PowerVM continues to improve Virtual I/O Server (VIOS) with Shared Storage Pools (SSP), which simplifies cloud management and efficiency of storage usage. PowerVM 2.2.6 includes a VIOS SSP performance enhancement known as flash acceleration. This feature can transparently increase a client's workload performance by utilizing SSP flash storage caching on the VIOS.
Background on Shared Storage Pools
VIOS SSP allows a group of VIOS nodes to form a cluster and provision virtual storage to client LPARs. The VIOS nodes in the cluster all have access to the same underlying physical disks, which are grouped into a single pool of storage. A virtual disk or LU can be carved out of that storage pool and mapped to a client LPAR as a virtual SCSI device. An LU may be thin or thickly provisioned, where thin provisioned LUs do not reserve blocks until they are written to, while thickly provisioned blocks reserve their storage when the LU is created.
Once an LU has been created in the pool, snapshots or clones of that LU can be created. The number of snapshots and clones created is limited only by the amount of available storage in the pool, and creating these objects happens nearly instantly. Snapshots are used for rolling back to previous points in time. Clones are used for provisioning new space efficient copies of an LU. These clones can be managed by PowerVC capture and deploy image management operations.
These VIOS SSP features allow rapid deployment of new client LPARs in a cloud computing environment. The storage pooling model of VIOS SSP also simplifies administration of large amounts of storage. The clustering aspect of VIOS SSP provides fault tolerance between VIOS multi-pathing pairs, and simplifies verification that other nodes can see the storage and are eligible for LPAR mobility operations.
What is SSP Flash Acceleration?
SSP flash acceleration enables a VIOS SSP node to use a flash caching device for read-only caching and maintains cache coherency between all VIOS nodes in the cluster. The flash caching device ideally is local to the server, such as built-in SSD or direct attached from serial attached SCSI (SAS), but may also be from a flash storage controller on the SAN. Read-only caching is performed across the entire storage pool including any storage tiers in the pool, where only the pool meta-data is excluded from caching.
When the storage pool caching is enabled, all read requests for the pool user data blocks are routed to the SSP caching software. If a specific user data block is found in the local VIO server cache, the I/O request is processed from the cache device. If the requested block is not found in the cache, or if it is a write request, the I/O request goes directly to the storage pool SAN device(s). Write requests will cause a cache invalidate to occur on other VIOS nodes that have the same block cached.
SSP flash acceleration is based on caching on the Virtual I/O Servers, while Power® flash caching or server-side caching is based on caching on the client logical partition. For more information on server-side caching, see the articles at
https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/com.ibm.aix.osdevice/caching_storage_data.htm
or
https://community.ibm.com/community/user/power/blogs/rob-gjertsen1/2020/06/25/aix-powervm-server-side-caching-enhancements
Both types of caching are entirely independent and may be used simultaneously.
Advantages of SSP Flash Acceleration
There are various benefits of SSP flash acceleration that include:
- An improvement in latency and throughput with read oriented and cache friendly workloads such as analytical and transactional workloads, and online transaction processing. Brokerage workloads such as DayTrader (https://www.ibm.com/support/knowledgecenter/en/linuxonibm/liaag/wascrypt/l0wscry00_daytrader.htm) that are highly read intensive have shown significant performance improvement with 50% more throughput and a 5X reduction in application latency.
- Better virtual machine (VM) density, without performance impacts.
- Allowing more efficient utilization and scaling of SAN infrastructure. The SAN offloading of read requests can increase write throughput on congested SANs.
- Transparency of acceleration to the client logical partitions that are unaware of caching on the Virtual I/O Servers. Clustered applications can be used on the client logical partitions, unlike server-side caching.
- Independent of client operating systems: caching is supported on AIX®, IBM® i, and Linux operating systems.
- Compatible with Live Partition Mobility (LPM).
- Independent of types of storage: there is no dependency on the type of flash storage for caching and SAN storage for SSP.
Using SSP Flash Acceleration
SSP flash acceleration is easy to configure and manage. The storage pool caching can be enabled or disabled dynamically while workloads are running on the client LPARs.
When caching is enabled for the storage pool, this starts caching on all VIO servers in the cluster that have a local cache defined. There are only two steps to enable caching from the VIOS CLI:
- First create a cache pool on each VIOS in the cluster with the cache_mgt command (https://www.ibm.com/support/knowledgecenter/8247-22L/p8hcg/p8hcg_cache_mgt.htm).
- Enable caching of the storage pool on the SSP cluster from a single VIOS node with the sspcache command (https://www.ibm.com/support/knowledgecenter/8247-22L/p8hcg/p8hcg_sspcache.htm).
When caching is disabled for the storage pool, this stops caching on all VIO servers in the cluster. This is performed with a single VIOS CLI sspcache command.
Concluding Thoughts
SSP flash acceleration can significantly improve the performance of read oriented workloads that are cache friendly and provides additional flexibility with caching at the VIOS level. This new SSP feature is also extremely easy to use. For additional information on flash acceleration, please see the Knowledge Center article (https://www.ibm.com/support/knowledgecenter/8247-22L/p8hb1/p8hb1_vios_ssp_flash_accl.htm).
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