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sql_hunter asked

Why Page file needs to be 1.5 times of the physical RAM?

Why Page file needs to be 1.5 times of the physical RAM? One of my server has 128 GB RAM and Page file set to 37.9 GB. But when I see the Perfmon counter Paging File: % Usage = 99.98. Memory: Committed Byte = 42012014009 Bytes (39 GB). According to my knowledge. Virtual memory = Physical RAM + Page files. So my Virtual Memory should be 128 + 37.8 = 167.1267371 GB. But I see my commit limit = 37417596928 (34 GB). Can anyone explain me this figure?
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Thanks G, Can you please let me know why does the Paging File: % Usage = 99.98 though my available RAM is 90 GB?
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Grant Fritchey answered
The recommendation is due to the highly volatile and demanding behavior of memory within SQL Server. However, it is somewhat old. It's from the days when people had 4-8gb of memory that was supporting data sets much, much larger than that. As the amount of memory you have increases, the exact size of the page file needed actually decreases. A lot of people are recommending a default page file size once you get past 32gb of ram. Your virtual memory measures are the measures of the page file, not the entire set of physical + page file. That measure you're seeing, Committed Bytes, is physical memory, not your page file. And remember, it reserves memory in the page file, but it doesn't always commit it. If you want to see memory allocations, look to dm_os_sys_memory.
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Remember, it's going to allocate space there, no matter what. So it's "used" even though it may or may not be anything stored there. Microsoft recommends a number of counters. The main one is pages/sec, not %used, although it's included: http://support.microsoft.com/kb/889654
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