Re: bcache fails after reboot if discard is enabled

[Dan Merillat <dan.merillat@gmail.com>]

On Sat, Apr 11, 2015 at 4:09 PM, Kai Krakow <hurikhan77@gmail.com> wrote:

> With this knowledge, I guess that bcache could probably detect its backing
> device signature twice - once through the underlying raw device and once
> through the md device. From your logs I'm not sure if they were complete

It doesn't, the system is smarter than you think it is.

> enough to see that case. But to be sure I'd modify the udev rules to exclude
> the md parent devices from being run through probe-bcache. Otherwise all
> sorts of strange things may happen (like one process accessing the backing
> device through md, while bcache access it through the parent device -
> probably even on different mirror stripes).

This didn't occur, I copied all the lines pertaining to bcache but
skipped the superfluous ones.

> It's your setup, but personally I'd avoid MD for that reason and go with
> lvm. MD is just not modern, neither appropriate for modern system setups. It
> should really be just there for legacy setups and migration paths.

Not related to bcache at all.  Perhaps complain about MD on the
appropriate list?  I'm not seeing any evidence that MD had anything to
do with this, especially since the issues with bcache are entirely
confined to the direct SATA access to /dev/sda4.

In that vein, I'm reading the on-disk format of bcache and seeing
exactly what's still valid on my system.  It looks like I've got
65,000 good buckets before the first bad one.  My idea is to go
through, look for valid data in the buckets and use a COW in
user-mode-linux to write that data back to the (copy-on-write version
of) the backing device.  Basically, anything that passes checksum and
is still 'dirty', force-write-it-out.  Then see what the status of my
backing-store is.  If it works, do it outside UML to the real backing
store.

Are there any diagnostic tools outside the bcache-tools repo? Not much
there other than show the superblock info.  Otherwise I'll just finish
writing it myself.