Hi, I think there could be a mistake in freespace/README. There are several places where it says about ~4000 slots per FSM page for the default BLKSZ:
""" For example, assuming each FSM page can hold information about 4 pages (in reality, it holds (BLCKSZ - headers) / 2, or ~4000 with default BLCKSZ), """ and: """ To keep things simple, the tree is always constant height. To cover the maximum relation size of 2^32-1 blocks, three levels is enough with the default BLCKSZ (4000^3 > 2^32). """ Let's determine the amount of levels in each FSM page first. I'm going to use Python for this. Note that range(0,13) returns 13 numbers from 0 to 12: ``` >>> sum([pow(2,n) for n in range(0,13) ]) 8191 >>> 8*1024 8192 ``` 13 levels are not going to fit since we need extra 24 bytes per PageHeaderData and a few more bytes for an int value fp_next_slot. Which gives us 12 levels and the number of slots: ``` >>> # there are pow(2,0) == 1 byte on the 1st level of the tree >>> pow(2,12 - 1) 2048 ``` The number of levels in the entire, high-level tree, seems to be correct: ``` >>> pow(2048,3) > pow(2,32) - 1 True ``` Hopefully I didn't miss or misunderstood anything. Thoughts? -- Best regards, Aleksander Alekseev
