scsi-target and the buffer cache

[Eric Anderson]

Nate Lawson wrote:

> 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.
>

I guess I would be using the user mode tool, unless there's another 
way?  Your comment on performance also makes me a little worried about 
that now - do you think I would see a large performance hit? 

Thanks!
Eric


-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.
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