Re: commit "xen/blkfront: use tagged queuing for barriers"

From: Jeremy Fitzhardinge
Date: Thu Aug 05 2010 - 19:12:13 EST

On 08/05/2010 10:19 AM, Christoph Hellwig wrote:
I'm pretty sure most if not all of the original Xen backends do the
same. Given that I have tried to implement tagged ordering in qemu
I know that comes down to doing exactly the same draining we already
do in the kernel, just duplicated in the virtual disk backend. That
is for a userspace implementation - for a kernel implementation only
using block devices we could in theory implement it using barriers,
but that would be even more inefficient. And last time I looked
at the in-kernel xen disk backed it didn't do that either.
blkback - the in-kernel backend - does generate barriers when it
receives one from the guest. Could you expand on why passing a
guest barrier through to the host IO stack would be bad for
performance? Isn't this exactly the same as a local writer
generating a barrier?
If you pass it on it has the same semantics, but given that you'll
usually end up having multiple guest disks on a single volume using
lvm or similar you'll end up draining even more I/O as there is one
queue for all of them. That way you can easily have one guest starve

Yes, that's unfortunate. In the normal case the IO streams would actually be independent so they wouldn't need to be serialized with respect to each other. But I don't know if that kind of partial-order dependency is possible or on the cards.

Note that we're going to get rid of the draining for common cases
anyway, but that's a separate discussion thread the "relaxed barriers"

Does that mean barriers which enforce ordering without flushing?

It's true that a number of the Xen backends end up implementing
barriers via drain for simplicity's sake, but there's no inherent
reason why they couldn't implement a more complete tagged model.
If they are in Linux/Posix userspace they can't because there are
not system calls to archive that. And then again there really is
no need to implement all this in the host anyway - the draining
is something we enforced on ourselves in Linux without good reason,
which we're trying to get rid of and no other OS ever did.

Userspace might not be relying on the kernel to do storage (it might have its own iscsi implementation or something).

Now where both old and new one are buggy is that that they don't
explicit cache flush (aka empty barrier) is silently dropped, making
fsync and co not preserve data integrity.
Ah, OK, something specific. What level ends up dropping the empty
barrier? Certainly an empty WRITE_BARRIER operation to the backend
will cause all prior writes to be durable, which should be enough.
Are you saying that there's an extra flag we should be passing to
blk_queue_ordered(), or is there some other interface we should be
implementing for explicit flushes?

Is there a good reference implementation we can use as a model?
Just read Documentation/block/barriers.txt, it's very well described
there. Even the naming of the various ORDERED constant should
give enough hints.

I've gone over it a few times. Since the blkback barriers do both ordering and flushing, it seems to me that plain _TAG is the right choice; we don't need _TAG_FLUSH or _TAG_FUA. I still don't understand what you mean about "explicit cache flush (aka empty barrier) is silently dropped". Who drops it where? Do you mean the block subsystem will drop an empty write, even if it has a barrier associated with it, but if I set PREFLUSH and POSTFLUSH/FUA then those will still come through? If so, isn't dropping a write with a barrier the problem?

It's one of the many backends written to the protocol specification,
I don't think it's fair to call it irrelevant. And as mentioned before
I'd be very surprised if the other backends all get it right. If you
send me pointers to one or two backends you considered "relevent" I'm
happy to look at them.

You can see the current state in git:// xen/dom0/backend/blkback is the actual backend part. It can either attach directly to a file/device, or go via blktap for usermode processing.

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