Dell EMC Unity: How to perform a rough calculation to determine usable space in a Dynamic Storage Pool. (User Correctable)
Summary: The article briefly describes the dynamic pool feature on Dell EMC Unity and how dynamic pool allocates the underlying drives.
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Instructions
Dynamic Pools is a Pool technology introduced for All Flash systems in Dell EMC Unity OE version 4.2 and later. Dynamic Pools replaces the previous Pool technology, now referred to as Traditional Pools, as the default Pool type for All Flash systems. Dynamic Pools utilize advanced RAID techniques and distributed sparing to offer better storage utilization and more simplified planning than Traditional Pools.
Supported Configuration:
The Dell EMC Unity system must be All Flash
The Dell EMC Unity system must be running OE 4.2 or greater
RAID width assignment during pool creation
Dynamic pools are the default pool type for All Flash system, when created within Unisphere.
Traditional pools can be only creating on a All flash system through UEMCLI.
The table below shows what RAID width the Dell EMC Unity system will select depending on the number of drives selected during initial pool creation:
How dynamic pools allocate underlying drives
In this scenario, the customer is creating a dynamic pool using 16x drives, each drive 1,920 GB in size, and has selected RAID 6 protection.
Based on the RAID width selection table above, a RAID 6 width of 12+2 will be assigned. When creating a dynamic pool, all 16x drives will be used for data, parity, and spare extent allocations. Because spare extent space for this dynamic pool will be equal to approximately 1x drives worth, we will be utilizing 15x drives worth of space to assign the data and parity extents. A dynamic storage pool will assign approximately 1x drives worth of spare extents for every 31x drives in the pool.
The calculation breaks down as fallows.
The usable space per drive after formatting will be 1,751 GB.
The amount of usable space to allocate data and parity extents.
1751 GB * 15 = 26,355 GB
Because RAID 6 data and parity rotates across the drives and we have a 12+2 width, the data and parity extents will rotate every 14x drives until we no longer have enough space to complete a full 12+2 stripe.
26,355 GB / 14 = 1882.5 GB
Because the usable space within the dynamic pool will be directly related to data extents, which is grouped rotations of 12, we must now multiply by 12.
1882.5 GB * 12 = 22,590 GB
In our example, the dynamic storage pool only displayed a usable space of 20.2 TB for the customer, leaving us with 2,230.14 GB of "unaccounted" space; however, the 2,230.14 GB of space was being consumed by RAID group meta-data, FLU meta-data, meta-data related to system drivers, etc..., which is why the system then excludes that space for the 'usable' space available to the user rather than displaying the full 22,590 GB and showing 2,230.14 GB of that already consumed.
Supported Configuration:
The Dell EMC Unity system must be All Flash
The Dell EMC Unity system must be running OE 4.2 or greater
RAID width assignment during pool creation
Dynamic pools are the default pool type for All Flash system, when created within Unisphere.
Traditional pools can be only creating on a All flash system through UEMCLI.
The table below shows what RAID width the Dell EMC Unity system will select depending on the number of drives selected during initial pool creation:
How dynamic pools allocate underlying drives
In this scenario, the customer is creating a dynamic pool using 16x drives, each drive 1,920 GB in size, and has selected RAID 6 protection.
Based on the RAID width selection table above, a RAID 6 width of 12+2 will be assigned. When creating a dynamic pool, all 16x drives will be used for data, parity, and spare extent allocations. Because spare extent space for this dynamic pool will be equal to approximately 1x drives worth, we will be utilizing 15x drives worth of space to assign the data and parity extents. A dynamic storage pool will assign approximately 1x drives worth of spare extents for every 31x drives in the pool.
The calculation breaks down as fallows.
The usable space per drive after formatting will be 1,751 GB.
The amount of usable space to allocate data and parity extents.
1751 GB * 15 = 26,355 GB
Because RAID 6 data and parity rotates across the drives and we have a 12+2 width, the data and parity extents will rotate every 14x drives until we no longer have enough space to complete a full 12+2 stripe.
26,355 GB / 14 = 1882.5 GB
Because the usable space within the dynamic pool will be directly related to data extents, which is grouped rotations of 12, we must now multiply by 12.
1882.5 GB * 12 = 22,590 GB
In our example, the dynamic storage pool only displayed a usable space of 20.2 TB for the customer, leaving us with 2,230.14 GB of "unaccounted" space; however, the 2,230.14 GB of space was being consumed by RAID group meta-data, FLU meta-data, meta-data related to system drivers, etc..., which is why the system then excludes that space for the 'usable' space available to the user rather than displaying the full 22,590 GB and showing 2,230.14 GB of that already consumed.
Additional Information
Relevant Knowledge Base articles and documents:
https://support.emc.com/docu85573_Dell-EMC-Unity:-Dynamic-Pools---Overview.pdf?language=en_US
https://support.emc.com/docu85573_Dell-EMC-Unity:-Dynamic-Pools---Overview.pdf?language=en_US
Affected Products
Dell EMC Unity FamilyProducts
Dell EMC Unity FamilyArticle Properties
Article Number: 000019417
Article Type: How To
Last Modified: 08 Oct 2024
Version: 3
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