Linus Torvalds wrote:
I'd assumed the move was primarily because of the difficulty of getting
correct semantics on a shared filesystem
.. not even shared. It was hard to get correct semantics full stop.
Which is a traditional problem. The thing is, the kernel always has some
internal state, and it's hard to expose all the semantics that the kernel
knows about to user space.
So no, performance is not the only reason to move to kernel space. It can
easily be things like needing direct access to internal data queues (for a
iSCSI target, this could be things like barriers or just tagged commands -
yes, you can probably emulate things like that without access to the
actual IO queues, but are you sure the semantics will be entirely right?
The kernel/userland boundary is not just a performance boundary, it's an
abstraction boundary too, and these kinds of protocols tend to break
abstractions. NFS broke it by having "file handles" (which is not
something that really exists in user space, and is almost impossible to
emulate correctly), and I bet the same thing happens when emulating a SCSI
target in user space.
Yes, there is something like that for SCSI target as well. It's a "local
initiator" or "local nexus", see
http://thread.gmane.org/gmane.linux.scsi/31288 and
http://news.gmane.org/find-root.php?message_id=%3c463F36AC.3010207%40vlnb.net%3e
for more info about that.
In fact, existence of local nexus is one more point why SCST is better,
than STGT, because for STGT it's pretty hard to support it (all locally
generated commands would have to be passed through its daemon, which
would be a total disaster for performance), while for SCST it can be
done relatively simply.
Vlad
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