4. Storage and Filesystems

The ext4 Filesystem

The ext4 file system is a scalable extension of the ext3 file system, which was the default file system of Red Hat Enterprise Linux 5. Ext4 is now the default file system of Red Hat Enterprise Linux 6

Because of delayed allocation and other performance optimizations, ext4's behavior of writing files to disk is different from ext3. In ext4, a program's writes to the file system are not guaranteed to be on-disk unless the program issues an fsync() call afterwards.

Further information on the allocation features of ext4 is available in the Storage Administration Guide

CIFS servers that require plaintext passwords

Some Common Internet File System (CIFS) servers require plaintext passwords for authentication. Support for plaintext password authentication can be enabled using the command:

echo 0x37 > /proc/fs/cifs/SecurityFlags

Warning

This operation can expose passwords by removing password encryption.

Event Tracing in GFS2

GFS2's event tracing is provided via the generic tracing infrastructure. The events are designed to be useful for debugging purposes. Note, however that it is not guaranteed that the GFS2 events will remain the same throughout the lifetime of Red Hat Enterprise Linux 6. Further details on GFS2's glocks and event tracing can be found in the following 2009 Linus Symposium paper: http://kernel.org/doc/ols/2009/ols2009-pages-311-318.pdf

mpi-selector

The mpi-selector package has been deprecated in Red Hat Enterprise Linux 6. environment-modules is now used to select which Message Passing Interface (MPI) implementation is to be used.

Note

The man page for the module command contains detailed documentation for the environment-modules package.

To return a list of what modules are available, use:

module avail

To load or unload a module use the following commands:

module load <module-name>
module unload <module-name>

To emulate the behavior of mpi-selector, the module load commands must be place in the shell init script (e.g. /.bashrc) to load the modules every login.

4.1. Technology Previews

fsfreeze: Red Hat Enterprise Linux 6 includes fsfreeze as a Technology Preview. fsfreeze is a new command that halts access to a filesystem on disk. fsfreeze is designed to be used with hardware RAID devices, assisting in the creation of volume snapshots. Further details on fsfreeze are in the fsfreeze(8) man page.
DIF/DIX support: DIF/DIX, is a new addition to the SCSI Standard and a Technology Preview in Red Hat Enterprise Linux 6. 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 receive, and stores both the data and the checksum. Conversely, when a read occurs, the checksum can be checked by the storage device, and by the receiving HBA.
The DIF/DIX hardware checksum feature must only be used with applications that exclusively issue O_DIRECT I/O. These applications may use the raw block device, or the XFS file system in O_DIRECT mode. (XFS is the only filesystem that does not fall back to buffered IO when doing certain allocation operations.) Only applications designed for use with O_DIRECT I/O and DIF/DIX hardware should enable this feature. Red Hat Enterprise Linux 6 includes the Emulex LPFC driver version 8.3.5.17, introducing support for DIF/DIX. For more information, refer to the Storage Administration Guide
Filesystem in Userspace: Filesystem in Userspace (FUSE) allows for custom filesystems to be developed and run in user-space.
LVM 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.
A known issue exists with this Technology Preview. I/O might hang if a device failure in the mirror is encountered. Note, that this issue is related to a failure of the mirror log device, and that no work around is currently known.
btrfs: Btrfs is under development as a file system capable of addressing and managing more files, larger files, and larger volumes than the ext2, ext3, and ext4 file systems. Btrfs is designed to make the file system tolerant of errors, and to facilitate the detection and repair of errors when they occur. It uses checksums to ensure the validity of data and metadata, and maintains snapshots of the file system that can be used for backup or repair. The btrfs Technology Preview is only available on the x86_64 architecture.
Btrfs is still experimental
Red Hat Enterprise Linux 6 Beta includes Btrfs as a technology preview to allow you to experiment with this file system. You should not choose Btrfs for partitions that will contain valuable data or that are essential for the operation of important systems.
LVM Application Programming Interface (API): Red Hat Enterprise Linux 6 Beta features the new LVM application programming interface (API) as a Technology Preview. This API is used to query and control certain aspects of LVM.
FS-Cache: FS-Cache is a new feature in Red Hat Enterprise Linux 6 Beta that enables networked file systems (e.g. NFS) to have a persistent cache of data on the client machine.