[Libguestfs] nbdkit blocksize filter, read-modify-write, and concurrency

[Richard W.M. Jones]

On Sat, May 21, 2022 at 01:21:11PM +0100, Nikolaus Rath wrote:
> Hi,
>
> How does the blocksize filter take into account writes that end-up
> overlapping due to read-modify-write cycles?
>
> Specifically, suppose there are two non-overlapping writes handled
> by two different threads, that, due to blocksize requirements,
> overlap when expanded.  I think there is a risk that one thread may
> partially undo the work of the other here.
>
> Looking at the code, it seems that writes of unaligned heads and
> tails are protected with a global lock., but writes of aligned data
> can occur concurrently.

I agree.

Assuming the underlying plugin is NBDKIT_THREAD_MODEL_PARALLEL and no
other filters impose thread model limits, the blocksize filter does
not limit the thread model, so the thread model of nbdkit would also
be NBDKIT_THREAD_MODEL_PARALLEL.

That means that two writes either on different connections or
pipelined on the same connection could happen at the same time.
“blocksize_pwrite” would be called concurrently for the two requests.

> However, does this not miss the case where there is one unaligned
> write that overlaps with an aligned one?
>
> For example, with blocksize 10, we could have:
> 
> Thread 1: receives write request for offset=0, size=10
> Thread 2: receives write request for offset=4, size=16
> Thread 1: acquires lock, reads bytes 0-4
> Thread 2: does aligned write (no locking needed), writes bytes 0-10
> Thread 1: writes bytes 0-10, overwriting data from Thread 2

I believe this analysis is correct.  (CC'd to Eric who knows a lot
more about this.)

However I don't think it's a bug.  If a client doesn't want writes to
squash each other, then it shouldn't send overlapping requests.  I bet
the same thing happens with an SSD.

NBD_CMD_FLAG_FUA is provided for clients that wish to ensure that a
write has been committed before sending another request.

Do you have an example of a client which sends overlapping requests
and depends on particular behaviour of the server?  You may be able to
get it to work by using nbdkit-noparallel-filter which can be used to
serialize nbdkit.

Rich.

-- 
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