28 Sep
2017
28 Sep
'17
6:53 p.m.
From: Theodore Ts'o
when a file was truncated and then rewritten, and
"truncate this file"
packet got reordered and got received after the "here's the new 4k of
contents of the file", Hilar[i]ty Enused.
This sounds _exactly_ like a bad bug found in the RVD protocol (Remote Virtual
Disk - a simple block device emulator). Disks kept suffering bit rot (damage
to the inodes, IIRC). After much suffering, and pain trying to debug it (lots
of disk writes, how do you figure out the one that's the problem), it was
finally found (IIRC, it wasn't something thinking about it, they actually
caught it). Turned out (I'm pretty sure my memory of the bug is correct), if
you had two writes of the same block in quick sucession, and the second was
lost, if the first one's ack was delayed Just Enough...
They had an unused 'hint' (I think) field in the protocol, and so they
recycled that to be a sequence number, so they could tell ack's apart.
Noel