TS7700 Cache thresholds and removal policies
This topic describes the boundaries (thresholds) of free cache space in a TS7700 and the policies that can be used to manage available (active) cache capacity in a grid configuration.
Cache thresholds for a disk-only TS7700 Cluster
There are three thresholds that define the capacity of cache partition 0 (CP0) in a TS7700 Tape Attach and the active cache capacity in a disk-only TS7700. (Because a disk-only TS7700 does not attach to a physical backend library, all of its virtual volumes are stored in the cache.)
- Automatic removal
- By default this state occurs when the cache is 3 TB below the
out-of-cache-resources threshold. In the automatic removal state,
the TS7700 automatically removes volumes from the disk-only cache
to prevent the cache from reaching its maximum capacity. This state
is identical to the limited-free-cache-space-warning state unless
the Temporary Removal Threshold is enabled. Note:
- To perform removal operations in a TS7700 Tape Attach Cluster, the size of
cache partition 0 (CP0) must be at least 10 TB. You can disable automatic removal within any
given disk-only TS7700 Cluster
by using the following library request command:
LIBRARY REQUEST library-name,SETTING,CACHE,REMOVE,{ENABLE|DISABLE}
- The default automatic removal threshold can be changed from the command line, by using the
following library request
command:
LIBRARY REQUEST library-name,SETTING,CACHE,REMVTHR,{VALUE}
- To perform removal operations in a TS7700 Tape Attach Cluster, the size of
cache partition 0 (CP0) must be at least 10 TB. You can disable automatic removal within any
given disk-only TS7700 Cluster
by using the following library request command:
- Limited free cache space warning
- This state occurs when there is less than 3 TB of free space left in the cache. When the cache
passes this threshold and enters the limited-free-cache-space-warning state, write operations can
use only an additional 2 TB before the out-of-cache-resources state is encountered. When a disk-only
TS7700 Cluster enters the
limited-free-cache-space-warning state, it remains in this state until the amount of free space in
the cache exceeds 3.5 TB. Messages that can be displayed on the management interface during the
limited-free-cache-space-warning state include:
- HYDME0996W
- HYDME1200W
Note:- LMTDTHR was introduced in White Paper - TS7700 Library Request Command.
- Host writes to the TS7700 Cluster and inbound copies continue during this state.
- Out of cache resources
- This state occurs when there is less than 1 TB of free space left
in the cache. When the cache passes this threshold and enters the
out-of-cache-resources state, it remains in this state until the amount
of free space in the cache exceeds 3.5 TB. When a disk-only TS7700
Cluster is in the out-of-cache-resources state, volumes on that cluster
become read-only and one or more out-of-cache-resources messages are
displayed on the management interface. These messages can include:
- HYDME0997W
- HYDME1133W
- HYDME1201W
Note: New host allocations do not choose a disk-only TS7700 Cluster in this state as a valid tape volume cache candidate. New host allocations that are issued to a disk-only TS7700 Cluster in this state choose a remote tape volume cache instead. If all valid clusters are in this state or unable to accept mounts, the host allocations fail. Read mounts can choose the disk-only TS7700 Cluster in this state, but modify and write operations fail. Copies inbound to this cluster are queued as deferred until the cluster exits this state.
State | Enter state (free space available) |
Exit state (free space available) |
Host message displayed |
---|---|---|---|
Automatic removal | <4 TB | >4.5 TB | CBR3750I when automatic removal begins |
Limited free cache space warning (CP0 for a TS7700 Tape Attach) | ≤3 TB or ≤15% of the size of cache partition 0, whichever is less | >3.5 TB or >17.5% of the size of cache partition 0, whichever is less | CBR3792E upon entering state CBR3793I upon exiting state |
Out of cache resources (CP0 for a TS7700 Tape Attach) | <1 TB or ≤5% of the size of cache partition 0, whichever is less | >3.5 TB or >17.5% of the size of cache partition 0, whichever is less | CBR3794A upon entering state CBR3795I upon exiting state |
Temporary removal1 | <(X + 1 TB)2 | >(X + 1.5 TB)2 | Console message |
Notelist:
|
Volume removal policies in a grid configuration
Removal polices determine when virtual volumes are removed from the cache of a TS7700 Cluster in a grid configuration. These policies provide more control over the removal of content from a TS7700 Cache as the active data reaches full capacity. To perform removal operations in a TS7700 Tape Attach Cluster, the size of cache partition 0 (CP0) must be at least 10 TB.
A TS7700 Tape Attach Cluster can have up to 7 tape-attached partitions. When a new partition is created, the resulting partition 0 (CP0) must have 2 TB free space. The temporary removal function, working with the volume removal policies you have defined, can be used to release the required space in CP0 (at least 2 TB plus the size of the new partition).
- Pinned
- The copy of the volume is never removed from this TS7700 Cluster. The pinning duration is not applicable and is implied as infinite. When a pinned volume is moved to scratch, it becomes a priority candidate for removal similarly to the next two policies. This policy must be used cautiously to prevent TS7700 Cache overruns.
- Prefer Remove - When Space is Needed Group 0 (LRU)
- The copy of a private volume is removed if an appropriate number of copies exists on peer clusters, the pinning duration (in x hours) has elapsed since last access, and the available free space on the cluster has fallen below the removal threshold. The order of which volumes are removed under this policy is based on their least recently used (LRU) access times. Volumes in Group 0 are removed before the removal of volumes in Group 1 except for any volumes in scratch categories, which are always removed first. Archive and backup data would be a good candidate for this removal group since it will not likely be accessed once written.
- Prefer Keep - When Space is Needed Group 1 (LRU)
- The copy of a private volume is removed if an appropriate number of copies exists on peer clusters, the pinning duration (in x hours) has elapsed since last access, the available free space on the cluster has fallen below a threshold, and LRU group 0 has been exhausted. The order of which volumes are removed under this policy is based on their least recently used (LRU) access times. Volumes in Group 0 are removed before the removal of volumes in Group 1 except for any volumes in scratch categories, which are always removed first.
Prefer Remove and Prefer Keep are similar to cache preference groups PG0 and PG1 with the exception that removal treats both groups as LRU versus using the volume size.
In addition to these policies, volumes that are assigned to a scratch category that have not been previously delete-expired are also removed from cache when the free space on a cluster has fallen below a threshold. Scratch category volumes, regardless of what their removal policies are, are always removed before any other removal candidates in volume size descending order. Pin time is also ignored for scratch volumes. Only when the removal of scratch volumes does not satisfy the removal requirements will Group 0 and Group 1 candidates be analyzed for removal. The requirement for a scratch removal is that an appropriate number of volume copies exist elsewhere. If one or more peer copies cannot be validated, the scratch volume is not removed.
- (Permanent) Removal Thresholds
- The default, or permanent, Removal Threshold is used to prevent a cache overrun condition in a
TS7700 Cluster that is configured as part of a grid. By default it is a 4 TB value (3 TB fixed plus
1 TB) that, when taken with the amount of used cache, defines the upper size limit for a TS7700
Cache or for a TS7700 Tape Attach CP0. Above this threshold, virtual volumes begin to be removed
from a TS7700 Cache. Virtual volumes are removed from a TS7700 Cache in this order:
- Volumes in scratch categories
- Private volumes least recently used, using the enhanced removal policy definitions
- Temporary Removal Thresholds
- The Temporary Removal Threshold lowers the default Removal Threshold to a value lower than the Stop Threshold in anticipation of a Service mode event.