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
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