Dell PowerFlex 4.6.x Technical Overview

Storage pools

A storage pool is a set of physical storage devices in a protection domain. A volume is distributed over all devices residing in the same storage pool. You can define a magnetic storage pool (for HDDs) or a high-performance storage pool (for SSDs). Storage pools support medium (1 MB) or fine (4 KB) granularity data layouts and allow enabling or disabling zero padding (the process of zeroing out all the bits of an SDS device).

Storage pools allow the managing of different storage tiers in PowerFlex. Each storage device belongs to one storage pool. The figure shows two storage pools.

When a volume is configured over the virtualization layer, it is distributed over all devices residing in the same storage pool. Each volume block has two copies on two different SDSs. This virtualization layer allows the system to maintain data availability following a single-point failure. If a fault set is configured, then each copy will also be on different fault sets. Two network failures render all the domains in a state where I/O errors are possible on any storage pool.

You must assign a media type setting to each storage pool. Supported types are: HDD, SSD, and transitional (allows for migration flows).

If all SDSs in a protection domain have two physical drives associated with them, such as one hard drive, and the other SSD, you should define two storage pools:

• Magnetic storage pool

  Consists of all HDDs in the protection domain

• High-performance storage pool

  Consists of all SSDs used for storage purposes in the protection domain

  NOTE:

  • Mixing different types of SSDs is not recommended. Creating a separate storage pool for each type is the recommended best practice, for example: SAS SSD, SATA SSD, NVMe SAS SSD.
  • PowerFlex might not perform optimally if there are large differences between the sizes of the fault units in the same storage pool. For example, if one device has a much larger capacity than the rest of the devices, performance may be affected. After adding devices, you can define how much of the device capacity is available to PowerFlex by using the SCLI modify_sds_device_capacity command.

Storage pools support the following data layouts for HDD or SSD media:

• Medium granularity: Space allocation occurs at 1 MB units.
  • Supports HDD and SSD media
  • Includes persistent checksum for data integrity
• Fine granularity:
  • Requires SSD media and SDPM or NVDIMM for acceleration
  • Space allocation occurs at 4 KB units
  • Includes persistent checksum for data integrity
  • Supports data compression which reduces the size of data that is stored on the disk
  • Supports thin-provisioned, zero-padded volumes

Each storage pool can work in one of the following modes:

• Zero-padding enabled

  Ensures that every read from an area previously not written to returns zeros. Some applications might depend on this behavior. Furthermore, zero padding ensures that reading from a volume will not return information that was previously deleted from the volume.

  This behavior incurs some performance overhead on the first write to every area of the volume since the area must be filled with zeros first.

  Fine granularity is always zero padded.

• Zero-padding disabled (default only for medium granularity)

  A read from an area previously not written to will return unknown content. This content might change on subsequent reads.

Some applications assume that when reading from areas not written to before, the storage will return zeros or consistent data. If you plan to use such applications, then zero padding must be enabled.

You can add storage pools during installation. In addition, you can modify storage pools post installation with most of the management clients.