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AIX Flash Cache

Flash Cache

On October 5, 2015, IBM made some significant new announcements, one of which was Flash Cache in AIX v7.2. It should be noted that AIX 7.2 only runs on POWER7 or higher servers. Since the announcement, IBM has back ported Flash Cache to AIX v7.1 tl4 SP2.

Flash Cache is also referred to as server side caching of data. It allows an LPAR to use SSDs or flash storage as a read-only cache to improve read performance for spinning disks. The cache can be significantly smaller than the data it is caching and can be direct attached or SAN based.

The supported flash memory technology for use with flash cache includes SSDs in an EXP24s drawer attached to the PCIe2/PCIe3 SAS RAID adapter with write cache, SSDs in the POWER servers system unit run by the integrated SAS controller and the IBM FlashSystem.

Once flash cache is set up, AIX decides what data is hot and stores a copy in the flash cache. The caching function can be enabled and disabled dynamically and is transparent to the workloads taking advantage of it. When caching is enabled, read requests for the target devices are sent to the caching software, which checks whether the block is in the cache. If it is, then the disk block is provided from the cache. All other reads and all writes will be sent through to the original disk.

Flash cache requires AIX v7.2, which in turn requires POWER7 or higher servers. The LPAR must have a minimum of 4GB of memory—but I would recommend much more memory than this, as the cache uses memory to keep track of read access and other things. I also would not recommend using it on small LPARs. Typically for a small LPAR you will get more benefit from adding memory or CPU. But for larger LPARs there can be significant benefits with the right workload.

In his AIXpert Blog on Flash Cache, Nigel Griffiths reminds us that there may not be a benefit if the SAN is already using internal caching for the data.


There are several terms that you need to understand when setting up and using flash cache.

  • Cache device—The SSD or flash device that will be used for caching
  • Cache pool—The group of cache devices set up to be used for caching
  • Cache partition—A logical cache device that exists in the cache pool
  • Target device—The storage device that is being cached

In addition, there are two components you need to be aware of:

  1. Cache Management—There’s a new command that is used to create, assign, destroy and report on the flash cache. The command is /usr/sbin/cache_mgt.
  2. Cache engine—This is the algorithm that determines what will be cached and that retrieves the data from the cache.

Setting up flash cache

There are several ways to implement flash cache. The flash can be direct attached to an AIX LPAR or it can be virtualized through a VIO server. Using a VIO server provides support for LPM, however, it is always possible to turn off caching prior to an LPM move, especially if the target hardware does not have a Flash cache. It should also be noted that the cache can only be provisioned to one LPAR or VIO server—as of right now it cannot be shared.

In order to test flash cache there are some prerequisites that have to be met for operating system and firmware—the key is to be current on both. Today you can only have one cache pool and one cache partition. However, there are no published limits on the pool size. For the system I was testing on, we had the following setup:

  • Power System E850 LPAR
  • LPAR: whole system using all 20 cores at 3.72 GHz
  • Memory: 512 GB
  • Flash cache: Four x 775GB SSDs—these were installed in the CEC with two on each side of the split bus. We did not have VIO servers so the flash cache was directly assigned to the LPAR.
  • Disks: 88 x 250 GB NetApp provided SAN LUNs as the targets—these are all in one huge filesystem that is used by SAS. Once the data is loaded it is primarily read only.
  • Our operating system was AIX

Jaqui Lynch is an independent consultant, focusing on enterprise architecture, performance and delivery on Power Systems with AIX and Linux.

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