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Managing Plancache Memory and Disk Usage

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Compiled query plan images, sometimes referred to as 'modules' or 'code modules', are stored in a plancache for later use. The plancache consists of both an in-memory plancache and an on-disk plancache, also known as the persistent plancache or PPC. Thus the plancache incrementally consumes both memory and disk storage space. This topic addresses managing that memory and storage space. Refer to Code Generation for more information on the plancache.

Compiled query plan images may be removed from the in-memory plancache through expiration or eviction. When a plan expires from the in-memory plan cache (called expiration), the plan is removed fully from the in-memory plancache so that when that plan is used again it must be fully reloaded from disk. When a plan is evicted (called eviction), the plan is removed from the in-memory plancache, but a lightweight entry is left behind, so that the query plan can be reloaded from disk in a much cheaper manner than for an expired plan. That is, an evicted plan is effectively still in the in-memory plancache, while an expired query plan is not.

Whether a plan expires or is evicted from the in-memory plancache, that plan remains in the on-disk plancache and is loaded back into memory the next time the query is run. Evicted plans will re-load more quickly than expired plans, but in neither case will the plan be re-compiled. Further, when a node restarts, the in-memory plancache starts off empty and plans are loaded back in from the on-disk plancache as queries are run.

Query plans are not recompiled after a plan is evicted or expires from the in-memory plancache or after a node restarts.

Query plans expire from the on-disk plancache as described in Set Plan Expiration Limits. When a plan expires from the on-disk plancache, it is completely removed from the plancache and the query must be recompiled when it is run again. In addition, query plans can be manually removed from the in-memory plancache as described in Drop a plan from the in-memory plancache. Finally, query plans can be deleted from the on-disk plancache as described in Delete Plancache Files.

Set Plan Expiration Limits

Plancache expiration policy can be set through the plan_expiration_minutes, disk_plan_expiration_minutes, and enable_disk_plan_expiration engine variables. Expiration for the in-memory plancache is always enabled.

Plans will expire from the on-disk plancache if both the enable_disk_plan_expiration is on and the time limit specified by disk_plan_expiration_minutes is reached (assuming the plan has not been read from disk during that time). This restriction also applies to temporary (temp) tables. SingleStore recommends re-using the same name for temporary tables to the degree possible.

Note

As engine variables are scoped at the node level (and not across the cluster), if you update any of these plan expiration variables, you must update the values across all nodes in your cluster. See Engine Variables for more information.

The following example shows how to set these variables to change the in-memory expiration limit to eight hours and set the on-disk expiration to one week.

SHOW VARIABLES LIKE '%plan_expiration%';
+------------------------------+-------+
| Variable_name                | Value |
+------------------------------+-------+
| disk_plan_expiration_minutes | 20160 |
| enable_disk_plan_expiration  | OFF   |
| plan_expiration_minutes      | 720   |
+------------------------------+-------+
3 rows in set (0.08 sec)
SET GLOBAL plan_expiration_minutes = 480;
Query OK, 0 rows affected (0.07 sec)
SET GLOBAL disk_plan_expiration_minutes = 10080;
Query OK, 0 rows affected (0.09 sec)
SET GLOBAL enable_disk_plan_expiration = true;
Query OK, 0 rows affected (0.08 sec)
SHOW VARIABLES LIKE '%plan_expiration%';
+------------------------------+-------+
| Variable_name                | Value |
+------------------------------+-------+
| disk_plan_expiration_minutes | 10080 |
| enable_disk_plan_expiration  | ON    |
| plan_expiration_minutes      | 480   |
+------------------------------+-------+
3 rows in set (0.07 sec)

Manage Eviction of Compiled Query Plans

Compiled query plan images (sometimes referred to as 'modules' or 'code modules') are evicted using LRU (least-recently used) when the in-memory plancache reaches a set memory limit. The compiled_images_eviction_memory_limit_percent and compiled_images_eviction_memory_limit_mb variables are used to specify the memory limit available for caching query plan images. Query plan images are evicted when the enable_compiled_images_eviction engine variable is set to ON, which is the default setting.

If the compiled_images_eviction_memory_limit_percent and compiled_images_eviction_memory_limit_mb variables are not set or are set to 0, compiled_images_eviction_memory_limit_percent will be set as a percentage of maximum_memory as shown in the following table.

maximum_memory (GB)

In-memory Plancache Memory Limit

Setting of compiled_images_eviction_memory_limit_percent

0-1

Prohibited ( maximum_memory minimum is 1GB)

1-64

10% of maximum_memory

64+

5% of maximum_memory

If the compiled_images_eviction_memory_limit_mb or compiled_images_eviction_memory_limit_percent variables are set, the memory limit for the in-memory plancache is derived at runtime as shown in the following table.

The notation _mb denotes compiled_images_eviction_memory_limit_mb and _percent denotes compiled_images_eviction_memory_limit_percent.

_percent

_mb

In-memory Plancache Memory Limit

0.0

0

The value is derived in accordance with the default allocation rule specified in the table above.

0.0

> 0

If _mb exceeds the maximum_memory , a value derived from the above table is used. Otherwise, the value of _mb is used as is.

> 0.0

Any value

The value of _percent is used.

If the compiled_images_eviction_memory_limit_percent variable is set to a value other than 0.0, the value of compiled_images_eviction_memory_limit_mb is ignored.

Note

Changes to maximum_memory may result in changes in the module cache limit, in accordance with the rules specified above.

Drop a plan from the in-memory plancache

In addition to setting a retention policy for in-memory and on-disk plans, you may also choose to drop plans from the plancache to either compile a new query plan or reduce memory usage. For running nodes, the following three commands can drop a plan from the plancache: DROP … FROM PLANCACHE , ALTER TABLE, and ANALYZE.

DROP ALL FROM PLANCACHE purges all previously compiled plans, both in-memory and on-disk, and generates fresh query plans for any new queries. It purges the plans locally, and not across the cluster.

Successful completion of a new ALTER TABLE will invalidate the plancache for all queries that operate on the affected table, requiring their recompilation

ANALYZEinvalidates stale plans from both the in-memory and the on-disk plancache, including plans which are on-disk but not in-memory. This invalidation ensures that all stale plans are discarded and that they are recompiled the next time the query is run.

Note

The ANALYZE command invalidates plans by marking them as invalid so they won’t be used in the future, but it does not actually delete all the files in the on-disk plancache. The files will still be on disk, so it is not possible to reduce disk usage of the on-disk plancache with this command. To delete plans which are only on-disk (other than waiting for them to expire using disk_plan_expiration_minutes), manually delete the files from the plancache directory. To purge all plans, both in-memory and on-disk, use the DROP ALL FROM PLANCACHE command.

Delete Plancache Files

Warning

The manual deletion of plancache files on disk is an offline operation and should be performed with caution. Please contact Support if assistance is needed. Use enable_disk_plan_expiration and disk_plan_expiration_minutes to set shorter expiration times if you frequently run out of disk space due to the size of your on-disk plancache.

To delete the physical plancache files stored on disk, perform the following actions:

  1. Stop each node using the sdb-admin stop-node command. The following example shows how to shut down a single node; however, you can use the option --all to shut down the whole cluster.

    sdb-admin stop-node --memsql-id <MemSQL_ID>

  2. Delete the contents of the plancache directory on each node. The default location is /var/lib/memsql/<ROLE>-<PORT>-<ID>/plancache/.

    sudo rm -rf /path/to/plancache/*

  3. Start the nodes back up using the sdb-admin start-node command. The following example shows how to start up a single node; however, you can use the option --all to start all nodes.

    sdb-admin start-node --memsql-id <MemSQL_ID>

The on-disk plancache should be cleared from the node as well as the in-memory plancache because of the node restart operation.

Change the plancache Directory

Each node in the cluster has a plancache directory which contains compiled plans used for code generation. The default location of the plancache directory is /var/lib/memsql/<node_ID>/plancache. You may consider changing the default directory for several reasons, including, but not limited to, moving to a larger file system, changing hardware to increase storage capacity, and restructuring directories as part of the clean-up process.

To change the plancache directory on one or more nodes, perform the following steps on each target node separately.

  1. Obtain a list of your cluster nodes by running the following command on the host where Toolbox is installed. Make a note of the node ID of the target node(s) and the IP address of the target host(s).

    sdb-admin list-nodes
    +------------+------------+-------------------------------------------+------+---------------+--------------+---------+----------------+--------------------+--------------+
| MemSQL ID  |    Role    |                   Host                    | Port | Process State | Connectable? | Version | Recovery State | Availability Group | Bind Address |
+------------+------------+-------------------------------------------+------+---------------+--------------+---------+----------------+--------------------+--------------+
| C0EE1AEC3B | Master     | ec2-54-234-59-65.compute-1.amazonaws.com  | 3306 | Running       | True         | 7.6.6   | Online         |                    | 0.0.0.0      |
| 6AF9490486 | Aggregator | ec2-34-229-205-7.compute-1.amazonaws.com  | 3306 | Running       | True         | 7.6.6   | Online         |                    | 0.0.0.0      |
| 2CDAC53371 | Leaf       | ec2-54-160-159-22.compute-1.amazonaws.com | 3306 | Running       | True         | 7.6.6   | Online         | 1                  | 0.0.0.0      |
| EF13284492 | Leaf       | ec2-54-242-64-33.compute-1.amazonaws.com  | 3306 | Running       | True         | 7.6.6   | Online         | 1                  | 0.0.0.0      |
+------------+------------+-------------------------------------------+------+---------------+--------------+---------+----------------+--------------------+--------------+

  2. Locate the node(s) on which plancache directory must be changed. Verify the location of the plancache directory on each node by running the following command.

    sdb-admin describe-node --memsql-id <node_ID> 
    +--------------------+---------------------------------------------------------------------------------+
| memsqlId           | EF13284492F0FDF0B35EC95B11523B9437B589B7                                        |
| host               | ec2-54-242-64-33.compute-1.amazonaws.com                                        |
| role               | Leaf                                                                            |
| port               | 3306                                                                            |
| bindAddress        | 0.0.0.0                                                                         |
| processState       | Running                                                                         |
| version            | 7.6.6                                                                           |
| memsqlConfig       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/memsql.cnf                 |
| datadir            | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/data                       |
| plancachedir       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/plancache                  |
| tracelogsdir       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/tracelogs                  |
| auditlogsdir       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/auditlogs                  |
| memsqld            | /opt/memsql-server-7.6.6-63e226ba05/memsqld                                     |
| nodeID             | 3                                                                               |
| pid                | 6967                                                                            |
| dpid               | 6974                                                                            |
| autoRestartEnabled | true                                                                            |
| isConnectable      | true                                                                            |
| recoveryState      | Online                                                                          |
| availabilityGroup  | 1                                                                               |
| config             | Omitted from table view. Use 'sdb-admin describe-node ... --property config'    |
| variables          | Omitted from table view. Use 'sdb-admin describe-node ... --property variables' |
+--------------------+---------------------------------------------------------------------------------+

  3. Using the sdb-admin stop-node command, stop each target node. Optionally, you can use the --all option to shut down the whole cluster.

    sdb-admin stop-node --memsql-id <node_ID>

    When prompted, choose to create a snapshot or skip it.

    Continue to stop the node(s) without taking a database snapshot?
Select 'Y' to stop the node(s) without taking a database snapshot.
Select 'N' to take a snapshot of all databases and then stop the node(s).
Taking a database snapshot will reduce the time it takes for the node(s) to recover after being restarted, but may increase the time it takes for the node(s) to stop. [Y/n]: Y
Toolbox is about to perform the following actions on host ec2-54-242-64-33.compute-1.amazonaws.com:
  · Run 'memsqlctl stop-node --memsql-id EF13284492'

Would you like to continue? [y/N]: y
✓ Stopped node on ec2-54-242-64-33.compute-1.amazonaws.com
Operation completed successfully

  4. After the node is stopped, log in to the target node’s host and copy the plancache directory recursively to the new path using the following command. The -p option preserves the directory’s attributes including mode, ownership, and timestamp.

    sudo cp -R -p </path/to/source/directory> </path/to/destination/directory>

    In this example, the plancache directory needs to be moved from /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/plancache to /var/lib/plancache_data. So the command will become:

    sudo cp -R -p /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/plancache /var/lib/plancache

    Important

    • SingleStore recommends to make a note of the directory’s permissions, user, and group prior to copying the files.

    • Make sure that the plancache directory’s permissions and ownership are intact. The default memsql user should be able to access the new plancache directory.

  5. On the host where Toolbox is installed, run the following command to update the new plancache directory path for the required node(s).

    sdb-admin update-config --memsql-id <node_ID> --key plancachedir --value /new/path/to/directory

    When prompted, confirm the update.

    Toolbox is about to run 'memsqlctl update-config --key plancachedir --value /var/lib/plancache' on the following nodes:
    - On host ec2-54-242-64-33.compute-1.amazonaws.com:
      + EF13284492

Would you like to continue? [y/N]: y
✓ Updated configuration on ec2-54-242-64-33.compute-1.amazonaws.com
Operation completed successfully

  6. Verify the changes by running the following command.

    sdb-admin describe-node --memsql-id <node_ID>
    +--------------------+------------------------------------------------------------------------------+
| memsqlId           | EF13284492F0FDF0B35EC95B11523B9437B589B7                                     |
| host               | ec2-54-242-64-33.compute-1.amazonaws.com                                     |
| role               | Leaf                                                                         |
| port               | 3306                                                                         |
| bindAddress        | 0.0.0.0                                                                      |
| processState       | Stopped                                                                      |
| version            | 7.6.6                                                                        |
| memsqlConfig       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/memsql.cnf              |
| datadir            | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/data                    |
| plancachedir       | /var/lib/plancache                                                           |
| tracelogsdir       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/tracelogs               |
| auditlogsdir       | /var/lib/memsql/aae1e686-42ab-432c-88bc-1086d894204a/auditlogs               |
| memsqld            | /opt/memsql-server-7.6.6-63e226ba05/memsqld                                  |
| nodeID             | --                                                                           |
| pid                | --                                                                           |
| dpid               | --                                                                           |
| autoRestartEnabled | --                                                                           |
| isConnectable      | --                                                                           |
| recoveryState      | --                                                                           |
| availabilityGroup  | --                                                                           |
| config             | Omitted from table view. Use 'sdb-admin describe-node ... --property config' |
| variables          | --                                                                           |
+--------------------+------------------------------------------------------------------------------+

  7. Start the node(s) for which the plancache directory has been updated. Optionally, you can use the --all option to start the entire cluster.

    sdb-admin start-node --memsql-id <node_ID>

    When prompted, confirm the operation.

    Toolbox is about to perform the following actions on host ec2-54-242-64-33.compute-1.amazonaws.com:
      · Run 'memsqlctl start-node --memsql-id EF13284492'
    

    Would you like to continue? [y/N]: y
    ✓ Started nodes on ec2-54-242-64-33.compute-1.amazonaws.com (1/1)
    ✓ Successfully started nodes on 1 host
    ✓ Successfully connected to node
    Operation completed successfully

  8. Verify the status of the updated nodes by running the sdb-admin list-nodes command.

    +------------+------------+-------------------------------------------+------+---------------+--------------+---------+----------------+--------------------+--------------+
    | MemSQL ID  |    Role    |                   Host                    | Port | Process State | Connectable? | Version | Recovery State | Availability Group | Bind Address |
    +------------+------------+-------------------------------------------+------+---------------+--------------+---------+----------------+--------------------+--------------+
    | C0EE1AEC3B | Master     | ec2-54-234-59-65.compute-1.amazonaws.com  | 3306 | Running       | True         | 7.6.6   | Online         |                    | 0.0.0.0      |
    | 6AF9490486 | Aggregator | ec2-34-229-205-7.compute-1.amazonaws.com  | 3306 | Running       | True         | 7.6.6   | Online         |                    | 0.0.0.0      |
    | 2CDAC53371 | Leaf       | ec2-54-160-159-22.compute-1.amazonaws.com | 3306 | Running       | True         | 7.6.6   | Online         | 1                  | 0.0.0.0      |
    | EF13284492 | Leaf       | ec2-54-242-64-33.compute-1.amazonaws.com  | 3306 | Running       | True         | 7.6.6   | Online         | 1                  | 0.0.0.0      |
    +------------+------------+-------------------------------------------+------+---------------+--------------+---------+----------------+--------------------+--------------+

  9. To safely remove your old plancache directory, create a backup of the directory and all its contents and then delete it. To perform these actions, run the following commands on the target host.

    sudo cp -R -p <path/to/source/directory> <path/to/backup/directory>
    sudo rm -r dirname

Last modified: August 8, 2024

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