8:01 p.m.
On 01/17/2018 04:38 PM, Eric Blake wrote:
On 01/17/2018 02:53 PM, Richard W.M. Jones wrote:
> Previously the file plugin supported ‘rdelay’ and ‘wdelay’ parameters
> for injecting delays (for testing) into read and write requests. This
> moves the functionality to a new delay filter so that it can be used
> with any plugin.
> ---
> +/* Write data. */
> +static int
> +delay_pwrite (struct nbdkit_next *next, void *nxdata,
> + void *handle,
> + const void *buf, uint32_t count, uint64_t offset)
> +{
> + write_delay ();
> + return next->pwrite (nxdata, buf, count, offset);
> +}
> +
> +/* Zero data. */
> +static int
> +delay_zero (struct nbdkit_next *next, void *nxdata,
> + void *handle, uint32_t count, uint64_t offset, int may_trim)
> +{
> + write_delay ();
> + return next->zero (nxdata, count, offset, may_trim);
If next->zero() fails with EOPNOTSUPP, that means we will delay once in
trying the underlying command, and again for each iteration of the
fallback loop as it calls delay_pwrite(). Is that okay, or do we want
to reproduce some fallback logic here and directly call next->pwrite on
EOPNOTSUPP so as to only have a single write delay in that case?
Actually, thinking about this more:
When I added zero support, I documented in commit ac3f294a that for the
file plugin, wdelay is indeed doubled on systems lacking efficient zero
support. But the fallback for handling EOPNOTSUPP is currently done at
the plugins.c level (ie. it is part of next->zero()) - so a filter
should never see a plugin returning EOPNOTSUPP. Or put another way,
your movement of wdelay from being part of the file plugin to now being
a separate filter actually FIXES the double delay. On the other hand, a
filter can itself return EOPNOTSUPP (rather than calling next->zero), in
which case we still need to support the fallback to .pwrite higher up
in the call stack
Along the same lines, with my FUA patches, if we keep the fallback of
calling .flush in connections.c, then the flush will pass through the
entire filter chain; but I had proposed moving the fallback flush into
plugins.c, at which point the fallback flush is done at the layer that
lacks FUA support. And maybe we want a flush delay to mirror the
existing rdelay and wdelay (simulating a long flush time can indeed be
useful in tests); where it definitely matters whether the flush delay is
triggered as part of FUA fallback, or only triggered on an actual
NBD_CMD_FLUSH from the client.
I'm also thinking ahead to expansions - for example, there are proposals
to add resize and block status commands to NBD. In your implementation,
if a filter does not define .can_flush or .flush, then its .can_flush
implementation merely returns whatever the underlying backend's version
would report. But as we add new callbacks, such as a new .can_resize,
we need to make sure we have sane defaults. For example, the offset
filter should probably not allow resizes (even if the underlying plugin
does). If the offset filter compiled in a version of nbdkit that lacks
the .can_resize hook is run by nbdkit that has added the hook, we
therefore want to default .can_resize to false as part of loading the
smaller filter struct into memory (and NOT default to falling through to
the plugin's .can_resize); a default based on fall-through should only
happen if the filter had a size large enough to admit that the user
explicitly omitted the callback in the filter, rather than it being NULL
because of version mismatch.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
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