Chapter 6. Storage

The fsfreeze tool is fully supported in Red Hat Enterprise Linux 6.5. The fsfreeze command halts access to a file system on a disk. fsfreeze is designed to be used with hardware RAID devices, assisting in the creation of volume snapshots. For more details on the fsfreeze utility, refer to the fsfreeze(8) man page.

pNFS allows traditional NFS systems to scale out in traditional NAS environments, by allowing the compute clients to read and write data directly and in parallel, to and from the physical storage devices. The NFS server is used only to control meta-data and coordinate access, allowing predictably scalable access to very large sets from many clients. Bug fixes to pNFS are being delivered in this release.

FUSE (Filesystem in User Space) is a framework that enables development of file systems purely in the user space without requiring modifications to the kernel. Red Hat Enterprise Linux 6.5 delivers performance enhancements for user space file systems that use FUSE, for example, GlusterFS (Red Hat Storage).

Most LVM commands require an accurate view of the LVM metadata stored on the disk devices on the system. With the current LVM design, if this information is not available, LVM must scan all the physical disk devices in the system. This requires a significant amount of I/O operations in systems that have a large number of disks.

The purpose of the lvmetad daemon is to eliminate the need for this scanning by dynamically aggregating metadata information each time the status of a device changes. These events are signaled to lvmetad by udev rules. If lvmetad is not running, LVM performs a scan as it normally would.

This feature is disabled by default in Red Hat Enterprise Linux 6. To enable it, refer to the use_lvmetad parameter in the /etc/lvm/lvm.conf file, and enable the lvmetad daemon by configuring the lvm2-lvmetad init script.

An implementation of LVM copy-on-write (cow) snapshots, previously available as a Technology Preview, is now fully supported in Red Hat Enterprise Linux 6.5. The main advantage of this implementation, compared to the previous implementation of snapshots, is that it allows many virtual devices to be stored on the same data volume. This implementation also provides support for arbitrary depth of recursive snapshots (snapshots of snapshots of snapshots …). Note that this feature is for use on a single system. It is not available for multi-system access in cluster environments. For more information, refer to the documentation of the -s, --snapshot option in the lvcreate man page.

Logical Volumes (LVs) can now be thinly provisioned to manage a storage pool of free space to be allocated to an arbitrary number of devices when needed by applications. This allows creation of devices that can be bound to a thinly provisioned pool for late allocation when an application actually writes to the pool. The thinly-provisioned pool can be expanded dynamically if and when needed for cost-effective allocation of storage space. This feature, previously available as a Technology Preview, is now fully supported. You must have the device-mapper-persistent-data package installed to use this feature. For more information, refer to the lvcreate(8) man page.

Scalability and ease-of-use of Device Mapper Multipath have been improved. These improvements include in particular:

Red Hat Enterprise Linux 6.5 introduces the Orlov block allocator that provides better locality for files which are truly related to each other and likely to be accessed together. In addition, when resource groups are highly contended, a different group is used to maximize performance.

The mdadm tool now supports the TRIM commands for RAID0, RAID1, and RAID10.

Red Hat Enterprise Linux 6 includes code in the megaraid_sas driver to enable LSI Syncro CS high-availability direct-atteched storage (HA-DAS) adapters. While the megaraid_sas driver is fully supported for previously enabled adaptaers, the use of this driver for Syncro CS is available as a Technology Preview. Support for this adapter will be provided directly by LSI, your system integrator, or system vendor. Users deploying Syncro CS on Red Hat Enterprise Linux 6 are encouraged to provide feedback to Red Hat and LSI. For more infomration on LSI Syncro CS solutions, please visit http://www.lsi.com/products/shared-das/pages/default.aspx.

Red Hat Enterprise Linux 6.5 introduces the safe offline interface for direct access storage devices (DASDs). Instead of setting a DASD device offline and returning all outstanding I/O requests as failed, with this interface, the user can set a DASD device offline and write all outstanding data to the device before setting the device offline.

Red Hat Enterprise Linux 6.5 supports Fixed Block Access (FBA) Extended Address Volumes (EAV) and EDEV installations. FBA Direct Access Storage Devices (DASDs) are mainframe-specific disk devices. In contrast to Extended Count Key Data (ECKD) DASDs, these disks do not require formatting and resemble the Logical Block Addressing (LBA) of non-mainframe disks. Despite this resemblance, the Linux kernel applies special handling during partition detection for FBA DASDs, resulting in a single, immutable partition being reported. While actual FBA DASD hardware is no longer available, the IBM z/VMhypervisor can simulate FBA DASD disks, backed by either ECKD or SCSI devices. EDEV storage then appears to the system as an FBA DASD (with one immutable partition), rather than an ECKD DASD.