[Stgt-devel] stgt backing store & SSC

[Mark Harvey]

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.

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.

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
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>
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