Re: scsi LLD implementation question

[James Bottomley <James.Bottomley@suse.de>]

On Fri, 2011-02-25 at 11:19 -0800, va stg2010 wrote:
> Thanks !
> There is a 32-bit tag  to refer back  the queue(for pending commands)
> when responses are posted back by HBA.   Suppose the HBA is  receiving
> commands from multiple initiators because the host has virtualized
> initiator environment, now in this case  one single large queue is
> good enough or there should be pending-commands-queue per virtual
> initiator or pending-command-queue per disks.

It doesn't have to be virtualised; this is the normal situation on a
SAN.  For a pending queue, the question is merely how to do efficient
lookup.  Some drivers use a fixed offset list (with the offset in the
completion tag) ... I suppose a radix tree would be a good way to go for
particularly large pending queues.

> Just want to minimize the  locking contentions  and  maximize
> parallelism in  SCSI  I/O to multiple disks from multiple virtual
> initiators via my HBA.

To be honest, fixes offset lists are best then ... they can operated
locklessly.

James