`deviceForPath` scanned `/sys/block` comparing device numbers and, when
no entry matched exactly, took the first one whose *major* agreed. Every
SATA disk is major 8. A filesystem's `st_dev` is its **partition**, so
the exact match never hits for anything on one, and the fallback then
resolved `/dev/sdb3` to whatever `/sys/block` listed first -- which is
alphabetical, which is `sda`.
On the machine this was found on that is a Samsung SSD sitting next to
the 6 TB spinning disk the library is actually on, so
`IsRotationalDisk` answered false and the scanner ran one worker per
core across a drive with one head. Matching on major alone cannot be
right on any machine with two disks, which is the case this exists for.
It goes through `/sys/dev/block/<major>:<minor>` instead -- a symlink
the kernel maintains to the device's own sysfs directory -- and climbs
to the parent when that turns out to be a partition. One readlink, no
scan, no ambiguity. The dev_t decode goes with it: Linux packs 12 bits
of major and 20 of minor split across the word, and masking the low
byte of each is right only for the first 256 of either.
`ProfileForPath` returns what the scanner needs to ask next, and the
new half is `queue_depth`: how many commands the drive will accept and
reorder at once. A SATA disk with NCQ enabled reports 31 or 32 and one
without reports 1, which is the difference between concurrency helping
and hurting. An absent file is read as "queues", because everything
that does not publish it -- NVMe, virtio, device-mapper -- is a device
where concurrency is fine.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MeQt5hgXg5YGoNZQ9ozG7L
yonluandClaude Opus 5
36af7090d9