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Thin Provisioning on DS8900

By CLINT HARDY posted Tue July 28, 2020 01:52 PM


The DS8900 was introduced 3Q2019 as an all flash storage array with industry leading low latency.  The transition to flash only arrays includes benefits far beyond just performance.   Flash storage technology allows for less energy use and reduces floor space requirements along with allowing for higher reliability and availability.  Combining these benefits with thin provisioning can continue to help in user cost reduction, while meeting storage requirements.


The DS8900 offers a critical platform for advancements in the thin provisioning environments and we plan on further improvements in the near future.  For now, let’s recap on the currently supported thin provisioning features on the DS8900.


DS8900 thin provisioning is an excellent high performing solution for space management.   This combined with small extents (21 Cylinder CKD, 16MB Fixed Block) allows for an efficient reclamation process when utilizing flash storage.  Small extents also offer the benefit of more even array utilization due to faster rotation at 16MB boundaries compared to 1GB boundaries.  The faster rotation also allows for better absorption of write bursts due to more effective NVS (nonvolatile write cache) space due to the additional spread across arrays.


The main consideration for the use of thin provisioning with small extents is the maximum configurable and physical capacity limits per machine type with small extents.  Many of the values for physical capacity are much higher than typical configurations, but if larger configurable and/or physical capacity is planned to be used, then it should be considered to use the DS8950 with at least 1TB of cache.   If necessary, this can be mitigated by creating separate storage pools for thin provisioned with small extents vs fully provisioned with large extents if more capacity is required.  The following table describes the limitations in capacity for extents based on cache size.


Cache Size

Max Virtual/Configurable Capacity with Large Extents

Max Physical Capacity With Small Extents

Max Virtual/Configurable Capacity with Small Extents

<= 512 GB

FB: 4096 TiB

CKD: 3652 TiB

FB: 512 TiB

CKD: 551 TiB

FB: 1024 TiB

CKD: 913 TiB

> 512 GB

FB: 8160 TiB - 16384 TiB

CKD: 7263 TiB - 14608 TiB

FB: 2048 TiB

CKD: 2205 TiB

FB: 3968 TiB - 4096 TiB

CKD: 3538 TiB - 3652 TiB


The larger cache sizes are required to manage the mapping tables for larger configurations of physical and virtual capacity.  This allows to take advantage or larger over provisioning and larger Safeguarded Copy backup capacity.   It also should be noted that configurable capacity will vary between the limits based upon the number and size of logical volumes.


Users must monitor and be careful when provisioning above the physical capacity limits in a storage pool to prevent running out of space and impacting production I/O.  Limitations can be set in place to limit the amount of over provisioning on a storage pool basis within the GUI and/or DSCLI.


Since the DS8900 is an all flash array, there are no additional considerations that need to be taken into account from a Global Metadata perspective like previous hybrid or all HDD DS8000 systems as discussed by @Nick Clayton in Configuring the DS8880 – Using Small Extents.


While space can be reclaimed directly by many host systems connected to the DS8900, the DS8900 also takes advantage of space reclamation in copy services relationships.  This can be done in the Z environment via the DFSMSdss™ SPACEREL command.


Metro Mirror, Global Mirror, and Global Copy relationships for thin provisioned CKD devices propagate the space mapping and host reclamation requests to the secondary volumes to ensure the capacity is matching the primary volumes.


Flash Copy and Global Mirror Flash Copy targets are a key use case for thin provisioned devices.  For Flash Copy, space is released during establish and withdraw of the relationship.  Global Mirror environments have ongoing space release/reclamation managed automatically by the DS8000.


Safeguarded thin provisioned backup capacity is also efficiently internally reclaimed.  This combined with efficiently packing changes on small granularity allows for even more efficient use of space.  This efficiently is what allows for a potential 500 backups of a given host addressable volume depending on the user change rate.


By providing thin provisioned solutions, users can take advantage and provision larger host addressable volumes than required and then allocate the capacity on demand.  This can be combined with thinly provisioning point and time backups utilizing flash copy or safeguarded copy.


1 comment



Fri October 16, 2020 02:12 PM

Nice article @CLINT HARDY​. Very informative and concise.