Administrator's Guide


Reclaiming Space in Sequential Access Storage Pools

Space on a sequential volume becomes reclaimable as files expire or are deleted from the volume. For example, files become obsolete because of aging or limits on the number of versions of a file. Space within an aggregate file also becomes reclaimable. An aggregate is a physical file that contains multiple logical files backed up or archived from a client in a single transaction. Space within the file becomes reclaimable space as logical files in the aggregate expire or are deleted by the client.

ADSM reclaims the space in storage pools based on a reclamation threshold that you can set for each sequential access storage pool. See the following sections:

"How ADSM Reclamation Works"

"Choosing a Reclamation Threshold"

"Reclaiming Volumes in a Storage Pool with One Drive"

"Reclamation for WORM Optical Media"

"Reclamation of Volumes with the Device Type of SERVER"

"Reclamation for Copy Storage Pools"

"How Collocation Affects Reclamation"

How ADSM Reclamation Works

When the percentage of reclaimable space on a volume exceeds the reclamation threshold set for the storage pool, the volume is eligible for reclamation. The server checks whether reclamation is needed at least once per hour and begins space reclamation for eligible volumes. You can set a reclamation threshold for each sequential access storage pool when you define or update the pool.

When multiple volumes are eligible for reclamation, ADSM reclaims the eligible volumes in random order. During space reclamation, the server copies files that remain on eligible volumes to other volumes in the same or a different storage pool. For example, Figure 22 shows the files from tapes 1, 2, and 3, being consolidated on tape 4.

Space within aggregate files is also reclaimed during the reclamation process. Unused space from expired or deleted logical files is removed as the aggregate file is copied to another volume during reclamation processing.

Figure 22. Tape Reclamation

Tape Reclamation


After ADSM moves all readable files to other volumes, one of the following occurs for the reclaimed volume:

Volumes that have a device type of SERVER are reclaimed in the same way as other sequential access volumes. However, because the volumes are actually data stored in the storage of another ADSM server, the reclamation process can consume network resources. See "Reclamation of Volumes with the Device Type of SERVER" for details of how ADSM reclaims these types of volumes.

Volumes in a copy storage pool are reclaimed in the same manner as a primary storage pool except for the following:

See "Reclamation for Copy Storage Pools" for details.

Choosing a Reclamation Threshold

The reclamation threshold indicates how much reclaimable space a volume must have before ADSM reclaims the volume. Space is reclaimable because it is occupied by files that have been expired or deleted from the ADSM database, or because the space has never been used.

The server checks whether reclamation is needed at least once per hour. The lower the reclamation threshold, the more frequently the server tries to reclaim space. Frequent reclamation optimizes the use of a sequential access storage pool's space, but can interfere with other processes, such as backups from clients.

If the reclamation threshold is high, reclamation occurs less frequently. A high reclamation threshold is useful if mounting a volume is a manual operation and the operations staff is at a minimum.

Each reclamation process requires at least two simultaneous volume mounts, that is, at least two mount points (drives). The two drives must be in the same device class, unless the volumes' storage pool has been defined with a reclamation storage pool. There must be a sufficient number of volumes, drives (if appropriate), and mount operators (if appropriate) to handle frequent reclamation requests. For more information about mount limit, see "Mount Limit". For information about how to use a reclamation storage pool for storage pools with only one mount point, see "Reclaiming Volumes in a Storage Pool with One Drive".

If you set the reclamation threshold to 50% or greater, ADSM can combine the usable files from two or more volumes onto a single new volume.

Setting the reclamation threshold to 100% prevents reclamation from occurring at all. You might want to do this to control when reclamation occurs, to prevent interfering with other server processes. When convenient for you and your users, you can lower the reclamation threshold to cause reclamation to begin.

Lowering the Migration Threshold

If you have been running with a high migration threshold and decide you now need to reclaim volumes, you may want to lower the threshold in several steps. Lowering the threshold in steps ensures that volumes with the most reclaimable space are reclaimed first. For example, if you had set the high migration threshold to 100%, first lower the threshold to 98%. Volumes that have reclaimable space of 98% or greater are reclaimed by the server. Lower the threshold again to reclaim more volumes.

If you lower the reclamation threshold while a reclamation process is active, the reclamation process does not immediately stop. If an onsite volume is being reclaimed, ADSM uses the new threshold setting when the process begins to reclaim the next volume. If offsite volumes are being reclaimed, ADSM does not use the new threshold setting during the process that is running (because all eligible offsite volumes are reclaimed at the same time).

Use the CANCEL PROCESS command to stop a reclamation process.

Reclaiming Volumes in a Storage Pool with One Drive

When a storage pool has only one mount point (that is, just one drive) available to it through the device class, data cannot be reclaimed from one volume to another within that same storage pool.

To enable volume reclamation for a storage pool that has only one mount point, you can define a reclamation storage pool for ADSM to use when reclaiming volumes. When ADSM reclaims volumes, ADSM moves the data from volumes in the original storage pool to volumes in the reclamation storage pool. ADSM always uses the reclamation storage pool when one is defined, even when the mount limit is greater than one.

If the reclamation storage pool does not have enough space to hold all of the data being reclaimed, ADSM moves as much of the data as possible into the reclamation storage pool. Any data that could not be moved to volumes in the reclamation storage pool still remains on volumes in the original storage pool.

The pool identified as the reclamation storage pool must be a primary sequential storage pool. The primary purpose of the reclamation storage pool is for temporary storage of reclaimed data. To ensure that data moved to the reclamation storage pool eventually moves back into the original storage pool, specify the original storage pool as the next pool in the storage hierarchy for the reclamation storage pool. For example, if you have a tape library with one drive, you can define a storage pool to be used for reclamation using a device class with a device type of FILE:

define stgpool reclaimpool fileclass maxscratch=100

Define the storage pool for the tape drive as follows:

define stgpool tapepool1 tapeclass maxscratch=100
reclaimstgpool=reclaimpool

And finally update the reclamation storage pool so that data migrates back to the tape storage pool:

update stgpool reclaimpool nextstgpool=tapepool1

Reclamation for WORM Optical Media

Because WORM optical media cannot be reused, storage pools that are assigned to device classes with a device type of WORM, WORM12, or WORM14 have a default reclamation value of 100. This prevents reclamation of WORM optical media.

To allow reclamation, you can set the reclamation value to something lower when defining or updating the storage pool. Reclaiming WORM volumes does not allow reuse, but does allow you to free library space by consolidating data on almost empty volumes to other volumes, and then ejecting the empty WORM volumes.

Reclamation of Volumes with the Device Type of SERVER

When virtual volumes (volumes in a primary storage pool with the device type of SERVER) are reclaimed, the client data stored on those volumes is sent across the network between the source server and the target server. As a result, the reclamation process can tie up your network resources. To control when reclamation starts for these volumes, consider setting the reclamation threshold to 100% for any primary storage pool using SERVER device type volumes. Lower the reclamation threshold at a time when your network is least busy, so that ADSM can reclaim volumes.

If you use virtual volumes for a copy storage pool, reclamation occurs without data being sent across the network. The ADSM server reclaims a volume as follows:

  1. The server determines which files on the volume are still valid.

  2. These valid files are obtained from a primary storage pool, or if necessary, from an onsite volume of a copy storage pool.

  3. The files are written to one or more new volumes in the copy storage pool and the database is updated.

  4. A message is issued indicating that the volume was reclaimed.

For information about using the SERVER device type, see Chapter 4. "Storing Data on Another Server".

Reclamation for Copy Storage Pools

Reclamation of primary storage pool volumes does not affect copy storage pool files.

Reclamation of volumes in copy storage pools is similar to that of primary storage pools. One difference, however, is that most volumes in copy storage pools may be set to an access mode of offsite, making them ineligible to be mounted. During reclamation, valid files on offsite volumes are copied from the original files in the primary storage pools. In this way, valid files on offsite volumes are copied without having to mount these volumes. For more information, see "Reclamation of Offsite Volumes".

Reclamation of copy storage pool volumes should be done periodically to allow reuse of partially filled volumes that are offsite. Reclamation can be done automatically by setting the reclamation threshold for the copy storage pool to less than 100%. However, you need to consider controlling when reclamation occurs because of how offsite volumes are treated. For more information, see "Controlling When Reclamation Occurs for Offsite Volumes".
Virtual Volumes:Virtual volumes (volumes that are stored on another ADSM server through the use of a device type of SERVER) cannot be set to the offsite access mode.

Reclamation of Offsite Volumes

As for volumes with other access values, volumes with the access value of offsite are eligible for reclamation if the amount of empty space on a volume exceeds the reclamation threshold for the copy storage pool. The default reclamation threshold for copy storage pools is 100%, which means that reclamation is not performed.

When an offsite volume is reclaimed, the files on the volume are rewritten to a read/write volume. Effectively these files are moved back to the onsite location. The files may be obtained from the offsite volume after a disaster, if the volume has not been reused and the database backup that you use for recovery references the files on the offsite volume.

The ADSM server reclaims an offsite volume as follows:

  1. The server determines which files on the volume are still valid.

  2. These valid files are obtained from a primary storage pool, or if necessary, from an onsite volume of a copy storage pool.

  3. The files are written to one or more volumes in the copy storage pool and the database is updated. If a file is an aggregate file with unused space, the unused space is removed during this process.

  4. A message is issued indicating that the offsite volume was reclaimed.

    For a single storage pool, the server reclaims all offsite volumes that are eligible for reclamation at the same time. Reclaiming all the eligible volumes at the same time minimizes the tape mounts for primary storage pool volumes.

If you have the Disaster Recovery Manager feature, see "Moving Reclaimed or Expired Volumes Back Onsite".

Controlling When Reclamation Occurs for Offsite Volumes

Suppose you plan to make daily storage pool backups to a copy storage pool, then mark all new volumes in the copy storage pool as offsite and send them to the offsite storage location. This strategy works well with one consideration if you are using automatic reclamation (the reclamation threshold is less than 100%).

Each day's storage pool backups will create some number of new copy storage pool volumes, the last one being only partially filled. If this partially filled volume is emptier than the reclaim percentage, this volume becomes eligible for reclamation as soon as you mark it offsite. The reclamation process would cause a new volume to be created with the same files on it. The volume you take offsite would then be empty according to the ADSM database. If you do not recognize what is happening, you could perpetuate this process by marking the new partially filled volume offsite.

One way to resolve this situation is to keep partially filled volumes onsite until they fill up. However, this would mean a small amount of your data would be without an offsite copy for another day.

If you send copy storage pool volumes offsite, it is recommended you control copy storage pool reclamation by using the default value of 100. This turns reclamation off for the copy storage pool. You can start reclamation processing at desired times by changing the reclamation threshold for the storage pool. To monitor offsite volume utilization and help you decide what reclamation threshold to use, enter the following:

query volume * access=offsite format=detailed

Depending on your data expiration patterns, you may not need to do reclamation of offsite volumes each day. You may choose to perform offsite reclamation on a less frequent basis. For example, suppose you ship copy storage pool volumes to and from your offsite storage location once a week. You can run reclamation for the copy storage pool weekly, so that as offsite volumes become empty they are sent back for reuse.

When you do perform reclamation for offsite volumes, the following sequence is recommended:

  1. Back up your primary storage pools to copy storage pools

  2. Turn on reclamation for copy storage pools by lowering the reclamation threshold

  3. When reclamation processing completes, turn off reclamation for copy storage pools by raising the reclamation threshold to 100%

  4. Mark any newly created, copy storage pool volumes as offsite and then move them to the offsite location

This sequence ensures that the files on the new copy storage pool volumes are sent offsite, and are not inadvertently kept onsite because of reclamation.

Using Storage on Another Server for Copy Storage Pools

Another resolution to this problem of partially filled volumes is to use storage on another ADSM server (device type of SERVER) for storage pool backups. If the other server is at a different site, the copy storage pool volumes are already offsite, with no moving of physical volumes between the sites. See Chapter 4. "Storing Data on Another Server" for more information.

Delaying Reuse of Reclaimed Volumes

You should delay the reuse of any reclaimed volumes in copy storage pools for as long as you keep your oldest database backup. Delaying reuse may help you to recover data under certain conditions during recovery from a disaster. For more information on delaying volume reuse, see "Delaying Reuse of Sequential Access Volumes".

How Collocation Affects Reclamation

If collocation is enabled and reclamation occurs, the server tries to reclaim the files for each client node or client file space onto a minimal number of volumes. Therefore, if the volumes are manually mounted, the mount operators must:

If collocation is disabled and reclamation occurs, the server tries to move usable data to new volumes by using the following volume selection criteria:

  1. The volume that contains the most data
  2. Any partially full volume
  3. An empty predefined volume
  4. An empty scratch volume


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