Topic:
What is the KEEP_DISK_SPACE variable for?

Description:
When DSL space available is misreported on SGI's.

More specifically, this can manifest itself on an SGI client where the DSL's are
mounted from a Sun server.  It is caused by an incompatibility in the way Sun
and SGI create and use filesystems.

Solution:
Sun can optionally use a parameter called "minfree" when creating filesystems to
reserve a percentage of the disk space for use only by root processes under
certain circumstances. By default, Sun sets the minfree parameter to 10% when
creating filesystems.  This means that a 10-GB disk will only show 9 GB
available when doing a 'df' on a Sun because 1 GB is being reserved by
minfree=10%.

SGI does not honor minfree settings and will report the full 10 GB of disk space
available for user processes.  Problems may occur when SGI's try to write in the
minfree space.  In in effort to compensate for this incompatibility, IESX
engineers added the variable KEEP_DISK_SPACE in the IESX environment.  If set to
true, the DSL free space reporter will automatically subtract the 10% minfree
amount from the size of the filesystem reported by the OS on an SGI.  This
variable has no affect on Sun workstations.  KEEP_DISK_SPACE is not configurable
and can only be set to true or false (10% or 0%).

Three scenarios exist for free space reporting on SGIs using Sun disks:

1.  Free disk space is reported accurately.  This will be the case if the DSL
filesystem was created with minfree=10% AND KEEP_DISK_SPACE=true or with
minfree=0% AND KEEP_DISK_SPACE not set or false.

2.  Free disk space is overestimated.  This will be the case if the DSL
filesystem was created with minfree set to any number at all and KEEP_DISK_SPACE
not set.  The amount of overestimation depends on what the minfree setting was.
On a 10-GB disk with minfree=10% and KEEP_DISK_SPACE=false, SGI workstations
will report 1 GB more free space than is actually available.

3.  Free disk space is underestimated.  This will be the case if the DSL
filesystem was created with minfree less than 10% and KEEP_DISK_SPACE=true. On a
10-GB disk with minfree=0% and KEEP_DISK_SPACE=true, SGI workstations will
report 1 GB less free space than is actually available. 



Last Modified on: 07-APR-00