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Buffer pool extension introduced in SQL server 2014. The buffer pool extension provides the seamless integration of a nonvolatile RAM extension to the Database Engine buffer pool to significantly improve I/O throughput. As we know the primary purpose of a SQL Server Database is to store and retrieve data, in this operation, commonly Data is read from disk into memory, once the data is changed, it is marked as dirty and the dirty pages are written to disk and flagged as clean data. clean pages may be flushed from memory when the data cache known as Buffer Pool comes under pressure and in this situation data got deleted from memory and SQL Server has to read it from disk the next time a user runs a query that requires the data.
Now there are no problems as data size is less than memory, but as data size is more than memory – It’s started creating issues, To resolve this, In SQL Server 2014 Buffer Pool Extensions introduced to solve problem if we have not enough memory. In the case of Buffer Pool Extensions, SQL Server uses the disk space to store clean buffers having unmodified data pages that out of RAM.
The buffer pool extension feature extends the buffer pool cache with nonvolatile storage (usually SSD). Because of this extension, the buffer pool can accommodate a larger database working set, which forces the paging of I/O’s between RAM and the SSDs. This effectively offloads small random I/O’s from mechanical disks to SSDs. Because of the lower latency and better random I/O performance of SSDs, the buffer pool extension significantly improves I/O throughput.

SSD storage is used as an extension to the memory subsystem rather than the disk storage subsystem. That is, the buffer pool extension file allows the buffer pool manager to use both DRAM and NAND-Flash memory to maintain a much larger buffer pool of lukewarm pages in nonvolatile random access memory backed by SSDs. This creates a multilevel caching hierarchy with level 1 (L1) as the DRAM and level 2 (L2) as the buffer pool extension file on the SSD. Only clean pages are written to the L2 cache, which helps maintain data safety. The buffer manager handles the movement of clean pages between the L1 and L2 caches.

The following illustration provides a high-level architectural overview of the buffer pool relative to other SQL Server components.

When enabled, the buffer pool extension specifies the size and file path of the buffer pool caching file on the SSD. This file is a contiguous extent of storage on the SSD and is statically configured during startup of the instance of SQL Server. Alterations to the file configuration parameters can only be done when the buffer pool extension feature is disabled. When the buffer pool extension is disabled, all related configuration settings are removed from the registry. The buffer pool extension file is deleted upon shutdown of the instance of SQL Server.

Best Practices

• The buffer pool extension size can be up to 32 times the value of max_server_memory.
• A ratio between the size of the physical memory (max_server_memory) and the size of the buffer pool extension of 1:16 or less. A lower ratio in the range of 1:4 to 1:8 may be optimal.

Limitations/Recommendations

• BPE feature is available for 64-bit SQL Server only.
• Its available with SQL Server 2014 Standard, Business Intelligence and Enterprise only.
• If BPE is enabled and you suppose have to modify the size of the file used by BPE on the non-volatile disk, then first need tp disable BPE and set the new size and enable BPE again. If the size is less than previously set, SQL Server must be restarted for the changes to take effect on the non-volatile disk.

Implementing SSD Buffer Pool Extension

— Enabling BPE as 50GB

ALTER SERVER CONFIGURATION 
  SET BUFFER POOL EXTENSION ON 
  (FILENAME = ‘F:\SSD_Data\VirendraTest.BPE’,SIZE = 50 GB)

— Disable BPE

ALTER SERVER CONFIGURATION SET BUFFER POOL EXTENSION OFF

— To See BPE status

SELECT
  (CASE WHEN ([is_modified] = 1 AND ([is_in_bpool_extension] IS NULL OR [is_in_bpool_extension] = 0)) THEN N’Dirty’
WHEN ([is_modified] = 0 AND ([is_in_bpool_extension] IS NULL OR [is_in_bpool_extension] = 0)) THEN N’Clean’
WHEN ([is_modified] = 0 AND [is_in_bpool_extension] = 1) THEN N’BPE’ END) AS N’Page State’,
(CASE WHEN ([database_id] = 32767) THEN N’Resource Database’ ELSE DB_NAME ([database_id]) END) AS N’Database Name’, 
COUNT(1) AS N’Page Count’
FROM sys.dm_os_buffer_descriptors 
GROUP BY [database_id], [is_modified], [is_in_bpool_extension]
ORDER BY [database_id], [is_modified], [is_in_bpool_extension]

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