[Stgt-devel] stgt backing store & SSC

[Pete Wyckoff]

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.

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