RAID 5/6 Support Finally Comes To Btrfs

[phoronix]

Phoronix: RAID 5/6 Support Finally Comes To Btrfs

It's been a long time coming, but the Btrfs file-system now finally supports RAID 5 and RAID 6 configurations for the next-generation Linux file-system...

http://www.phoronix.com/vr.php?view=MTI5Mjc

[mark_]

I think you overuse the verb "finally" [1]. It sounds impatient and in it's overuse it sounds like a little child is begging and nagging for something until it gets it. Just leave it out and the articles gain quality. Less is more.

[1] https://www.google.com/search?q=phoronix+finally

[jwilliams]

Considering the glacial pace of btrfs development, the word "finally" is certainly apt in this case.

[waucka]

What do we gain from this? btrfs RAID doesn't work quite like ordinary RAID, so btrfs RAID1 doesn't have the two-disk limitation that ordinary RAID1 does. Does this allow for increased performance?

[DanL]

RAID 1 is simple mirroring and wastes space compared to RAID 5: http://en.wikipedia.org/wiki/Raid_5#RAID_5

When I worked for Sun's storage division, RAID 5 was wildly popular. Unfortunately, zfs hadn't really caught on yet (would have made my life a lot easier).

[ryao]

What do we gain from this? btrfs RAID doesn't work quite like ordinary RAID, so btrfs RAID1 doesn't have the two-disk limitation that ordinary RAID1 does. Does this allow for increased performance?

RAID 1 has no two disk limitation. It appears that btrfs does from Chris Mason's email. You can read It for performance information.

With that said, Chris claims to have implemented a LRU stripe cache. That would imply double buffering with the Linux disk cache, which wastes memory. He also says that this is a clone of what md raid already does, which implies that it suffers from the RAID write hole. It also means that the md raid code is being duplicated, which is generally a bad thing for maintainability.

[patstew]

What do we gain from this? btrfs RAID doesn't work quite like ordinary RAID, so btrfs RAID1 doesn't have the two-disk limitation that ordinary RAID1 does. Does this allow for increased performance?

The most important advantage of btrfs/zfs raid is that the filesystem checksums allow it to work out which set of data is the correct one when there is some corruption (as opposed to a full-on disk failure), unlike normal raid-1 or 5.

[axero]

But what about RAID7 (like raidz3 in zfs) and support for ditto blocks?

[varikonniemi]

What do we gain from this? btrfs RAID doesn't work quite like ordinary RAID, so btrfs RAID1 doesn't have the two-disk limitation that ordinary RAID1 does. Does this allow for increased performance?

* Planned support for N-way mirroring (triple mirror raid1) isn't
included yet.

From my understanding the benefit of BTRFS RAID is that is is object-level raid so there are no 1 week rebuilds after the 10TB stack blows a disk, only the lost files are written out somewhere safe.

[benmoran]

I'm really looking forward to when this hits mainline. I don't know if this was stated yet, but another advantage of file system aware RAID like btrfs is potentially faster scrubbing. You only need to scrub the blocks that are occupied by file data, which is awesome for massive arrays. Another advantage to btrfs raid is that it's super easy to mount. You can just mount whichever disk you want in the array, and the entire array is assembled automatically. This can certainly have its uses.