kd_write() went through bread(), so every block a write allocated was
read from the disk first and then replaced whole. That was a quarter of
the disk operations of a SchFS write. bgetblk() is bread() without the
read, as Linux's getblk() is.
1 MB on AHCI under QEMU: 2.0 -> 2.8 MB/s in 4 KB writes, 4.5 -> 5.6
MB/s in 64 KB ones. 3 MB of random data written and copied through the
cache's evictions reads back with the same md5.
The 63 KB/s seen earlier is the legacy IDE driver on QEMU's default
machine, at about 10 ms per emulated PIO write; the same disk on AHCI
(q35) is 40 times faster.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
b213ab5778