I guess many have heard the phrase «640K ought to be enough for anybody», which Bill Gates is mistakenly thought to be the author of. Although RAM is constantly becoming cheaper, still its capacity is always not enough for database server. SQL Server features intelligent caching of frequently requested data, but the data size often exceeds the available amount of memory, which sometimes is difficult to expand. One of the interesting options appeared in SQL Server 2014 is Buffer Pool Extension, which enables cache of data pages on a state solid drive (SSD) in case the RAM is not enough. In this article I present a thorough overview of this technology using SQL Server 2014 CTP1.
Buffer Pool is the largest area to store data pages in SQL Server. Any page read from the disk is placed in Buffer Pool first and later is used for other purposes (for data reading or modifying). There are two types of pages in Buffer Pool. A «clean» data page is the one that was not modified. Consequently, the modified but not written back to the disk with checkpoint or lazy writer processes data pages are termed as “dirty” ones. Buffer Pool Extension allows you to increase the size of Buffer Pool using an SSD. This will be useful if you have an opportunity to add a solid-state drive in your server, but you can not expand RAM or put all frequently requested data on the SSD due to some restrictions. For example, you can use any cheap SSD as a cache for BPE without setting any reliability requirements, while to store the whole or a part of the database more expensive and reliable systems are needed.
To enable this option you should perform ALTER SERVER CONFIGURATION and specify the location and size of the file that will be used for the BPE feature
set buffer pool extension
on ( filename = 'Z:\Cache.bpe' , size = 64 gb )
Immediately after executing this command the file is created and used. SQL Server can use it only to cache “clean” data pages. You can’t simply change the location of the file or its size. To do this you need to disable BPE and enable it again specifying other parameters. To disable Buffer Pool Extension the following command is used:
set buffer pool extension off;
So, in which cases this option may appear to be useful and what advantages it has:
- It is recommended for OLTP systems, where data reading requests prevail.
- There is no risk of data loss, because BPE caches only “clean” pages. When a disk fails the BPE just becomes disabled.
- You do not need to make any changes in the application as everything works properly as soon as you turn on the option.
But there are also some limitations that should be taken into account before starting to use this option:
- The option is available only in Enterprise Edition.
- You can not specify a file smaller than the current size of the RAM for the BPE feature. According to my tests in SQL Server 2014 CTP1 you’ll get an error like this:
Msg 868, Level 16, State 1, Line 18
Buffer pool extension size must be larger than the current memory allocation threshold 2048 MB. Buffer pool extension is not enabled.
- 128 GB is the maximum supported memory, which means those who have systems with a larger amount of memory can not use the option. And that’s pretty bad moment, because large memory is available to customers of even entry-level servers.
- The size of BPE can exceed the size of RAM by up to 32 times. Consequently, the maximum allowed BPE size is 32 x 128 GB = 4 TB. However, the recommended BPE size is from 4X to 10X the RAM.
- It goes without saying that the BPE file should be placed on a fast state solid drive, otherwise the option becomes completely useless. Despite there are no reliability requirements, I should notice that if your drive fails suddenly and you lose BPE the system performance will suffer provided that it depends heavily on this option.
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