Buffer Pool Extension (BPE) - a new feature introduced in Microsoft SQL Server 2014 – effectively extends SQL Server’s usable memory from physical memory onto solid state disk drives (SSDs). This provides very cost-effective, increased performance - allowing customers to scale-up their databases as demand grows. This document outlines the usage of BPE, describing best practices for use with SanDisk SSDs, along with the testing models used to characterize the performance gains possible.
Microsoft incorporated a new feature in Microsoft SQL Server 2014 that is known as a Buffer Pool Extension (BPE). With this feature enabled, the buffer manager is capable of extending Microsoft SQL Server’s usable memory from physical memory onto solid state disk drives (SSDs) for cost-effective increased performance. This expands the available memory, allowing customers to scale-up their databases, as demand for data access grows.
Given that SQL Server 2014 is the latest version of the widely installed SQL Server database, this change in Microsoft SQL Server technology will impact IT deployments of SQL Server workloads, even as it impacts the expectations of the business organizations that are leveraging SQL Server database.
This document will outline the usage of this BPE feature, as it describes SanDisk’s testing of its usage models, alone with describing best practices for BPE’s use with SanDisk SSDs.
SQL Server 2014 exerts intensive pressure on the file system on which the database, backup, and indexes are stored. During normal operations, the SQL Server database makes use of the buffer manager in order to decrease the amount of database file I/O.
By adding the BPE functionality, Microsoft is expanding the buffer pool by moving data onto solid-state disks. In this way, Microsoft has provided a performance increase by using a combination of DRAM and NAND flash memory to provide SQL Server instance with a much larger pool of “lukewarm” pages in non-volatile random access memory that is backed by the SSD. The word “lukewarm” means that the data is ready to be transferred more quickly by the SSD than it would be from other types of storage, such as mechanically driven HDDs.
Multiple SSDs may be used in the buffer pool extension (BPE). The new structure provides multi-level caching hierarchy with level one (L1) as the DRAM and level two (L2) as a file that is stored on the SSD. The file stored on the SSD can be configured by the administrator to any given size, all the way up to full capacity. Only “clean pages” are written to the L2 cache, a process that ensures the safety of an organization’s data in the event of an unexpected server failure.
The buffer pool extension may be enabled or disabled at any time the SQL Server remains operational, and it can scale up to 32 times the value of max_server_memory. Importantly, Microsoft recommends a ratio for the buffer pool extension of 1:16 or less.
SQL Server can change its memory requirements dynamically based on available resources. However, it is recommended that the administrator configure a fixed amount of memory, using all available DRAM that is left, when the server is fully loaded and idle.
The Buffer Pool Extension file is created to a specified size and subsequently written to in sequential order as data as passed to the server. This data is also randomly read, as needed, by the SQL Server database instead of retrieving the data that is stored on the spinning disks. This increases the performance of the server by up to 30%.
IBM x3650 M4 type 7915
Two external enclosures containing:
SanDisk Optimus SSD in external enclosure used for Buffer Pool Extension
To configure the memory options for a SQL Server database, open the SQL Server Management Studio and do the following:
The following command examples will enable and disabled the buffer pool extension.
ALTER SERVER CONFIGURATION SET BUFFER POOL EXTENSION ON (FILENAME = ‘Z:\BPECACHE.
BPE’, SIZE =370 GB)
ALTER SERVER CONFIGURATION SET BUFFER POOL EXTENSION OFF
Note that the file name Z:\BPECACHE.BPE is merely an example taken from our local configuration. This means that you may choose to specify a different drive and file name to suit your needs.
Once you have the buffer pool extension enabled, you may periodically want to check the performance status in the performance monitor or by using the following commands:
SELECT * FROM sys.dm_os_performance_counters
WHERE object_name LIKE ‘%Buffer Manager%’
ORDER BY counter_name
These commands can be entered into a script and executed from the power shell using the following command line which will allow you to automate pulling this information as needed to facilitate performance reports that are useful in further tuning your server. In this example the commands were entered into a file named BPEstat.sql and executed as follows:
SQLCMD.exe –E –I .\BPEstat.sql >dumpstats.txt
This will create a text file containing the performance counters related to the buffer manager. You may also choose to display the results on screen, but keep in mind that the fields are long and that they will likely wrap on your power-show window. The output file can be quickly cleaned up with further scripting to provide an overview report as shown here from an idle (over 24 hours) test server in Figure 1.
Figure 1: Output of SQLCMD.exe –E –I .\BPEstat.sql >dumpstats.txt
It may also be useful to have a look at all of the information stored in the Buffer Pool Extension and this can be achieved using the following commands:
SELECT * FROM sys.dm_os_buffer_descriptors;
Scripting this as above will provide a long report depending upon the size of your Buffer Pool Extension, and as such I have truncated ours to a few lines just to show you what you should be expecting.
Figure 2: Output of SELECT * FROM sys.dm_os_buffer_descriptors; GO
For a clean and decoded report of what the Buffer Pool Extension contains, use the following script:
select db_name(database_id) databaseName,
WHEN page_id = 0 THEN ‘File Header Page m_type 15’
WHEN page_id = 1 OR page_id % 8088 = 0 THEN ‘PFS m_type 11’
WHEN page_id = 2 OR page_id % 511232 = 0 THEN ‘GAM m_type 8’
WHEN page_id = 3 OR (page_id - 1) % 511232 = 0 THEN ‘SGAM m_type 9’
WHEN page_id = 6 OR (page_id - 6) % 511232 = 0 THEN ‘DCM m_type 16’
WHEN page_id = 7 OR (page_id - 7) % 511232 = 0 THEN ‘BCM m_type 17’
WHEN page_id = 9 AND file_id = 1 THEN ‘Boot Page m_type 13’
WHEN page_id = 10 AND DB_ID() = 1 THEN
‘config page > sp_configure settings only present in master m_type 14’
(select type_desc from sys.allocation_units where
sys.dm_os_buffer_descriptors.allocation_unit_id) as allocation_type_desc,
cast(free_space_in_bytes as numeric)/1024 as free_space_in_mega_bytes,
This will provide an extremely detailed and decoded view of the activity involving the Buffer Pool Extension. As you will see when you execute the script, the output of this view is far too detailed to include in this whitepaper. That’s why it is best to export this to a text file, or to send it directly to a wide-format line printer for review.
The information contained in this report will include databaseName, fileName, page_type, page_level, allocation_type_desc, page_type, row_count, free_space_in_bytes, free_space_in_mega_bytes, is_ modified, numa_node, read_microsec, is_in_bpool_extension. This report will give you a much clearer picture of your level of activity for the Microsoft SQL Server database. It is recommended that you periodically execute this view as you gather performance information during the setup of your Buffer Pool Extension.
As seen in Figure 3, the Performance Monitor provides real-time graphing of any of the counters you choose. Simply select add counter and scroll down the SQLServer:Buffer Manager as shown below.
Figure 3: Using Performance Monitor to add counters.
It’s worth noting that if you add all counters from this selection you’re going to end up with a significant amount of information that will be difficult to decipher on the fly. However, it is useful to create a Data Collector Set to record a large sample.
Buffer Pool Extension will benefit read-heavy OLTP activity, particularly in servers that have lower amounts of physical memory, whereas OLAP will see little or no increase in performance. It was also found that an increase in the number of solid-state disks (SSDs) did not necessarily increase the performance on our test database, which contained 75,000 customers and a size of approximately 850GB. For deployments using more physical memory and multiple SSDs to store a Buffer Pool Extension file that was the same size as the database, a significant performance increase can be achieved, but it would not be determined to be cost-effective to do so.
The following figures will illustrate the performance characteristics that were observed during extensive testing.
There is a massive performance increase in the number of page reads per second as seen in Figure 4. The more read-intensive the database is, the greater the benefit will clearly be when the Buffer Pool Extension is enabled and configured appropriately.
Figure 4: Page Reads/Sec with Buffer Pool Extension enabled vs. disabled
Figure 5: The Percentage of Processor Usage is lower even under the peak of operations with the Buffer Pool Extension enabled.
Figure 6: Average Disk I/O time in milliseconds shows significant performance improvements, which will directly translate into better Quality of Service.
Figure 7: Transactions per Second (TPS), as observed from Performance Monitor, shows more consistent performance with the Buffer Pool Extension enabled during heavy mixed read/write activity.
The sample, shown above, was taken several hours into a TPC-e-like workload (similar to an industry standard TPC-e benchmark test) during a checkpoint. Checkpoints have a highly mixed workload. The best performance is achieved when the SQL Server database retrieves pages from the buffer, and not the spinning disks. As the limits of the buffer are reached, the data is moved to the disks to create free space for new pages. This generally occurs during a checkpoint operation. In the figure above, the transaction count dips dramatically during this activity. However, there is still a significant transaction account occurring, and a much faster recovery to the normal Quality of Service (QoS) when using the Buffer Pool Extension (BPE).
While the Buffer Pool Extension is largely Random Reads in an OLTP type of workload, as the file is being populated, it is being written to up to 75% of the time – and up to 99.3% of the writes are sequential writes.
Figure 8: Average disk I/O write time is also lower with the Buffer Pool Extension enabled, because SSDs have a much faster write time compared to spinning disks.
Figure 9: Average time in milliseconds to achieve a disk write
The Lazy Write process, like the checkpoint, pushes pages from the buffer to the physical disks. As logic dictates and as seen in the figure below, the more buffer space you have, the more pages can be stored there. These pages can be subsequently written by the Lazy Write process—and this happens at a perceived performance gain to the user. This is another advantage seen when using the Buffer Pool Extension: There is more buffer to store pages that are then written by a background process while the SQL I/O continues to take place in the foreground.
Figure 10: Lazy Writes captured over several minutes of sampling
The amount of the Buffer Pool Extension that is in use will increase over time. During testing, the amount of Buffer Pool Extension used was observed to be as high as 100%. This will vary, depending on the page lock setting and on the amount of pressure the database is under, as well as the size of the Buffer Pool Extension. As the size of the Buffer Pool Extension in use increases, so the Extension Page Reads/sec increases. This is due to the increasing amount of useful data that is being held in the extension file after writing it to the database. In an effort to reduce file-system pressure, this data is retrieved from the buffer, rather than from the disks themselves.
Figure 11: Buffer Pool Extension Page Reads vs Percentage of Buffer Pool Extension in use
The result of these improvements is shown in a TPC-E type workload of the type described earlier. The amount of performance increase will vary, depending upon the amount of physical memory that is available, the size of the Buffer Pool Extension, the database and log sizes, and the type of workload that is being executed.
Figure 12: Shows the percentage of performance increase measured in Transactions per Second with Buffer Pool Extension enabled vs. Buffer Pool Extension disabled
The SQL Server 2014 Buffer Pool Extension (BPE) used with SanDisk Enterprise SSDs provides a costeffective performance increase, particularly when compared to deployments with lower amounts of physical memory. This scalable performance increase is created by leveraging the BPE functionality provided in MS SQL Server 2014.
Using BPE requires minimal configuration and deployment time -- and it can be easily expanded to meet future requirements. Importantly for IT customers in the enterprise data center, all of this can be achieved without taking the server that is hosting Microsoft SQL Server offline. This allows users to gain performance improvements by implementing them on-the-fly, while maintaining operational status. This makes the process of expanding memory for SQL Server deployments seamless and invisible to the end-users who are accessing the data.
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