Chapter 49. Storage

Red Hat Enterprise Linux 7 includes a new multiple-queue I/O scheduling mechanism for block devices known as blk-mq. The scsi-mq package allows the Small Computer System Interface (SCSI) subsystem to make use of this new queuing mechanism. This functionality is provided as a Technology Preview and is not enabled by default. To enable it, add scsi_mod.use_blk_mq=Y to the kernel command line.

Although blk-mq is intended to offer improved performance, particularly for low-latency devices, it is not guaranteed to always provide better performance. In particular, in some cases, enabling scsi-mq can result in significantly worse performance, especially on systems with many CPUs. (BZ#1109348)

Since Red Hat Enterprise Linux 7.1, storage array management with libStorageMgmt, a storage array independent API, has been fully supported. The provided API is stable, consistent, and allows developers to programmatically manage different storage arrays and utilize the hardware-accelerated features provided. System administrators can also use libStorageMgmt to manually configure storage and to automate storage management tasks with the included command-line interface.

The Targetd plug-in is not fully supported and remains a Technology Preview. (BZ#1119909)

DIF/DIX is a new addition to the SCSI Standard. It is fully supported in Red Hat Enterprise Linux 7 for the HBAs and storage arrays specified in the Features chapter, but it remains in Technology Preview for all other HBAs and storage arrays.

DIF/DIX increases the size of the commonly used 512 byte disk block from 512 to 520 bytes, adding the Data Integrity Field (DIF). The DIF stores a checksum value for the data block that is calculated by the Host Bus Adapter (HBA) when a write occurs. The storage device then confirms the checksum on receipt, and stores both the data and the checksum. Conversely, when a read occurs, the checksum can be verified by the storage device, and by the receiving HBA. (BZ#1072107)