Performance loss when writing large files over NFS, on kernel 4.12.14

This document (000019615) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 15 SP1
SUSE Linux Enterprise Server 12 SP5
SUSE Linux Enterprise Server 12 SP4

Situation

While writing a large file to an NFS mount point, if the OS's "dirty_background_ratio" or "dirty_background_bytes" is reached, write performance drops off excessively.

This is observed on kernel 4.12.14, which is present in SLES 12 SP4 / SP5 and SLES 15 SP1.

Resolution

Update the kernel with public maintenance. The list below shows some of the kernel flavors and versions where the fix was first released.

For SLES 15 SP1:
kernel-default-4.12.14-197.40.1
kernel-azure-4.12.14-8.30.1

For SLES 12 SP4:
kernel-default-4.12.14-95.51.1
kernel-azure-4.12.14-6.40.1
kernel-rt-4.12.14-8.18.1

For SLES 12 SP5:
kernel-default-4.12.14-122.20.1
kernel-azure-4.12.14-16.13.1

If your SUSE kernel flavor is not on this list, the following information may be helpful:

Generally speaking, SUSE Linux Enterprise support packs which were under general maintenance received this fix in a kernel release in late April 2020.

To check your currently installed kernel for this fix, for example, if you use "kernel-default":

rpm -q --changelog kernel-default | grep 1163403

If this returns results, the fix if present.

Cause

When the amount of data waiting to be written (the write cache, or dirty cache) reaches the dirty background threshhold, additional threads are spawned to help clear out data from the dirty cache. This was causing too many calls for small and sometimes redundant writes and commits to be performed. The code has been modified to insure that a suitable number of dirty pages are collected before writing them to NFS.

Disclaimer

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