On 10/21/19 9:34 PM, Zygo Blaxell wrote: > On Mon, Oct 21, 2019 at 05:27:54PM +0200, Edmund Urbani wrote: >> On 10/16/19 9:42 PM, Zygo Blaxell wrote: >>> For raid5 I'd choose btrfs -draid5 -mraid1 over mdadm raid5 >>> sometimes--even with the write hole, I'd expect smaller average data >>> losses than mdadm raid5 + write hole mitigation due to the way disk >>> failure modes are distributed. >> What about the write hole and RAID-1? I understand the write hole is most >> commonly associated with RAID-5, but it is also a problem with other RAID >> levels. > Filesystem tree updates are atomic on btrfs. Everything persistent on > btrfs is part of a committed tree. The current writes in progress are > initially stored in an uncommitted tree, which consists of blocks that > are isolated from any committed tree block. The algorithm relies on > two things: > > Isolation: every write to any uncommitted data block must not > affect the correctness of any data in any committed data block. > > Ordering: a commit completes all uncommitted tree updates on > all disks in any order, then updates superblocks to point to the > updated tree roots. A barrier is used to separate these phases > of updates across disks. > > Isolation and ordering make each transaction atomic. If either > requirement is not implemented correctly, data or metadata may be > corrupted. If metadata is corrupted, the filesystem can be destroyed.
Ok, the ordering enforced with the barrier ensures that all uncommitted data is persisted before the superblocks are updated. But eg. a power loss could still cause the superblock to be updated on only 1 of 2 RAID-1 drives. But I assume that is not an issue because mismatching superblocks can be easily detected (automatically fixed?) on mount. Otherwise you could still end up with 2 RAID-1 disks that seem consistent in and of themselves but that hold a different state (until the superblocks are overwritten on both). > The isolation failure affects only parity blocks. You could kill > power all day long and not lose any committed data on any btrfs raid > profile--as long as none of the disks fail and each disk's firmware > implements write barriers correctly or write cache is disabled (sadly, > even in 2019, a few drive models still don't have working barriers). > btrfs on raid5/6 is as robust as raid0 if you ignore the parity blocks. I hope my WD Reds implement write barriers correctly. Does anyone know for certain? -- *Liland IT GmbH* Ferlach ● Wien ● München Tel: +43 463 220111 Tel: +49 89 458 15 940 off...@liland.com https://Liland.com <https://Liland.com> Copyright © 2019 Liland IT GmbH Diese Mail enthaelt vertrauliche und/oder rechtlich geschuetzte Informationen. Wenn Sie nicht der richtige Adressat sind oder diese Email irrtuemlich erhalten haben, informieren Sie bitte sofort den Absender und vernichten Sie diese Mail. Das unerlaubte Kopieren sowie die unbefugte Weitergabe dieser Mail ist nicht gestattet. This email may contain confidential and/or privileged information. If you are not the intended recipient (or have received this email in error) please notify the sender immediately and destroy this email. Any unauthorised copying, disclosure or distribution of the material in this email is strictly forbidden.
