[Gluster-devel] regarding GF_CONTENT_KEY and dht2 - perf with small files

[Jeff Darcy]

>       Background: Quick-read + open-behind xlators are developed to help
> in small file workload reads like apache webserver, tar etc to get the
> data of the file in lookup FOP itself. What happens is, when a lookup
> FOP is executed, GF_CONTENT_KEY is added in xdata with max-length and
> posix xlator reads the file and fills the data in xdata response if this
> key is present as long as the file-size is less than max-length given in
> the xdata. So when we do a tar of something like a kernel tree with
> small files, if we look at profile of the bricks all we see are lookups.
> OPEN + READ fops will not be sent at all over the network.
> 
>       With dht2 because data is present on a different cluster. We can't
> get the data in lookup. Shyam was telling me that opens are also sent to
> metadata cluster. That will make perf in this usecase back to where it
> was before introducing these two features i.e. 1/3 of current perf
> (Lookup vs lookup+open+read)

Is "1/3 of current perf" based on actual measurements?  My understanding
was that the translators in question exist to send requests *in parallel*
with the original lookup stream.  That means it might be 3x the messages,
but it will only be 1/3 the performance if the network is saturated.
Also, the lookup is not guaranteed to be only one message.  It might be
as many as N (the number of bricks), so by the reasoning above the
performance would only drop to N/N+2.  I think the real situation is a
bit more complicated - and less dire - than you suggest.

> I suggest that we send some fop at the
> time of open to data cluster and change quick-read to cache this data on
> open (if not already) then we can reduce the perf hit to 1/2 of current
> perf, i.e. lookup+open.

At first glance, it seems pretty simple to do something like this, and
pretty obvious that we should.  The tricky question is: where should we
send that other op, before lookup has told us where the partition
containing that file is?  If there's some reasonable guess we can make,
the sending an open+read in parallel with the lookup will be helpful.
If not, then it will probably be a waste of time and network resources.
Shyam, is enough of this information being cached *on the clients* to
make this effective?