xv6 operating system - Implementation of triple indirection

Question

The xv6 mkfs.c file declare the variables:

int nblocks = 985;
int nlog = LOGSIZE;
int ninodes = 200;
int size = 1024;

That declaration should work properly with inode that have 12 direct blocks and 1 indirect block, what i don't understand is why nblocks is defined as 985? The number of blocks in one inode is 140 (128 for the indirect + 12 direct), so i don't fully understand where is 985 came from and also the size of 1024.

If i would understand that part i think i will be able to change the variables to support triple indirection.

Thanks!

Answer 1

An inode takes only 32 bytes on disk (sizeof(struct inode)). Only when writing to an inode it starts to take more space (blocks) on disk.

Notice this block of code:

bitblocks = size/(512*8) + 1;
usedblocks = ninodes / IPB + 3 + bitblocks;
...
assert(nblocks + usedblocks + nlog == size);