Thansk Ceylo. I agree the the allocated size should satisfy the alignment constraints. What confused me is that some allocated size is way bigger than the requested buffer length. Below are some rows of the allocated size and buffer length(The left side is the allocated size and the right side is the corresponding buffer length). 49152 40000 49152 40000 131072 12000 131072 12000 As you see the third allocated size is more than 10x of the logical size. So I supposed there might be some other reason. Maybe one allocated size can be shared by multiple mtlbuffer I guess. By the way, I did some tests on my ipad. When the allocated size came to 1.5GB and with about 200MB logical buffer, the application didn't crash.