With the release last year of thin provisioning for z/OS, organizations are looking to exploit thin provisioning on their new DS8880 to improve efficiency, operational simplicity and reduce costs. More and more users are also using PPRC to migrate from old to new storage systems as this can be faster and simpler than using software tools especially for large environments. This post aims to provide an overview of the considerations for migrating from fully provisioned to thin provisioned devices using PPRC.
Effective use of thin provisioning for either production or FlashCopy use requires the DS8880 to be configured using small extents. This is a general recommendation when configuring a new DS8880, as it provides a range of other benefits and avoids complications if there was a desire to switch to thin provisioned devices in future. Benefits of small extents include:
- A more even distribution of workload over ranks, especially for bursty sequential workloads.
- Increased efficiency of Easy Tier, as idle small extents can be demoted to a lower tier.
- Prepares for future use of thin provisioning or space efficient FlashCopy.
Note : The use of Small Extents does limit the capacity of the DS8880, (~500TB with 256GB or less memory and ~2PB with 512GB or greater memory) but these limits are unlikely to be hit in most configurations. There is also a modest (~15%) reduction in maximum sequential write throughput of a single rank, but most production workloads will likely see more benefit from the improved workload distribution and so still see an overall benefit in performance.
In R8.2 we released the ability to do PPRC from a fully provisioned volume to a thin provisioned volume to enable migration to thin provisioning using PPRC. However as very few users would be migrating from old DS8800s to new DS8880s we also enabled this ability on the DS8870 microcode. Minimum microcode levels for migrating from fully provisioned to thin provisioned devices as as follows:
- DS8870 - 87.51.81.0
- DS8880 - 88.20.112.0
Important Note : 88.30.139.0 is required on the DS8880 to enable migration back from a thin provisioned DS8880 volume to a full provisioned DS8870 volume.
When migrating from fully provisioned devices on DS8870 to thin provisioned devices on DS8880 it is necessary to specify that the target devices are allowed to be space efficient. This is achieved with:
When doing a migration in this manner, the new devices will become fully provisioned when the migration is performed. The space release functionality provided in DFSMSdss using the SPACEREL command can be used if desired to release the space once the migration has been completed. As R8.3 provides full support for space release in a Metro Mirror environment, a larger range of users can now take advantage of this functionality.
We are also seeing user moving to thin provisioned devices during their migration even if they do not immediately plan on exploiting this functionality on the new DS8880s. As there is no way to convert in place from fully provisioned to thin provisioned this prepares them to exploit the function sometime later during the 4+ year lifespan of the new systems.
A new capability in R8.3 enables a user to set the over-provisioning ratio limit on the DS8880 to 1:1 which will prevent them from accidentally over-provisioning their storage pools and running out of space. If they were not looking to immediately exploit over-provisioning then this provides a safety mechanism to prevent operational errors when defining new devices.
With the ability to use PPRC to migrate to thinly provisioned devices on a new DS8880, everyone should consider thin provisioning as part of their implementation!
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