4. Storage

4.1. Storage Input/Output Alignment and Size

Recent enhancements to the SCSI and ATA standards allow storage devices to indicate their preferred (and in some cases, required) I/O alignment and I/O size. This information is particularly useful with newer disk drives that increase the physical sector size from 512 byes to 4K bytes. This information may also be beneficial for RAID devices, where the chunk size and stripe size may impact performance.
The Red Hat Enterprise Linux 6 provides the ability to read and utilize this information, and optimize how data is read and written from storage devices.

Note

The Storage Administation Guide features a chapter that covers I/O Limits in further detail.

4.2. Dynamic Load Balancing with DM-Multipath

Device Mapper Multipathing (DM-Multipath) creates a single conceptual device from the multiple cables, switches and controllers that connect servers to storage arrays. This enables centralized management of connection devices (also known as paths) and makes it possible to balance loads over all available paths.
DM-Multipath in Red Hat Enterprise Linux 6 introduces two new options when dynamically balancing load over paths. Paths can now be dynamically selected depending on either the queue size of each path or previous I/O time data.

Note

The DM Multipath book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise Linux 6.

4.3. Logical Volume Manager (LVM)

Volume management creates a layer of abstraction over physical storage by creating logical storage volumes. This provides greater flexibility over just using physical storage directly. Red Hat Enterprise Linux 6 manages logical volumes using the Logical Volume Manager (LVM).

Important

system-config-lvm is a graphical user interface provided in Red Hat Enterprise Linux to manage logical volumes. The functionality provided by system-config-lvm is in the process of transitioning to a more maintainable tool named gnome-disk-utility (also referred to as palimpsest). As a result, Red Hat will be very selective in updating system-config-lvm. As gnome-disk-utility reaches feature parity with system-config-lvm, Red Hat reserves the right to remove system-config-lvm during the life of Red Hat Enterprise Linux 6.

Note

The Logical Volume Manager Administration document describes the LVM logical volume manager, including information on running LVM in a clustered environment.

4.3.1. LVM Mirror Improvements

LVM supports mirrored volumes. By creating mirrored logical volumes, LVM ensures that data written to an underlying physical volume is mirrored onto a separate physical volume.
4.3.1.1. Snapshots of Mirrors
The LVM snapshot feature provides the ability to create backup images of a logical volume at a particular instant without causing a service interruption. When a change is made to the original device (the origin) after a snapshot is taken, the snapshot feature makes a copy of the changed data area as it was prior to the change so that it can reconstruct the state of the device. Red Hat Enterprise Linux 6 introduces the ability to take a snapshot of a mirrored logical volume.
4.3.1.2. Merging Snapshots
Red Hat Enterprise Linux 6 introduces the ability to merge a snapshot of a logical volume back into the origin logical volume. This allows system administrators to revert any changes that have occurred on a logical volume by merging back to the point preserved by a snapshot.
For more information about the new snapshot merge feature, consult the lvconvert manpage.
4.3.1.3. Four-Volume Mirrors
LVM in Red Hat Enterprise Linux 6 supports creating a logical volume with up to four mirrors.
4.3.1.4. Mirroring mirror logs
LVM maintains a small log (on a separate device) which it uses to keep track of which regions are in sync with the mirror or mirrors. Red Hat Enterprise Linux 6 provides the ability to mirror this log device.

4.3.2. LVM Application Library

Red Hat Enterprise Linux 6 features the new LVM Application Library (lvm2app), allowing the development of LVM based storage management applications.