is allocated memory taken from Virtual memory reserved or its different.

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is allocated memory taken from Virtual memory reserved or its different.

SQLDBA 0

if you see the screenshot , the buffle pool memory clerk, the allocated memory, Virtual memory reserved and virtual memory committed.

I am confused if the allocated memory taken from virtual memory, or allocated memory is taken from OS and Virtual memory reserved is reserved from OS but not used yet.

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Answer 1

Erland Sommarskog 122K MVP Volunteer Moderator

No, we don't see the screenshot.

The allocated memory is taken from the virtual memory, but hopefully never exceeds the available physical memory.

The virtual memory is just how much memory that is addressable in total. And which can exceed the physical memory, being backed up by the page file. But the whole point of the buffer cache is to have the data more quickly accessible than having to read it from disk. So reading data from data file and then store it in the page file is not a smart move, so SQL Server does not want to do that. And if the OS decides to evict SQL Server's memory to the page file, SQL Server writes a warning to the errorlog file.

The equation changes if the SQL Server account has the Windows privilege "Lock pages in memory". With this permission, SQL Server cannot be paged out.

SQLDBA 0 Reputation points

2025-07-12T06:06:45.4866667+00:00

Thanks Erland Sommarskog for the Answer,

"In the screenshot, the buffer-allocated memory is 5,482,120 KB, while the reserved virtual memory is 6,483,488 KB. My question is: Is the allocated memory taken from the reserved virtual memory? Or, if we sum them (5,481,120 + 6,483,488), does this represent the total memory consumed by the buffer pool alone?"
Erland Sommarskog 122K Reputation points MVP Volunteer Moderator

2025-07-12T09:44:04.9466667+00:00
I spied on SSMS using Profiler when running this report, and I found that the query that populates this table includes a grand_total column that is not displayed. The formula for this column is interesting:

(sum(pages_kb + (CASE WHEN type <> ''MEMORYCLERK_SQLBUFFERPOOL'' THEN virtual_memory_committed_kb ELSE 0 END) + shared_memory_committed_kb)) as grand_total

pages_kb is the same as Allocated memory (KB).

So for the buffer pool, Allocated memory is the total consumption + any value there may be in Shared Memory Committed. But for other clerks, we should add Allocated and Virtual Memory Committed to get the total consumption.

Answer 2

Bruce (SqlWork.com) 78,236 Volunteer Moderator

windows supports virtual paged memory. virtual memory can be larger than physical memory. memory is written to the page file when swapped out. so the max virtual memory size is based on the pageful size and the max addressable memory.

when looking at an application, it can reserve virtual memory pages which makes the memory available. it committed when the page is actually accessed.

SQLServer does it own internal memory management using buffer pools rather than say the C/C++ heap. When you display memory usage, it displays how the buffer pool is used. it tries allocate all its memory at startup. so allocated memory is how much memory it has allocated (this is virtual memory). the buffer pool has used and free blocks.

Erland Sommarskog 122K Reputation points MVP Volunteer Moderator

2025-07-10T21:16:39.0366667+00:00

it tries allocate all its memory at startup.

No, it allocates memory when needed. But once it has allocated space in the buffer pool, it will hold on to it, unless the operating system signals that there is memory pressure.

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is allocated memory taken from Virtual memory reserved or its different.

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