When we extended nfsd's maximum IO size to 1MB, the servers configured with lots of nfsd threads started failing to create them all, because more memory was then required per thread. For the short term we decided to document the change and allow people to manually adjust the maximum IO size. However as noticed at https://bugzilla.redhat.com/show_bug.cgi?id=765751#c21 we'd get a more reasonable defaults on 32-bit servers if the heuristic used to choose a default maximum IO size was based on the amount of low memory available instead of on the total amount of RAM available. Patches doing this have since gone upstream: 508f92275624fc755104b17945bdc822936f1918 "nfsd: fix default iosize calculation on 32bit" and 87b0fc7deb5feccf93b022f6a976e8441152dbb2 "nfsd: cleanup setting of default max_block_size".
This request was evaluated by Red Hat Product Management for inclusion in a Red Hat Enterprise Linux release. Product Management has requested further review of this request by Red Hat Engineering, for potential inclusion in a Red Hat Enterprise Linux release for currently deployed products. This request is not yet committed for inclusion in a release.
We've seen a problem with these patches upstream: when a server is upgraded, the wsize may decrease. Clients don't renegotiate new wsize after a reboot. Therefore they continue trying to send larger writes, which may fail. The problem may be worked around by either unmounting all clients before the server upgrade or remounting any affected clients after the upgrade. For now, I think this isn't worth applying unless we think of a way to avoid the problem.
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