NFS tasks stall or hang for extended periods of time

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Environment

SUSE Linux Enterprise Server 11 Service Pack 4 (SLES 11 SP4)

SUSE Linux Enterprise Server 11 Service Pack 3 (SLES 11 SP3)

SUSE Linux Enterprise Server 12 (SLES 12)

SUSE Linux Enterprise Server 12 Service Pack 1 (SLES 12 SP1)

Situation

While a variety of NFS work is going on at an individual NFS client machine, that client machine may experience extended periods of time when some of the NFS tasks appear to be making no progress, while others continue. This has been seen in (but may not be limited to) cases where large reads are going on (and making progress), but other smaller activities are attempted (small writes, directory lists, etc).

This may be seen on:

SLES 11 SP3 with kernel 3.0.101-0.47.50 or higher

SLES 11 SP4

SLES 12

SLES 12 SP1

Resolution

Update the kernel as outlined below:

For SLES 12 SP1: Update to kernel 3.12.57-60.35.1 or higher.

For SLES 12: Update to kernel 3.12.55-52.42.1 or higher.

For SLES 11 SP4: Update to kernel 3.0.101-71.1 or higher.

For SLES 11 SP3 with LTSS contract: Update to LTSS kernel 3.0.101-0.47.79 or higher.

For SLES 11 SP3 without an LTSS contract: SP3 is out of maintenance. For a supported solution, it is necessary to have either a SP3 Long Term ServicePack Support (LTSS) contract, or update to SP4. However, by chance (not by policy) there is a PTF for x86_64 platforms running SP3, available at:

https://ptf.suse.com/f2cf38b50ed714a8409693060195b235/sles11-sp3/10157/x86_64/20160331/

Cause

A change which came from the upstream linux community (commit c05eecf636101dd4347b2d8fa457626bf0088e0a) altered the way which sunrpc schedules high and low priority tasks. The intention was to favor strong ordering requirements. However, the unintended result was that low priority tasks might wait an unreasonable period of time if a large amount of high priority tasks are present.

A better balance needed to be struck, to treat low priority tasks fairly, so they can still get reasonably prompt attention.

Disclaimer

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