[stgt] [PATCH] sg-based backing store

[FUJITA Tomonori]

On Mon, 06 Oct 2008 16:25:23 +0200
Doron Shoham <dorons at voltaire.com> wrote:

> Alexander Nezhinsky wrote:
> > Backing store based on SCSI Generic driver (sg), v.3.
> > allows to route scsi commands to the native scsi devices
> > unchanged. It uses Direct I/O (dio) to avoid memcopy
> > to the buffer cache.
> > 
> > Using sg allows asynchronous execution, commands are 
> > written to a sg-backed char device, completions are
> > read from the same device, which can be polled 
> > asynchronously.
> > 
> > This bs provides significant performance improvement when 
> > working with native scsi devices. In a setup, where 
> > the scsi devices are exported by a tgt with bs_null,
> > and both links (from initiator to target and from the 
> > target to the "backing-store" target) are iSER/IB
> > sustained bandwidth of 1450 MB/s for READ and
> > 1350 MB/s for WRITE is achieved. This to be compared to
> > 700-800 MB/s when running with bs_rdwr in the same setup.
> > Some improvements are seen with IOPS as well:
> > 60 kIOPS for READ, 38 kIOPS for WRITE
> > (compared to 31/35KIOPS with bs_rdwr).
> > 
> 
> Hi,
> 
> I applied this patch and it works, but I noticed that the serial number
> reported by the device (using sg_inq) returned 'beaf11' instead of the real physical serial number.
> 
> Does it mean that the inquiry command is not transferred to the device and handled internally?
Yeah, this just uses sg for I/Os. You are talking about passthrough.
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