Chapter 5. Storage

Enhancements to device-mapper

Several significant enhancements to device-mapper have been introduced in Red Hat Enterprise Linux 6.6:
  • The dm-cache device-mapper target, which allows fast storage devices to act as a cache for slower storage devices, has been added as a Technology Preview.
  • The device-mapper-multipath ALUA priority checker no longer places the preferred path device in its own path group if there are other paths that could be used for load balancing.
  • The fast_io_fail_tmo parameter in the multipath.conf file now works on iSCSI devices in addition to Fibre Channel devices.
  • Better performance can now be achieved in setups with a large number of multipath devices due to an improved way in which the device-mapper multipath handles sysfs files.
  • A new force_sync parameter in multipath.conf has been introduced. The parameter disables asynchronous path checks, which can help limit the number of CPU contention issues on setups with a large number of multipath devices.

dm-era Technology Preview

The device-mapper-persistent-data package now provides tools to help use the new dm-era device mapper functionality released as a Technology Preview. The dm-era functionality keeps track of which blocks on a device were written within user-defined periods of time called an era. This functionality allows backup software to track changed blocks or restore the coherency of a cache after reverting changes.