Hello,
It looks to me that you have issue with the RAM, too much st and the disk queue cannot be drained quickly enough. It is not clear to me what is the status of the request when you get all the status back.
Do you see any ep_tmp_oom_errors ( temp OOM per sec.) in cbstats or the admin console (at the bucket level) ?
Have you tried with the latest release Couchbase 2.2 to configure more writers and readers?
http://docs.couchbase.com/couchbase-manual-2.2/#using-multi--readers-and-writers
This could probably help.
regards
Tug
@tgrall