Tuning object retrieval response time

The OAM response time for retrieval of objects might be a key factor in the performance of your application, especially if the application is intended for interactive use. The minimum time to retrieve an object from an unmounted cartridge in an optical library is typically 15 to 30 seconds. These times increase when the resource is attached to a different OAM in an OAMplex and cross-system processing is required. Understand the retrieval response time requirements for your applications and monitor the actual response time achieved.

The key to providing the required response time is to assign objects to storage classes that have an adequate INITIAL ACCESS RESPONSE SECONDS value defined and to management classes that do not cause a transition to a slower storage class until the frequency of retrieving the objects is reasonably low. The primary attributes of a management class that can be used to control transition are TIME SINCE CREATION and TIME SINCE LAST USE.

Restriction: Do not use TIME SINCE LAST USE if the UPD=N option is used on the OAM1,CBRINIT statement in IEFSSNxx PARMLIB member.

Use the OSREQ QUERY function to obtain the estimated retrieval response time for an object. The OSREQ QUERY function also provides descriptive information concerning objects residing on the following storage media:

DB2 sublevel storage on a direct access storage device (DASD)
An optical disk volume inside of an optical library
An optical disk volume on the shelf
A tape volume inside an IBM automated tape library dataserver
A shelf-resident tape volume (a tape volume that resides outside an IBM automated tape library dataserver)
File system sublevel storage, which could be a zFS file system on DASD or a NFS file system on a wide variety of internal storage within an NFS server.

OAM returns this descriptive information, along with the primary, backup, and secondary backup retrieval order keys, in the Query Element List. QUERY searches the object directory for a match on the specific name in the NAME keyword and returns a single query element. You can perform a generic search for each object whose name matches the partially qualified name specified in the NAME keyword. The search returns a query element for each object found. The output of a QUERY might be used as the input to an OSREQ RETRIEVE request.

Additionally, for library-resident objects whose retrieval can be predicted in advance, you might want to fetch the objects before they are needed for interactive use. This can be done by performing the following activities:

Using the OSREQ macro to change objects’ storage classes to storage classes having a nonzero value for INITIAL ACCESS RESPONSE SECONDS (stored on removable media—tape or optical)
Running the storage management cycle to move the objects to the proper level of the object storage hierarchy

Tip: You can use the PERIODIC attribute of the management class to prefetch the objects to DASD to improve performance on subsequent retrievals. For example, on the first day of each month, you can assign the objects to a management class that moves the objects to a faster storage class just before you use the objects. See page PERIODIC for a discussion of the attributes for this parameter.