Re: data loss+inode recovery using RAID6 write journal

Nick Black
2016-10-26T18:51:33Z

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Shaohua Li left as an exercise for the reader:
> Thanks for the testing. We can't improve the quality of the new feature if
> nobody tests it. Yep, the write journal isn't mature yet, but I can't imagine
> the data loss. With write journal, data is written to the ssd first, then to
> raid disks and IO is finished at that time. So if IO is finished, the data
> should be in raid disks. The only possible way to data loss is the recovery.
> But it's also possible filesystem/writeback hasn't flushed data to disk yet.
> I'm wondering if you can reproduce it with/without journal, so we can narrow
> down it a bit.

I doubt it can be replaced without the journal -- like I said, I've been
using mdadm RAID[56] for over a decade, and never seen such a problem.

I'll attempt to reproduce with the journal enabled. Assuming I can, I can
try to reproduce without, but I doubt it'll be fruitful. Are there any
debugging options / flags I should enable prior reproducing in order to get
a more complete report? Some state I should dump from my array and
filesystems? Feel free to be technical.

I owe a lot to Linux MD RAID, and am happy to put some effort into running
this down.

I'll report whether I can at least reproduce ASAP.

--nick