IBM 990 manual Memory allocation, Partial Memory Restart

6947ch02.fm Draft Document for Review April 7, 2004 6:15 pm

￿Partial Memory Restart

In the rare event of a memory card failure, Partial Memory Restart enables the system to be restarted with only part of the original memory. In a one-book system, the failing card will be deactivated, after which the system can be restarted with the memory on the remaining memory card.

In a system with more than one book, all physical memory in the book containing the failing memory card is taken offline, allowing you to bring up the system with the remaining physical memory in the other books. In this way, processing can be resumed until a replacement memory card is installed.

Memory error-checking and correction code detects and corrects single-bit errors, or 2-bit errors from a chipkill failure, using the Error Correction Code (ECC). Also, because of the memory structure design, errors due to a single memory chip failure are corrected.

Memory background scrubbing provides continuous monitoring of storage for the correction of detected faults before the storage is used.

The memory cards use the latest fast 256 Mb, and 512 Mb, synchronous DRAMs. Memory access is interleaved between the memory cards to equalize memory activity across the cards.

Memory cards have 8 GB, 16 GB, or 32 GB of capacity. All memory cards installed in one book must have the same capacity. Books may have different memory sizes, but the card size of the two cards per book must always be the same.

The total capacity installed may have more usable memory than required for a configuration, and Licensed Internal Code Configuration Control (LIC-CC) will determine how much memory is used from each card. The sum of the LIC-CC provided memory from each card is the amount available for use in the system.

Memory allocation

Memory assignment or allocation is done at Power-on Reset (POR) when the system is initialized. Actually, PR/SM is responsible for the memory assignments; it is PR/SM that controls the resource allocation of the CPC. Table 2-1 on page 28 shows the distribution of physical memory across books when a system initially is installed with the amounts of memory shown in the first column. However, the table gives no indication of where the initial memory is allocated. Memory allocation is done as evenly as possible across all installed books.

PR/SM has knowledge of the amount of purchased memory and how it relates to the available physical memory in each of the installed books. PR/SM has control over all physical memory and therefore is able to make physical memory available to the configuration when a book is non-disruptively added. PR/SM also controls the reassignment of the content of a specific physical memory array in one book to a memory array in another book. This is known as the Memory Copy/Reassign function.

Due to the memory allocation algorithm, systems that undergo a number of MES upgrades for memory can have a variety of memory card mixes in all books of the system. If, however unlikely, memory should fail, it is technically feasible to Power-on Reset the system with the remaining memory resources (see “Partial Memory Restart” on page 54). After Power-on Reset, the memory distribution across the books is now different, as is the amount of memory.

Capacity Upgrade on Demand (CUoD) for memory can be used to order more memory than needed on the initial model, but that is required on the target model; see “Memory upgrades”

54IBM eServer zSeries 990 Technical Guide