SCF Reference Manual for the Storage Subsystem (G06.28+, H06.05+, J06.03+)

2. Change one or more of the “Disk Attributes for the ALTER DISK Command” (page 230). For
example, this command changes the revive attributes for the disk:
-> ALTER $DATA00, REVIVERATE 10, REVIVEPRIORITY 100
3. Verify the change is entered into the system configuration database:
-> INFO $DATA00, DETAIL
4. See “Resetting a Disk” (page 103).
Configuring the Size of Disk Cache
The CACHE attribute specifies the disk cache configuration for an in-use volume.
CAUTION: Configuring the cache too small or too large can cause severe performance problems.
Considerations for ALTER DISK, CACHE and Disks
• Using the ALTER DISK, CACHE command causes all disk caches to be flushed when the cache
configuration is changed.
• A cache that is too large can degrade system performance, causing:
Excessive page faulting in the processor◦
◦ A significant number of read faults on the cache
◦ File-system errors indicating lack of memory (such as error 35 or 36)
◦ Processor halts due to lack of memory (such as halt %11501)
• Repeat this procedure as often as necessary to achieve:
Cache Read Hits: High◦
◦ Cache Read Misses: Low
◦ Cache Faults: Low
◦ Audit Forces: Low
• There can be up to eight disk processes for each volume. Each disk process in the volume
disk-process group requires a minimum configuration for each cache block size. These
minimums are determined by the disk process during initialization (that is, after a system-load
or when a volume is brought up). The minimum values are based upon a number of factors,
such as the physical memory page size and the number of disk processes. If the specified
number of blocks is below the minimum, the disk process uses the calculated minimum number.
• To avoid wasting unused space within a physical memory page, all allocations round up to
the next whole physical page boundary. For example, assuming a 16K physical page size,
if you specify 100 for the 512-byte cache, the actual value is rounded up to 128 (8 physical
pages). While the pages underlying the cache are not swappable, they are available to the
memory manager when not in use.
• The maximum tested cache allocation is 900 MB. If you allocate more cache than can be
supported by the available physical and processor memory, the disk process overrides the
requested values by setting the cache sizes to default values (.5K,18), (1K,18), (2K,18),
(4K,1024).
• On occasion, the memory needs of the rest of the system might prevent the disk process from
actually using the allocated cache.
86 Configuring Disks