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Depending on a workload for a DASD (ECKD), you can modify the
subsystem operating mode of the hosting ESS for a particular
DASD.
- Novell/SUSE SLES8
- The Linux DASD driver provides an API (ioctl interface), that allows
an adjustment of the ESS subsystem operating modes.
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- Novell/SUSE SLES9
- A new Linux command
tunedasd is introduced, that simplifies to
set the subsystem operating modes.
Changing the ESS subsystem operating mode, i.e. changing the
cache management algorithm for a DASD only makes sense, if there is
lot of I/O traffic to that particular DASD. Otherwise it is not
recommended to change the mode, except the default mode is still
set to sequential (see below).
The ESS allows the following subsystem operating modes:
- Normal Cache Replacement
- Bypass Cache
- Inhibit Cache Loading
- Sequential Access
- Sequential Prestage
- Record Access
- Normal Cache Replacement
- This is the recommended mode. The cache is managed using the
ESS standard algorithms. This is the default value for FCP/SCSI
and the default value for ECKD DASD in Novell/SUSE SLES9 and Novell/SUSE
SLES8, starting with Service Pack 3 + Security update for Linux
kernel, Release 20040112.
- Sequential Access/Prestage
- Default value (Access) for Linux ECKD DASD drives in Novell/SUSE
SLES7 and earlier versions of SLES8. Results in poor performance
when random I/O is done.
- Inhibit Cache Loading/Record Access
- Special modes, that should be used carefully. Performance is
good for random I/O.
- Bypass Cache
- Special mode, that should be used carefully. Performance is
moderate for random I/O.
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