Dell PowerFlex 4.6.x Technical Overview

Checksum protection

PowerFlex calculates and validates the checksum value for the payload during transit to protect data in-flight. Checksum protection is applied to all I/Os.

This feature addresses errors that change the payload during the transit through PowerFlex. PowerFlex protects data in-flight by calculating and validating the checksum value for the payload at both ends.

• During write operations, the checksum is calculated when the SDC receives the write request from the application. This checksum is validated just before each SDS writes the data on the storage device.
• During read operations, the checksum is calculated when the data is read from the SDS device, and is validated by the SDC before the data returns to the application. If the validating end detects a discrepancy, it will initiate a retry. The checksum will be done in the granularity of a sector (1/2 KB).
• Pools with fine granularity with or without compression, have a persistent checksum by default. This cannot be changed.

Each I/O goes through compression, and the checksum is calculated before it is written to the disk. There are two types of checksum:

• Fine granularity layout saves checksum data before and after processing to guarantee data integrity (compressed or not).
• There are also system checksums for metadata.

This feature applies to all I/Os: application, rebuild, rebalance, and migrate. The checksum is also kept in RMcache (read memory cache), protecting every block that is maintained in SDS memory against memory corruption. The checksum feature can be enabled at the protection domain level, and defined at the storage pool level. The feature is T10/DIF-ready.

The fine granularity data layout has a default checksum whether it is compressed or not.