Slow I/O under load with RHEL 5.7 compared to RHEL 4 and RHEL 6

Solution Unverified - Updated -


  • A significant slowdown in I/O in RHEL5 (all releases, including RHEL5.9) when the machine is under load.
  • The exact same workload on RHEL4 or RHEL6 does not show this problem.
  • The slowdown occurs both writing to NFS or to a local disk.
  • Happens on a freshly rebooted machine as well (so not because of the pagecache contents)
  • Are there any kernel tunables?


  • Red Hat Enterprise Linux 5 -all release streams up to and including RHEL5.9.z (2.6.18-348.15.1.el5 released July 22 2013) and RHEL5.10GA-beta. Later versions or RHEL5 may be affected too, at the time of writing no fix has been committed.
  • RHEL4 and RHEL6 and later are specifically NOT affected.
  • we're waiting on details of the customer benchmark/workload.

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