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+++ This bug was initially created as a clone of Bug #820019 +++

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.

This can decrease the maximum IO size on upgrade.  A few upstream users have reported problems when the upgrade without remounting clients: in that case, clients will continue trying to use the old (larger) IO size but the server will drop those (now too-large) requests.

We could do something more sophisticated.

For now let's leave the rhel6 behavior alone.  We can reconsider if it turns out to be a problem for someone.