Blob Cache Size Engine Variables

The size of the blob cache is controlled by the following engine variables:

Updates to the maximum_blob_cache_size_mb and maximum_blob_cache_size_percent engine variable function as follows:

• Changing maximum_blob_cache_size_percent changes the value of maximum_blob_cache_size_mb and changes the size of the blob cache.

• Changing maximum_blob_cache_size_mb changes the value of maximum_blob_cache_size_mb and the size of the blob cache but does not update maximum_blob_cache_size_percent.

• The maximum_blob_cache_size_percent engine variable can be "out of date" if you set maximum_blob_cache_size_mb after setting maximum_blob_cache_size_percent.

• On a restart, maximum_blob_cache_size_percent takes precedence, and sets maximum_blob_cache_size_mb to the corresponding value.

Details related to the blobs cached in the blob cache and events on those blobs are found in the information schema views MV_CACHED_BLOBS and MV_BLOB_CACHE_EVENTS.

Automatic Resizing of Blob Cache

When the disk space on the disk it’s mounted on starts getting filled up and stored files/objects need more space, the blob cache starts shrinking itself. The blob cache can shrink down to 70% of its maximum size.  Anything that uses disk space (logs, trace logs, plancache, audit logs) even things outside the engine can steal space from the blob cache.

Example:

//Assume you are using a 1 TB disk and

maximum_blob_cache_size_mb = 800GB  // for example
maximum_blob_cache_size_percent = 0
maximum blob cache size = 800GB

SET maximum_blob_cache_size_percent = 0.50;

//Now

maximum_blob_cache_size_mb = 500GB
maximum_blob_cache_size_percent = 0.50
maximum blob cache size = 500GB

SET maximum_blob_cache_size_mb = 800GB;

maximum_blob_cache_size_mb = 800GB
maximum_blob_cache_size_percent = 0.50
maximum blob cache size = 800GB

// On restarting the node

maximum_blob_cache_size_mb = 500GB
maximum_blob_cache_size_percent = 0.50
maximum blob cache size = 500GB