Linux DB2 9.5 and above performance degradation with VxFS CIO enabled
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Article ID: 100026741
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Updated On:
Description
Error Message
Degraded DB2 performance when CIO is enabled and DB2 9.5 and greater.
Cause
The Linux memory map semaphore (mmap_sem) controls changes to an process memory mappings. The semaphore can be held by any number of shared readers or one exclusive writer. When a process has multiple threads, there can be contention on the process global mmap_sem. This behavior is present in DB2 9.5 and above when VxFS Concurrent I/O (CIO) is enabled. When an application is concurrent aware, there is not need to synchronize access to the mmap_sem semaphore.
Resolution
The fix was to introduce a new mount option "nommapcio". This will reject/fail all mmap calls on files opened with CIO ( through the advisory and not the cio mount option ). This will ensure files opened in this manner will no longer need to use the mmap_sem semaphore.
AVAILABLE PATCHES CONTAINING THE FIX
VxFS 5.1SP1RP1HF5 and later
-Install patch per instructions in the README
- Reboot the system to load new kernel drivers
For VxFS 6.0 and greate,r the mount option = nommapcio is available as part of the package.
Please note that any file systems used by DB2 must be mounted with the 'nommapcio' mount option, i.e.:
# mount -t vxfs -o nommapcio
Applies To
Linux and DB2 9.5+
Issue/Introduction
DB2 performance degradation on Linux with VxFS CIO enabled due to lock contention on mmap_sem.
Additional Information
ETrack: 2314212
ETrack: 2293722
JIRA: null
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