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Performance issue with Linux NFS clients when ClearCase VOB database is located on a NAS device



This technote provides a workaround for a problem with certain Linux NFS clients that negatively impacts IBM Rational ClearCase performance when the VOB database is located on a NAS device.


Slow performance on any VOB access. The NFS code places a synchronous read or write lock when accessing the VOB, which causes these newer kernels to have very bad performance compared to Red Hat 5.x and SUSE 10.x kernels.

This problem is known to affect RedHat 6.0 to 6.4 versions (2.6.32 kernels) and SUSE Linux Enterprise Server 11 SP2 (3.0.13 kernels), although other versions are possibly affected as well.

Review technote 1295636: Known issues with Linux VOB hosts accessing VOB storage on NAS over NFS for more information.




Linux VOB servers with VOB storage located on a NAS device.

Diagnosing The Problem

Resolving The Problem

IBM is closely working with the various vendors to resolve the issue.

IBM has developed a work around which involves moving the almd files (used for VOB database concurrency control) to local disk storage on the VOB server.

IMPORTANT: This workaround requires the patch added to Rational ClearCase Fix Pack 7 ( for 8.0, that implements remote almd file storage when the almd_path environment variable is specified.

The workaround was also added to ClearCase and back ported to ClearCase

  1. Log in to the VOB server as VOB owner

  2. Create a directory on your local disk and ensure that the VOB owner has full access.
    This directory will be used to store all the almd files for VOBs hosted on this server. Make sure that users are able to write to the directory - with chmod to set the sticky bit and permissions of the directory.

    For example:

    % mkdir /var/clearcase/almd_stg    

  3. Stop ClearCase on the VOB server as root

    # /opt/rational/clearcase/etc/clearcase stop

  4. Append (or create) the /var/adm/rational/clearcase/config/db.conf file, specifying the -almd_path <path-to-dir> option.

    For example:

    % echo "-almd_path /var/clearcase/almd_stg" > /var/adm/rational/clearcase/config/db.conf

  5. Start ClearCase on the VOB server

    % /opt/rational/clearcase/etc/clearcase start

Next time you access a VOB, you will notice two new files in the directory you specified for almd_path.


NOTE: If you need to do an unpgrade. make sure you save the db.conf file so that you retain the almd_stg setting.

NOTE: fix_prot will not change the permissions of the almd files in the local directory. You will need to use chmod and chown to change permissions and ownership.

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Document Information

Modified date:
16 June 2018