DragonFlyBSD

sbin/newfs_hammer: optimize blockmap initialization loop

This patch optimizes blockmap initialization loop of freemap zone by newfs_hammer command.

newfs_hammer does not need to read (either actually read(2) or get cached malloc(3)ed buffer) layer 1 structure everytime within the loop while filling in layer 2 structure. The layer 1 that covers current phys_offset needs to be updated only when layer 1 bits of phys_offset has changed. In other words, phys_offset changes within lower 42 bits (lower 4TB address space) do not affect the layer 1, and newfs_hammer can reuse (keep the pointer of) the buffer until phys_offset reaches to the next layer 1 given that the buffer does not get released while running in this loop.

This reduces reading layer 1 approximately from (size_of_volume / 8MB + 1) times to (size_of_volume / 4TB + 1) times. For exammple, if the volume size is < 4TB it reads layer 1 only once for that volume which is good enough as a single layer 1 structure covers 4TB of address space.

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Following test shows there is no diff between blockmap layout made by /sbin/newfs_hammer and this patch. It doesn't make newfs much faster as newfs is basically i/o bound process rather than cpu bound, however it should still make sense to do this shortcut.

1. /sbin/newfs_hammer -L TEST /dev/ad1 /dev/ad2 /dev/ad3 > /dev/null
2. hammer -f /dev/ad1:/dev/ad2:/dev/ad3 blockmap > out1
3. ~/work/dragonfly/sbin/newfs_hammer/newfs_hammer -L TEST /dev/ad1 /dev/ad2 /dev/ad3 > /dev/null
4. hammer -f /dev/ad1:/dev/ad2:/dev/ad3 blockmap > out2
5. diff out1 out2 #

Since I don't have storages large enough to test whole 60 bits of address space, I've made a unittest that mimics and asserts layer 1 offset calculation within the loop (attached to this issue). It could take a while to finish as it runs through whole 60 bits.

1. gcc -Wall -g unittest.c -o unittest
2. ./unittest
   ................................................................................................................................................................................................................................................................
   success