An update on bcachefs [LWN.net]

An update on bcachefs

Posted May 24, 2018 7:18 UTC (Thu) by vadim (subscriber, #35271)
In reply to: An update on bcachefs by rahvin
Parent article: An update on bcachefs

Probably because BTRFS got merged too early and had serious gotchas for the cool features.

Eg, the "raid1 volumes only mountable once RW if degraded" issue was there until very recently and is not the kind of thing one wants to deal with on a RAID setup.

Compression until recently came with warnings.

Compression is still quite half-assed on the UI side. Why can't I easily check how well a file is being compressed? Why do I have to 'defrag' to compress, on a SSD? I don't care if it's fragmented, I just want it compressed. There's no need to move already compressed data around.

Snapshots are in theory very cool, but in practice easily cause horrible performance issues. Eg, I unwisely had installed snapper and allowed DNF to make a couple dozen snapshots of my filesystem. Cleaning that up took an entire day, on an SSD, with the first snapshots taking hours to remove and completely locking up the machine. I don't want to imagine how that would work on an HDD.

Despite my best efforts so far I haven't figured out a way to do a btrfs scrub in such a way that it doesn't make the entire desktop lag, on a computer with a SSD.

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An update on bcachefs

Posted May 25, 2018 2:03 UTC (Fri) by zlynx (guest, #2285) [Link]

One thing to point out about slow btrfs scrubs is that it depends on your IO scheduler. Use the standard CFQ or BFQ. If you use deadline or noop then of course the scrub is going to use up all the IO because nothing will stop it.

It still causes some extra latency. On my email server I use multi-queue BFQ and I can feel a bit of extra delay when loading each IMAP message from it during a scrub, but I don't think it's that serious.

And on my laptop with NVMe SSD the btrfs scrub is done in about 30 seconds, so it isn't a big deal there either. It reads and verifies at hundreds of megabytes per second.