ac_fs/ac_fs_copy: Increase the size of IO_SIZE
ac_fs/ac_fs_copy: Increase the size of IO_SIZE

Increase the amount of IO we do at a time to 8MiB. This is what Linus
used[0]. This did seem to give a nice wee speedup (at the expense of
increased CPU usage).

Copying a 5.8GiB file from a hard drive in a USB3.0 enclosure to a SATA
connected SSD, using a 2MiB IO_SIZE produces

0.03user 17.41system 1:27.24elapsed 20%CPU (0avgtext+0avgdata 1700maxresident)k
5767296inputs+11969368outputs (1major+83minor)pagefaults 0swaps

and using a 8MiB IO_SIZE

0.03user 21.45system 1:06.99elapsed 32%CPU (0avgtext+0avgdata 1676maxresident)k
11969368inputs+11969368outputs (0major+82minor)pagefaults 0swaps

[0]: https://lore.kernel.org/lkml/CA+55aFxHt8q8+jQDuoaK=hObX+73iSBTa4bBWodCX3s-y4Q1GQ@mail.gmail.com/

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac