Re: Simulating disk failure with a writeback cache

[Josef Bacik <jbacik@fb.com>]

On Mon, Dec 19, 2016 at 3:55 PM, Kent Overstreet 
<kent.overstreet@gmail.com> wrote:
> On Mon, Dec 19, 2016 at 10:27:34AM -0500, Josef Bacik wrote:
>>  Oh sorry for some reason I only saw this reply but not your 
>> previous one
>>  about seeing the last version of the file.  Yeah that's kind of 
>> annoying,
>>  I'll make a note to fix the test to do a wipefs before doing the 
>> log replay.
>>  Does using wipefs work as well as dd'ing the first bit of the disk 
>> for you?
>>  Thanks,
> 
> That worked for the short test, but it won't work in general - the 
> issue is that
> in bcache/bcachefs, btree nodes are also log structured, so if we 
> just blow away
> the journal we'll still find btree node entries from the future (and 
> it'll
> complain loudly on finding btree node entries far in the future of 
> the most
> recent journal entry).
> 
> For the bug I'm hunting now I'm gonna see if I can reproduce it with 
> a small
> enough filesystem that I can just dd over the entire device between 
> replays.

Ok that works fine in the short term for you, but ideally you wouldn't 
have to fix the test every time you wanted to run it on bcachefs.  Do 
you have a suggestion of how to make this generic enough to support you 
as well without dd'ing the whole drive?  Thanks,

Josef