[Stgt-devel] stgt backing store & SSC

[Mark Harvey]

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,
};
struct blk_header {
        uint32_t blk_type;
        uint32_t blk_size;
        uint32_t disk_blk_size;
        loff_t blk_number;
        loff_t prev_blk;
        loff_t curr_blk;
        loff_t next_blk;
};


> > In the lu_init for the ssc device, you can change the BS.  Then in
> > your READ/WRITE command handler, you can make calls into your own
> > BS that knows how to store this information.  (Look out for the
> > allocation size hack in target.c for TYPE_SPT; you may need
> > something similar.)
> >
> > In general, the current separation of backing store from device only
> > really works for block storage.  We need an OSD-only backing store
> > for OSD too (not in tree).  May be cleaner just to rid the concept
> > of backing store from everything except the device code.
> >
> >               -- Pete
> > _______________________________________________
> > Stgt-devel mailing list
> > Stgt-devel at lists.berlios.de
> > https://lists.berlios.de/mailman/listinfo/stgt-devel
> --
> Ming Zhang
>
>
> @#$%^ purging memory... (*!%
> http://blackmagic02881.wordpress.com/
> http://www.linkedin.com/in/blackmagic02881
> --------------------------------------------
>
>