An update on bcachefs [LWN.net]

An update on bcachefs

Posted May 24, 2018 7:53 UTC (Thu) by Cyberax (✭ supporter ✭, #52523)
In reply to: An update on bcachefs by khim
Parent article: An update on bcachefs

This page cheerfully says that it's OK: https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_... Back then this page also looked nice and cheerful: https://btrfs.wiki.kernel.org/index.php?title=RAID56&...

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An update on bcachefs

Posted May 25, 2018 19:26 UTC (Fri) by khim (subscriber, #9252) [Link] (3 responses)

Ok, I'll bite. You say that "back then" page looked nice and cheerful. Here's the full history.

When exactly RAID56 wasn't marked with red and word "Unstable"?

When you are ignoring warnings written in red... bad things happen... that's just how life is it...

An update on bcachefs

Posted May 25, 2018 20:54 UTC (Fri) by Cyberax (✭ supporter ✭, #52523) [Link] (2 responses)

"Write hole still exists" is a far cry from "will corrupt your data irrevocably on rebuild".

Also, I wasn't aware of this page's existence. RAID56 search on Google gives another page that looked OK.

An update on bcachefs

Posted May 31, 2018 21:45 UTC (Thu) by Wol (subscriber, #4433) [Link] (1 responses)

To be fair, fixing the write hole is tricky - in fact it seems to me that it is inherent and unavoidable in a simple raid 5 implementation.

Even md raid has only just fixed it, with the addition of a journal (a feature that might still be experimental). The journal is intended primarily as an optimisation, flushing updates to SSD before saving them to the full raid on spinning rust. It fixes the write hole almost as a side-effect.

(Note, understanding the journal is on my to-do list, I need to learn more about it before I can document it ... :-)

Cheers,
Wol

An update on bcachefs

Posted Jun 2, 2018 17:52 UTC (Sat) by koverstreet (subscriber, #4296) [Link]

Except btrfs is copy on write - the raid5 hole exists because of doing updates in place on existing stripes, so why btrfs is doing it that way I have no idea.