Re: [Libguestfs] [PATCH nbdkit 2/9] cache: Add cache-on-read mode.

On 12/28/18 12:45 PM, Richard W.M. Jones wrote: > The same as qemu's copyonread flag, this caches read requests. > --- > filters/cache/nbdkit-cache-filter.pod | 11 +++++ > filters/cache/cache.c | 37 +++++++++++++-- > tests/Makefile.am | 4 +- > tests/test-cache-on-read.sh | 66 +++++++++++++++++++++++++++ > 4 files changed, 114 insertions(+), 4 deletions(-) Makes sense. > +=item B<cache-on-read=true> > + > +Cache read requests as well as write requests. Any time a block is > +read from the plugin, it is saved in the cache (if there is sufficient > +space) so the same data can be served more quickly later. Is it worth mentioning that this is fine for a client that is expected to be the only writing client of a given server; but that it can result in stale data if the server can modify the data via other means?

(In particular, since we don't implement NBD_FLAG_CAN_MULTI_CONN, we already admit that two clients trying to write in parallel are not guaranteed to see consistent read results after a flush - while caching read data only makes that more apparent)

> +++ b/tests/test-cache-on-read.sh What you have here looks okay, but I have a possible idea for an additional test: use the delay filter to prove that repeated reads of a region of the disk only suffer a one-time read penalty, rather than a penalty per read.