Migration candidates

HSM for Windows™ client chooses larger and older files as candidates for threshold migration.

Files that are frequently modified or accessed are poor candidates for migration. HSM for Windows client assumes that the last access date or modification date or creation date is an indicator of how dynamic a file is. Hence, HSM for Windows client chooses migration candidates that have a greater age, as measured by access, modification, or creation date. You configure which of these dates (access, modification, or creation) HSM for Windows client uses to determine file age. You also configure the minimum age for a migration candidate. Among files that meet the minimum age, and are the same size, HSM for Windows client migrates only the oldest files.

Small files are also not good candidates for migration because migrating a small file frees up less space than migrating a large file. There is a transaction cost for every file migration and recall. The transaction cost is the same regardless of file size, even though migrating larger files frees up more space. Hence, HSM for Windows client chooses large files for migration candidates. You can configure the minimum size for a migration candidate, but among files with the same age, HSM for Windows client migrates only the largest files.

You can also configure the weight (importance) of age, relative to size, for migration candidates. For example, assume that your volume contains some large files that tend to be dynamic. You can decrease the chance that the files will be migrated by increasing the weight of file age for migration candidates.

To find the migration candidates, HSM for Windows client scans the volume. HSM for Windows client scans all directories in the volume in an orderly manner, but typically not all at once. A scan continues until enough migration candidates are found. The next scan starts where the previous scan finished, until, over time, the entire volume can be scanned. Further scans traverse the volume again and again. You can configure how often to scan for migration candidates.

If not enough migration candidates are found, HSM for Windows client can scan the entire volume in a single scan. If the entire volume is scanned without yielding sufficient candidates, HSM for Windows client issues a warning. At the next scan, it is possible that the size or age of some files will qualify them for migration.

The size and age of the files in the most recent scan are compared with the files in the migration pool. The comparison yields a new ranked list of migration candidates. The oldest and largest files are the first on the list.

A scan begins in these situations:

• The configured time interval since the last scan elapses.
• You manually start a scan.
• Before a threshold migration, the pool does not contain enough files to reduce the space usage from the high threshold to the low threshold.
• During a threshold migration, the pool of migration candidates becomes empty.

Migration candidates are stored in a pool, ready to be migrated when space usage reaches the high threshold. Before a migration, there must be enough migration candidates in the pool to reduce the space usage from the high threshold to the low threshold.

The pool contains more files than are needed, in case some candidates are no longer valid by the time of the next threshold migration. A file in the pool can loose migration eligibility for several reasons:

• The file was deleted from the file system.
• The file was modified, and it no longer meets the minimum age or the minimum size for migration.
• The configured minimum age or minimum size for migration was increased.

Periodically HSM for Windows client validates the files in the pool. Files that are no longer valid are eliminated from the pool. If the pool does not contain enough files to reduce the space usage from the high threshold to the low threshold, a scan begins. You can configure the frequency of the validation.