Questions and answers
The following questions and answers describe ASM functionality:
- Q:
- Does ASM use I/O load balancing?
- A:
- Yes, ASM does its own I/O load balancing.
When selecting a local page data set to fulfill a write request, ASM attempts to avoid overloading page data sets. ASM also attempts to favor those devices or channel paths that are providing the best service. If SCM demonstrates a performance advantage, then SCM is selected over any page data set.
- Q:
- How does the auxiliary storage shortage prevention algorithm in SRM prevent shortages?
- A:
- It does so by swapping out address spaces that are accumulating paging space at a rapid rate. Page space is not immediately freed, but another job or TSO/E session (still executing) will eventually complete and free page space. SRM also prevents the creation of new address spaces and informs the operator of the shortage so that he can optionally cancel a job.
- Q:
- Is running out of auxiliary storage (paging space) catastrophic?
- A:
- No, not necessarily; it might be possible to add more page data sets with the PAGEADD operator command, optionally specifying the NONVIO system parameter. It may be necessary to reIPL to specify an additional pre-formatted and cataloged page data set. (See the description of the PAGE parameter of the IEASYSxx member in z/OS MVS Initialization and Tuning Reference.)
- Q:
- Can we dynamically allocate more paging space?
- A:
- Yes. Additional paging space may be added with the PAGEADD operator
command if the PAGTOTL parameter allowed for expansion (see the description
of the PAGTOTL parameter of the IEASYSxx member in z/OS MVS Initialization and Tuning Reference
and
the PAGEADD command in z/OS MVS System Commands).
If you are using storage-class memory (SCM), you can dynamically allocate additional paging space to SCM using the CONFIG ONLINE command.
- Q:
- Can we remove paging space from system use?
- A:
- Yes. Use the PAGEDEL command for local page data sets.
- Q:
- How does ASM select slots?
- A:
- ASM selects slots when writing out pages to page data sets based
on whether the write request is an individual request or a group request.
For an individual write request, such as a request to write stolen
pages (those pages changed since they were last read from the page
data set), ASM selects any available slots. For a group write request,
such as a request that results from a VIO move-out of groups of pages
to page data sets, ASM attempts to select available slots that are
contiguous. ASM also attempts to avoid scattering requests across
large page data sets.
If you are using storage-class memory (SCM), ASM pages to contiguous blocks of SCM, if available.
- Q:
- How does ASM select a local page data set for a page-out?
- A:
- ASM selects a local page data set for page-out from its available
page data sets. ASM selects these data sets in a circular order within
each type of data set, subject to the availability of free space and
the device response time.
If you are using storage-class memory (SCM), ASM selects paging space first from available contiguous blocks of SCM, and then from available noncontiguous blocks of SCM.
- Q:
- What factors should I consider when allocating storage-class memory (SCM) to a partition?
- A:
-
- Continue to define page data sets on DASD, which provides improved availability compared to failure scenarios that could consume all of your paging space.
- Configure approximately the same amount of paging space for storage-class memory (SCM) on Flash Express® cards as you have defined for page data sets on DASD. For many configurations, a single pair of Flash Express cards provides enough paging space for an entire z/OS® partition.
- Using 1 MB pageable large pages with SCM can improve system performance by paging a smaller number of larger pages to SCM than would be paged to 4 KB page data sets on DASD. If contiguous space is not available on SCM, 1 MB large pages are demoted to 256 4 KB blocks and paged to either 4 KB page data sets or to SCM, based on response time.
- Because SCM is not persistent across IPLs, PLPA data is also required for warm starts. The PLPA copy on page data sets is used for warm starts, and the PLPA copy on SCM is used for resolving page faults. In addition, local page data sets must accommodate all VIO paging.
- For additional SCM configuration options, refer to Space calculation examples, Estimating total size of paging data sets and the IEASYSxx system parameter list in z/OS MVS Initialization and Tuning Reference.
- Q:
- Will data-in-virtual users increase the need for paging data sets?
- A:
- Data-in-virtual does provide applications with functions that would encourage extensive use of virtual storage. Depending on the extent of the usage of data-in-virtual, paging data set requirements may increase.