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36 Dell EMC SC Series: Red Hat Enterprise Linux Best Practices | CML1031
On RHEL 6 systems, dm-multipath might not get installed and configured during the OS installation. This
prevents dm-multipath to capture and manage the Boot-From-SAN volume after the OS is installed. To
correct this:
1. Install and configure device-mapper-multipath package. See section 3.6.
2. Rebuild initramfs image with multipath.
# dracut –v –f –a multipath –include /etc/multipath /etc/multipath
3. Reboot the system.
4. multipath –ll now shows the boot-from-SAN volume. The pvs command shows the multipath device
instead of the sdX device for the OS volume group.
3.12 Snapshots
A snapshot is a point-in-time copy of a volume. Taking a snapshot on a volume makes the pages on the
volume read-only. Any changes to the existing page (data) get written to new pages. Unlike other storage
systems, snapshots do not require any space reservation on the SC Series system. It provides a fast and
space-efficient way to protect data and applications on Linux systems.
3.12.1 Snapshot guidelines
When using snapshots, consider the following guidelines:
• All volumes of an application or database must be protected as a set using the consistency group
feature. The consistency group will ensure that the snapshot is taken at the exact same time on all
volumes in that group.
• Snapshots do not replace application-level backups. However, they can be used in conjunction with
other backup solutions to offer additional protection to the data. They also can offload backup
processing to an alternate system. For example, an administrator can take snapshots on the primary
system, create view volumes from the snapshots, and mount them on an alternate system to run
backup jobs or other resource-intensive processing.
• Snapshots can be taken on demand manually or automatically based on a single or multiple
schedules.
• To ensure data integrity on the snapshots, quiesce the application to achieve a consistent state
before taking the snapshot. See the following example to quiesce an xfs filesystem:
# xfs_freeze –f / #Freeze a xfs system mounted on /
# xfs_freeze –u / #Unfreeze the / file system
For a database application such as Oracle, it is recommended to put the database in hot backup
mode before taking a snapshot and end backup mode after the snapshot is taken. Consult the
corresponding application documentation for details, or the Dell EMC SC Series Arrays and Oracle
best practices document.
• Set an expiration policy on snapshots that meets the recovery point objective. Expired snapshots are
deleted automatically to reclaim space. However, the snapshots are protected from deletion if there
are existing view volumes created from them. These expired snapshots are retained until all the
associated view volumes are deleted.
• Applications can access the point-in-time snapshot data only via view volumes. A view volume is a
volume created from an existing snapshot.