Chapter 3. Virtualization
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For more information regarding Active Memory Sharing, see IBM PowerVM Virtualization
Active Memory Sharing, REDP-4470.
3.4.7 Active Memory Deduplication
In a virtualized environment, the systems might have a considerable amount of duplicated
information that is stored on RAM after each partition has its own operating system, and
some of them might even share the same kinds of applications. On heavily loaded systems,
this behavior might lead to a shortage of the available memory resources, forcing paging by
the Active Memory Sharing partition operating systems, the Active Memory Deduplication
pool, or both, which might decrease overall system performance
Active Memory Deduplication allows the POWER Hypervisor to map dynamically identical
partition memory pages to a single physical memory page within a shared memory pool. This
way enables a better usage of the Active Memory Sharing shared memory pool, increasing
the system’s overall performance by avoiding paging. Deduplication can cause the hardware
to incur fewer cache misses, which also leads to improved performance.
Active Memory Deduplication depends on the Active Memory Sharing feature being available,
and it consumes CPU cycles that are donated by the Active Memory Sharing pool’s VIOS
partitions to identify deduplicated pages. The operating systems that are running on the
Active Memory Sharing partitions can “hint” to the POWER Hypervisor that some pages
(such as frequently referenced read-only code pages) are good for deduplication.
To perform deduplication, the hypervisor cannot compare every memory page in the Active
Memory Sharing pool with every other page. Instead, it computes a small signature for each
page that it visits and stores the signatures in an internal table. Each time that a page is
inspected, a look-up of its signature is done in the known signatures in the table. If a match is
found, the memory pages are compared to be sure that the pages are really duplicates. When
a duplicate is found, the hypervisor remaps the partition memory to the existing memory page
and returns the duplicate page to the Active Memory Sharing pool.
From the LPAR perspective, the Active Memory Deduplication feature is not apparent. If an
LPAR attempts to modify a deduplicated page, the Power hypervisor grabs a free page from
the Active Memory Sharing pool, copies the duplicate page contents into the new page, and
maps the LPAR’s reference to the new page so that the LPAR can modify its own unique
page.
For more information regarding Active Memory Deduplication, see Power Systems Memory
Deduplication, REDP-4827
3.4.8 Operating system support for PowerVM
Table 3-3 shows operating system support for virtualization features.
Table 3-3 Virtualization features supported by AIX, IBM i, and Linux
Feature
AIX 6.1 TL9
SP1
AIX 7.1 TL03
SP1
IBM i 7.1
TR 9
IBM i 7.2
TR 1
RHEL 6.6
SLES 11 SP3
Virtual SCSI
Yes
Yes
Yes
Yes
Yes
Yes
Virtual Ethernet
Yes
Yes
Yes
Yes
Yes
Yes
Shared Ethernet Adapter
Yes
Yes
Yes
Yes
Yes
Yes
Virtual Fibre Channel
Yes
Yes
Yes
Yes
Yes
Yes
