RHEL 4U6 x86_64 freezed by heavy I/O on PE2950

[roderick.castillo at metanomics.de]

Please see
http://www.ibm.com/developerworks/linux/linux390/perf/tuning_how_fixed_io_buffers.html
Could it be that following question is related to the issue?:
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Enable the fixed I/O buffer feature in RHEL4:

    Add the option 'fixedbuffers' in the file /etc/modprobe.conf.

    Example:

        options dasd_mod dasd=<list of all DASD devices>,fixedbuffers 

    To activate the usage of fixed buffers you need to rebuild the initrd 
using the tool mkinitrd.
    To update the initrd for the current running kernel:

       1. cd /boot
       2. save the old initrd
       3. mkinitrd -f initrd-$(uname -r).img $(uname -r)
       4. ensure that the name of the new initrd matches the entry in 
/etc/zipl.conf

    You need to run zipl (zipl -V is more informative) and reboot your 
system.
 
Note:

    When the new feature is enabled it applies to all ECKD and FBA disks, 
but not to disks that are accessed using the DIAG discipline. You can 
specify the new 'fixedbuffers' keyword on the kernel parameter line when 
the DASD driver is build into the kernel, or on the command line, when the 
dasd_mod module is loaded.
How to verify the activation:
    To check whether the parameter was recognized correctly you can have a 
look at the boot messages in the message log. When the fixed I/O buffers 
are enabled successfully you find the following line in the log:

        dasd_devmap: turning on fixed buffer mode 

    A second way to check that the fixed I/O buffers are enabled is to 
view the file /proc/slabinfo. If the feature is enabled successfully the 
slabinfo file contains information for a slab cache called 
'dasd_page_cache'. 
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Thanks
Rick