Buffer Pool in SQL Server

The buffer pool contains and manages SQL Server’s data cache. Information on its contents can be
found in the sys.dm_os_buffer_descriptors DMV. For example, the following query returns the
amount of data cache usage per database, in MB:

SELECT count(*)*8/1024 AS 'Cached Size (MB)'
,CASE database_id
WHEN 32767 THEN 'ResourceDb'
ELSE db_name(database_id)
END AS 'Database'
FROM sys.dm_os_buffer_descriptors
GROUP BY db_name(database_id),database_id
ORDER BY 'Cached Size (MB)' DESC

Refer: SQL Internal and troubleshooting ebook.