Cache Parameters - IBM System Storage DS3500 Introduction And Implementation Manual

Draft Document for Review March 28, 2011 12:24 pm
scan completes within the time setting allocated. After the media scan has completed, it will
start over again and reset its time for completion to the current setting. This media scan
setting can be reduced, however if the setting is too low, priority will be given to media scan
over host activity to ensure that the scan completes in the allocated time. This scan can
impact on performance, but improve data integrity.
Media scan must be enabled for the entire storage subsystem. The system wide enabling
specifies the duration over which the media scan will run. The logical drive enabling specifies
whether or not to do a redundancy check as well as media scan.
A media scan can be considered a surface scan of the hard drives, whereas a redundancy
check scans the blocks of a RAID 3, 5, or 6 logical drive and compares it against the
redundancy data. In the case of a RAID 1 logical drive, then the redundancy scan compares
blocks between copies on mirrored drives.
We have seen no effect on I/O with a 30 day setting unless the processor is utilized in excess
of 95%. The length of time that it will take to scan the LUNs depends on the capacity of all the
LUNs on the system and the utilization of the controller.
Note: If you change the media scan duration setting, the changes will not take effect until
the current media scan cycle completes or the controller is reset.

3.3.10 Cache parameters

Cache memory is an area of temporary volatile storage (RAM) on the controller that has a
faster access time than the drive media. This cache memory is shared for read and write
operations.
Efficient use of the RAID controller cache is essential for good performance of the DS3500.
See 9.5.2, "Change Cache Settings" on page 247 for detailed explanation about the cache
parameter settings.
Cache blocksize selection
On the DS3500, the cache blocksize is a variable value that can be set to 4 K, 8 K, or 16 K.
The general default setting is 8 K. The main goals with setting this value are to minimize the
number of cache IO's needed, and at the same time, not waste space. This value is a storage
subsystem wide parameter, and when set, it is the value to be used by all cache operations.
For example, if the I/O of greatest interest is taken from your database operations during the
day rather than from your weekly backups, you want to tune this value to handle the high
transactions best. Knowing that the higher transactions will have smaller I/O sizes, using the
4 K settings is generally best for transaction intense environments.
Best practice: Set the cache blocksize to 4 K for the DS3500, normally for transaction
intense environments.
In a throughput intense environment, as we discussed earlier, you want to get as much data
into cache as possible. In this environment it is generally best to use the 16 K blocksize for the
cache.
Best practice: Set the cache blocksize to 16 K for the DS3500 system, normally for
throughput intense environments.
Chapter 3. IBM System Storage DS3500 Storage System planning tasks
7914DS3KPlanning_090710.fm
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