication
> > > I was thinking a separate 'cache' file will be required and 'hashed'
> > > some way for this.
> > >
> > > I can not really see any advantage of storing a fixed deduplication
> > > method in the SSC block header.
> >
> > dedup is just another compression method. even with compression u can
> > have different algorithms and with encryption u have different
> > algorithms or ways or all these just kinds of data transformation.
> >
> > just need a type to mark what kind of data transformation it uses and
> > each transform provide a pluggable way to fetch/store data.
> >
> > for example like de-dup. u can store a token here and then when need to
> > read this data, u check the header to see if overread or under read
> > because of the block size issue. (forgot how SSC call these...:-()
> >
> > then with token u can contact another data service to find data and
> > return.
> >
> > so once u define the infrastructure, these can be added later...
> >
> >
> > >
> > > I have not thought this thru... So any input on this will be welcome.
> > >
> > > Cheers
> > > Mark
> > >
> > > >
> > > > --
> > > > Ming Zhang
> > > >
> > > >
> > > > @#$%^ purging memory... (*!%
> > > > http://blackmagic02881.wordpress.com/
> > > > http://www.linkedin.com/in/blackmagic02881
> > > > --------------------------------------------
> > > >
> > > >
> > --
> > Ming Zhang
> >
> >
> > @#$%^ purging memory... (*!%
> > http://blackmagic02881.wordpress.com/
> > http://www.linkedin.com/in/blackmagic02881
> > --------------------------------------------
> >
> >
-- 
Ming Zhang

@#$%^ purging memory... (*!%
http://blackmagic02881.wordpress.com/
http://www.linkedin.com/in/blackmagic02881
--------------------------------------------