Nate Lawson wrote:
Scott Long wrote:
Eric Anderson wrote:
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
The way the target mode stack works in FreeBSD is that the kernel
provides some of the basic services, but the actual target emulator
is meant to live in userland. The userland program responds to
events from the kernel via the select interface. This generally
works pretty well. However, it does mean that control has to
cross the kernel-userland boundary at least once for every event.
What I'd suggest doing is prototyping your target emulator in userland
and evaluating the performance there, and then moving it to the kernel
if you _really_ need more performance.
Agree 100%. While having it in usermode means there are boundary
crossings that increase per-transaction latency, the actual bulk data
transfer is via zero-copy IO and you should be able to exceed the data
transfer rates of several 10K RPM drives on decent hardware.
Ok, great.. Now, will scsi_target work ok with raw devices, or only
files? (although I'm not sure theres all that much difference really).
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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