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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.

A plan expires from the in-memory plancache (called expiration) after the time interval specified by the plan_expiration_minutes engine variable has elapsed. When a plan expires from the in-memory plan cache, the plan is removed fully from the in-memory plancache so when that plan is used again it must be fully reloaded from disk.

Plans are evicted from the in-memory plan cache (called eviction) when the plancache memory usage goes over the thresholds specified by compiled_images_eviction_memory_limit_mb and compiled_images_eviction_memory_limit_percent, as described below. Once the plancache goes over the threshold, the engine evicts the least recently used query plan images until the memory use of the plancache falls below the threshold.

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 reload more quickly than expired plans, but in neither case will the plan be recompiled. 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 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.

Query plans can be manually removed from the in-memory plancache as described in Drop Plans from the Plancache. All plans can be removed from the in-memory and on-disk plancache as described in Drop All Plans from the Plancache. Finally, query plans can be manually 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 and disk_plan_expiration_minutes engine variables.

Expiration for the in-memory plancache is always enabled. Expiration for the on-disk plancache is managed with the enable_disk_plan_expiration engine variable, which defaults to ON.

Plans will expire from the on-disk plancache if the following three conditions are met: 

  • The enable_disk_plan_expiration engine variable is set to 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), and

  • The plan is not in the in-memory plancache.

This process also applies to temporary (temp) tables. SingleStore recommends reusing the same name for temporary tables to the degree possible.

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  | ON    |
| 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; /* ensure this variable is set to 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

Note

Plan eviction and the information in this section is applicable when interpreter_mode is set to interpret_first or compile.

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)

compiled_images_eviction_memory_limit_percent Value

In-memory Plancache Memory Limit

0-1

Prohibited (maximum_memory minimum is 1GB)

1-64

10

10% of maximum_memory

64+

5

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 Plans from the 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 can be used to purge previously compiled plans and generate fresh query plans. It can be run on a specific plan or more generally. Refer to DROP … FROM PLANCACHE for more information.

Note

DROP ALL FROM PLANCACHE marks plans for deletion but does not delete them. As a result, there may be extra plans on disk because the engine generates new plans to replace the invalidated plans which remain on disk until they are garbage collected. You can tell if the disk plan garbage collector is behind by checking the nodes' "memsql.log" files for lines, including the phrase "Exiting stale disk plan garbage collection after 20 seconds of work". If the disk plan garbage collector is behind, it gives up after 20 seconds and waits until the next passage of disk_plan_expiration_minutes minutes to continue.

Successful completion of a new ALTER TABLE will invalidate the plancache for all queries that operate on the affected table, requiring their recompilation. The invalidation of the plancaches happens asynchronously. That is, new queries will compile and use new query plans, while currently running queries will use the old query plans. Once no queries are using the old query plans, they are marked as ready to be purged by the garbage collector.

ANALYZE invalidates 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.

Drop All Plans from the Plancache

To remove all plans from the in-memory and on-disk plancaches, SingleStore recommends the practice of temporarily setting plan_expiration_minutes and disk_plan_expiration_minutes to 0. Setting these engine variables to 0 will cause the plans in the plancaches to be marked as invalid and subsequently deleted by the garbage collector.

To forcibly drop all plans in the in-memory and on-disk plancaches:

Run SHOW VARIABLES and save the current values for the plan expiration minutes engine variables.

SHOW VARIABLES LIKE '%plan_expiration_minutes'; /* be sure to save these values */

Set the plan expiration minutes engine variables to 0 and invoke SLEEP to give the garbage collector time to delete the plans. A larger value for SLEEP may be needed depending on the size of the plancache and the speed of your disks.

SET GLOBAL plan_expiration_minutes=0;
SET GLOBAL disk_plan_expiration_minutes=0;
SELECT SLEEP(300);

Reset the plan expiration minutes engine variables to their previous values.

SET GLOBAL plan_expiration_minutes=<oldvalue>;


SET GLOBAL disk_plan_expiration_minutes=<oldvalue>;

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

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Last modified: April 7, 2025

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Verification instructions

Note: You must install cosign to verify the authenticity of the SingleStore file.

Use the following steps to verify the authenticity of singlestoredb-server, singlestoredb-toolbox, singlestoredb-studio, and singlestore-client SingleStore files that have been downloaded.

You may perform the following steps on any computer that can run cosign, such as the main deployment host of the cluster.

  1. (Optional) Run the following command to view the associated signature files.

    curl undefined

  2. Download the signature file from the SingleStore release server.

    • Option 1: Click the Download Signature button next to the SingleStore file.

    • Option 2: Copy and paste the following URL into the address bar of your browser and save the signature file.

    • Option 3: Run the following command to download the signature file.

      curl -O undefined

  3. After the signature file has been downloaded, run the following command to verify the authenticity of the SingleStore file.

    echo -n undefined |
        cosign verify-blob --certificate-oidc-issuer https://oidc.eks.us-east-1.amazonaws.com/id/CCDCDBA1379A5596AB5B2E46DCA385BC \
          --certificate-identity https://kubernetes.io/namespaces/freya-production/serviceaccounts/job-worker \
          --bundle undefined \
          --new-bundle-format -
    Verified OK