Microsoft Word Hardware Reconfiguration Methodology V final2

Filesystem checking for backups and data restoration

1. Filesystems

All filesystems, no matter how robust, modern, or technologically savvy are all at risk for data loss and/or corruption. The most likely time for this to occur is during disk data writes that simultaneously occur with a power outage or fluctuation. Although modern filesystems are able to reduce this danger by incorporating filesystem journaling, the risk is always present. In addition, unrecoverable hardware errors and kernel panics may also be a likely culprit. This section therefore examines various filesystem-checking tasks that should be performed prior to undertaking any backup-related action.

2. Issues

Filesystem checking, although very important for backups are equally important for data restoration. Restoring files from a good backup to a damaged filesystem could result in the overwriting of existing files or even further corrupt the filesystem. Similarly, a corrupt backup due to filesystem damage will likely result in missing files upon restoration, assuming that the archive is not corrupt because of the damaged filesystem. Some readers may assume that these issues are exaggerated; however, a simple lookup on the web for filesystem errors with respect to backups and restoration will yield much useful information.

3. Periodic checks

Filesystems must be checked periodically; even the simplest filesystem inconsistency can cause file damage. It is also possible that severely damaged filesystems may be missing files or they may become inaccessible due to damaged permissions and/or ACL’s. The only way to verify the

status of a filesystem and its files is to perform a filesystem check. Linux provides the necessary tools for all of its supported filesystem formats, the most notable of which is Fsck. However, certain filesystems are more sensitive than others are. For example, Ext2 is far more sensitive to power outages and incomplete data writes as compared to its modern enhancement, the Ext3 filesystem. Although similar, Ext3 employs a journaling mechanism that ensures safe data writes resulting in a more consistent filesystem.

In addition, certain backup tools are more sensitive to filesystem errors than others are. Dump and Restore are particularly sensitive as they require (Dump) and recreate (Restore) filesystem data structures. These data structures are stored using filesystem i-nodes and damage to them can result in damaged or inconsistent backups. It is also possible for backups to crash and result in incomplete archives due to filesystem errors; this can occur for all the backup tools (except DD). However, Tar and Cpio are generally not as severely affected by errors as Dump and Restore are. Nevertheless, all these tools can suffer from filesystem inconsistencies. DD, however, is not susceptible to filesystem errors as it will simply copy over all filesystem errors to its image file that can when mounted (DD images can be mounted) be checked.

4.