Here is a report of a repeatable unrecoverable problem.
11.0 release amd64 r306420
kern.geom.debugflags=0 (unmodified)
# dd if=/dev/zero of=/dev/ada0s1 seek=2048 count=1 bs=1m
1+0 records in
1+0 records out
1048576 bytes transferred
# reboot
# <reboot: Device not configured>
The same goes for any other uncached access to the
filesystem in ada0s1a (Note: / is on s1a, dd occurs past
that), ie all first use of commands that read disk.
Repeating scrolling the console...
"
vnode_pager_generic_getpages_done: I/O read error 5
vm_fault: pager read error, pid 1 (init)
"
Requires HW reset button, then after system comes back
up, all layout, filesystems, and data are fine.
Repeatable every time.
Also,
# echo '<random_string>' | dd of=/dev/ada0s1 seek=2048 count=1
bs=1m conv=sync
does get written to disk and is readable upon reboot,
# dd if=/dev/ada0s1 skip=2048 count=1 bs=1m
# <random_string>
wherein that read does not trigger the fault,
only the write of the string does.
All the layout offsets and sizes add up sequentially, no overlap,
the relavant portions are below, disk is <~= 250G,
gpart, fdisk, boot0cfg, bsdlabel all concur without error,
and were done with the live release tool versions.
What am I overlooking, or is this kernel behaviour a bug?
=> 63 x ada0 MBR (XG)
63 1 - free - (512B)
64 8388608 ada0s1 freebsd [active] (4.0G)
8388672 109051904 ada0s2 freebsd (52G)
=> 0 8388608 ada0s1 BSD (4.0G)
0 4194304 ada0s1a freebsd-ufs (2.0G) /
4194304 4194304 - free - (2.0G)