560 - Understanding storage device access limitations during ESE backups

The ESE Agent uses the backup selection and profiling settings to determine which files and folders will be backed up.

To understand the limitations in this process, take note of the following terms:

• Storage device: In the context of Redstor data backups, a physical disk or a UNC network storage location.
• Volume: A single accessible storage area resident on one or more partitions of one or more physical disks (a.k.a. a logical drive or a "mount"). A single volume can therefore span multiple physical disks and a single physical disk can contain multiple volumes.
• Drive letter: As one of the methods of making the file system (the logical and hierarchical grouping of data) on a volume accessible, the Windows operating system assigns a drive letter to it, e.g. C:, D:, E:, etc.
• Mount point: A reference in a volume that points to another volume.

 The limitations are:

1. During the backup process, up to four volumes on four different storage devices on can be backed up in parallel.
2. However, when two or more volumes share the same storage device, these volumes will be processed consecutively.
3. The same principle applies to volumes mounted within volumes. Where mount points refer to items on volumes that reside on the same storage device, the volumes will also be processed one after the other.

Note: The physical disk performance depends on the read pattern. A sequential read pattern tends to be faster than a random read pattern. This is especially true for hard disk drives, but it also affects solid state drives. If two volumes are processed simultaneously it will effectively degrade disk performance because the read pattern will be more random than sequential. When the ESE application processes a big file it will use more than one thread to process that file, but only one of those threads will be tasked to read data. ESE will use additional threads to process the data and send the result to the Storage Platform.

Example scenario:

• Volumes C: and E: will be processed simultaneously because they do not share the same physical disks. Therefore D: and the "\\fileserver\share" network share will be processed thereafter.
• Volume MyDrive has no drive letter assigned to it but is indirectly included in the backup selection via a mount point from E: and shares the same physical disk. Volumes D: and MyDrive will be backed up simultaneously but E: will be backed up separately from MyDrive.

Also take note of the following:

• Mounted drive images like ISO files are treated as removable disks and will not be backed up.
• Progress bars in the Agent are displayed per volume being backed up.
• Cyclical references of mount points between volumes are not valid if included in the backup selection and will result in a failed backup, e.g. C: points to D: (via mount point) which points back to C: (via mount point).