scsi-target and the buffer cache

[Nate Lawson]

Eric Anderson wrote:
> I'm curious about whether a target mode device would use the buffer 
> cache or not.  Here's a scenario:
> 
> Host A: has fibre channel host adapter, in target mode, large memory 
> pool, and another fiber channel host adapter connecting to fibre channel 
> block device.
> Host B: Fibre channel host adapter, connecting to Host A.  'sees' the 
> target mode block device created by Host A.
> 
> Will Host A use the buffer cache to cache blocks between the real block 
> device, and the shared target mode device?
> What about if Host A put a filesystem on the block device, created a 
> single file the size of the filesystem, and shared that filesystem via a 
> target mode device to Host B?
> What I'm wanting is a box (FreeBSD?) that can be placed between a fibre 
> channel block device (like a RAID array), and a fibre channel host using 
> that block device, and act as a block cache for that device, using the 
> FreeBSD's memory.  If it had a significant amount of memory, this could 
> be very useful.

If you use the example scsi_target usermode 
(usr/share/examples/scsi_target), then the buffer cache will be used 
since its reads/writes are from usermode like normal.  If you don't want 
that behavior, you can set O_DIRECT in the open() call of the backing 
store file.

If you chose to modify the kernel side, you'd have to make sure your 
accesses were through the VOP layer and then it would be cached.

You should check to be sure the target mode performance meets your 
expectations also.

-- 
Nate