Sector Size Support

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Overview

Many applications manage large blocks of data—much larger than the legacy hard drive sector size of 512Bytes. Advanced Format Drives (AFDs), which use 4-kilobyte (KiB) sectors, are more efficient for such workloads.

DataCore SANsymphony supports both 512-byte (512B) and 4-kilobyte (4K) sector sizes, allowing virtual disks to be created independently of the sector size used by the underlying storage pools.

Sector sizes are important when creating disk pools, virtual disks, and performing operations on virtual disks. This topic provides information about supported sector sizes, native I/O, and sector size emulation in disk pools and virtual disks.

4 KiB Sector Support

Refer to host operating system documentation to verify that hosts support virtual disks with a 4 KiB sector size. Virtual disks with a 4 KiB sector size are not supported with Windows Server versions prior to 2012. For more information on the VMware ESXi and 4KiB sector size, refer to the Knowledge Base article for more information.

Supported Sector Sizes

The following sector sizes are supported for virtual disks:

  • 4KiB (4K)
  • 512Bytes (512B)

When creating a virtual disk, you can select either supported sector size in the DataCore Management Console.

Native and Emulated I/O Operations

I/O processing in SANsymphony is determined by the relationship between the virtual disk sector size and the storage pool sector size. Depending on whether these sector sizes match or differ, I/O operations are handled either natively or through automatic sector size emulation.

  • When the sector sizes match, I/O is processed directly without translation. See When I/O is Native.
  • When the sector sizes differ, SANsymphony performs sector size emulation to translate I/O between the virtual disk and the storage pool. See Sector Size Emulation.

When I/O is Native

During the creation of a virtual disk in the DataCore Management Console, native operation occurs when the sector size of the virtual disk (VD) matches the sector size of the selected storage pool. In this case, SANsymphony performs I/O directly, without sector size emulation.

Matching sector sizes include the following combinations:

  • 512B virtual disk on a 512B pool
  • 4KiB virtual disk on a 4KiB pool

In these configurations, I/O is passed directly to the storage pool without translation.

If the virtual disk sector size differs from the storage pool (for example, 4KiB VD on a 512B pool), SANsymphony performs sector size emulation. See Sector Size Emulation.

Sector Size Emulation

Sector size emulation occurs when the virtual disk sector size and the storage pool sector size do not match. SANsymphony automatically translates I/O between the virtual disk and the storage pool.

In this scenario, SANsymphony acts as a translation layer between the host and the underlying storage. When the host issues I/O using the virtual disk’s sector size, SANsymphony converts those requests to match the physical sector size of the storage pool. For example, multiple 512-byte requests can be combined into a single 4KiB operation, or a 4KiB request can be split into multiple 512-byte operations, depending on the configuration.

This process is fully transparent to the host and applications. It ensures proper I/O alignment, compatibility between 512B and 4KiB environments, and data integrity across storage devices that use different sector sizes.

Supported Emulations

  • 4KiB virtual disk on a 512B pool
  • 512B virtual disk on a 4KiB pool
4KiB Virtual Disk on a 512B Pool

When this combination is selected, the virtual disk is presented to the host as a 4KiB disk while the underlying storage pool uses 512B sectors, and SANsymphony automatically aggregates multiple 512B physical sectors into a single 4KiB logical sector.

512B Virtual Disk on a 4KiB Pool (512-byte Emulation (512e))

Starting with PSP21, SANsymphony supports 512-byte virtual disks on 4K-native storage pools. This configuration is commonly referred to as 512e (512-byte emulation). The virtual disk operates in 512-byte emulation (512e) mode.

When this combination is selected, the virtual disk is presented to the host as a 512e disk while the storage pool uses 4 KiB sectors. SANsymphony automatically translates I/O between the host and the storage pool.

This allows hosts or applications that require 512B disks to run on modern 4K-native storage, including deployments that use 4K-native drives or controllers.

512-byte Emulation (512e) Mirrored Virtual Disk

Mirrored virtual disks can use storage from pools with different sector sizes. SANsymphony automatically handles the differences and ensures the virtual disk remains consistent for the host.

The following table shows an example of a 512B virtual disk mirrored across two pools with different sector sizes.

Example 1: 512-byte Emulation (512e) Virtual Disk Mirrored Across Two 4KiB Pools

Mirror Source Pool Sector Size Virtual Disk Sector Size (to Host)
SSV1 4KiB 512B
SSV2 4KiB 512B

Example 2: 512-byte Emulation (512e) Virtual Disk Mirrored Across Two Pools with Different Sector Sizes.

Mirror Source Pool Sector Size Virtual Disk Sector Size (to Host)
SSV1 4KiB 512B
SSV2 512B 512B

SANsymphony applies sector size emulation independently for each mirror source when sector sizes differ. Read and write operations are automatically translated to match the sector size of each underlying pool. This ensures that the virtual disk presents a consistent sector size to the host, regardless of the underlying pool sector sizes.

  • The virtual disk sector size is consistent across all mirrors.
  • Matching sector sizes result in native I/O.
  • Mismatched sector sizes are handled automatically through sector size emulation.
  • Read and write operations are translated as required to maintain proper I/O alignment.
  • Data consistency is maintained across all mirrors, even when the underlying pools have different sector sizes.

Sector Size Rules and Limitations

The following rules and limitations apply to disk pools and virtual disks based on sector size configuration.

Disk Pools

  • The sector size of a pool is selected when the pool is created and cannot be changed. (See Creating Disk Pools.)
  • Only physical disks with matching sector sizes can be added to a pool.
  • Physical disks with different sector sizes cannot be mixed within the same pool.
  • Capacity Optimization cannot be enabled on disk pools created with a 4KiB sector size.

Virtual Disks

  • The sector size of a virtual disk cannot be changed after creation.
  • Snapshots and rollbacks must use the same sector size as the original virtual disk.
  • For replication, the source and destination virtual disks must use the same sector size.
  • The sector size of the disk pool selected for the CDP history log must be greater than or equal to the sector size of the disk pool used by the virtual disk.
  • 4KiB virtual disks are not supported for VMware VVOLs or protocol endpoints.
  • The sector size of pass-through disks must match the virtual disk sector size.

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