Re: BTRFS list of grievances

[Roman Mamedov <rm@romanrm.net>]

On Fri, 27 Sep 2024 13:20:14 +0200
waxhead <waxhead@dirtcellar.net> wrote:

> 1. FS MANAGEMENT
> ================
> BTRFS is rather simple to manage. We can add/remove devices on the fly, 
> balance the filesystem, scrub, defrag, select compression algorithms 
> etc. Some of these things are done as mount options, some as properties 
> and some by issuing a command that process something.

I will add my annoyance or rather a showstopper.

Consider a RAID1 of two 20TB disks. One disk disconnects and the system
operates on just the remaining one for a few days.

Side note: will Btrfs even agree to operate in such state without constant
stream of errors to dmesg?

Then the disk is reconnected to the system.

For a start, are we even able to cleanly forget an abruptly disappeared drive
in RAID1, and then re-add it back when the same disk it reappears (under a
different /dev/sdX location)? Without remounting and reboot?

Secondly, it feels like you'll be extremely lucky not to die a fiery death of
"parent transid mismatch errors" right away with Btrfs, after this.

Or if not, then how do you get from there to a consistent state? Run a scrub,
make the system reread the entire 40 TB of data, correcting errors and lack of
duplication where necessary.

Meanwhile, mdadm RAID1: thanks to the Write-intent bitmap, after a re-add the
RAID resyncs just the small changed areas from the continuously running disk
to the temporarily-absent one, and the array consistency is almost instantly
restored, in many cases just with a few GBs read and written.

Or maybe I underestimate the current Btrfs capabilities here?

-- 
With respect,
Roman