Add zap_prefetch() interface

[behlendorf]

Provide a generic interface to prefetch ZAP entries by name. This
function is based of the existing zap_prefetch_uint64() version which
is used by the deduplication code.

Signed-off-by: Brian Behlendorf behlendorf1@llnl.gov

[behlendorf]

@prakashsurya Could you please review this. I'd like to merge it to get the ground work in place for potentially improving Lustre's prefetch.

[prakashsurya]

My only gripe is adding a function without any internal consumers of it, but other than that, LGTM.

Why use MT_EXACT? I'm not too familiar with those flags meaning or usage (apart from looking at zap.h), so I'm curious why that over the others.

[behlendorf]

I agree, having an internal consumer would be best. I'm sure there are places where we could call it as a micro-optimization, but it might be difficult to quantify if it actually helps. One place where might help significantly is in zfs_readdir() when you're iterating over a fat zap.

As for MT_EXACT it's the only valid flag which zap_name_alloc() accepts when zp->zap_normflags isn't set. This flags are to handle how a lookup should be normalized.

[behlendorf]

I'm bumping this to a future release until we have a known consumer.

[behlendorf]

Closing this issue until we have a known consumer. The patch will remain available if we decide it's needed.