Re: What mechanisms protect against split brain?

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> On 2022/6/8 18:15, Wang Yugui wrote:
> > Hi, Forza, Qu Wenruo
> >
> > I write a script to test RAID1 split brain base on Qu's work of raid5(*1)
> > *1: https://lore.kernel.org/linux-btrfs/53f7bace2ac75d88ace42dd811d48b7912647301.1654672140.git.wqu@suse.com/T/#u
> 
> No no no, that is not to address split brain, but mostly to drop cache
> for recovery path to maximize the chance of recovery.
> 
> It's not designed to solve split brain problem at all, it's just one
> case of such problem.
> 
> In fact, fully split brain (both have the same generation, but
> experienced their own degraded mount) case can not be solved by btrfs
> itself at all.
> 
> Btrfs can only solve partial split brain case (one device has higher
> generation, thus btrfs can still determine which copy is the correct one).
> 
> >
> > #!/bin/bash
> > set -uxe -o pipefail
> >
> > mnt=/mnt/test
> > dev1=/dev/vdb1
> > dev2=/dev/vdb2
> >
> >    dmesg -C
> >    mkdir -p $mnt
> >
> >    mkfs.btrfs -f -m raid1 -d raid1 $dev1 $dev2
> >    mount $dev1 $mnt
> >    xfs_io -f -c "pwrite -S 0xee 0 1M" $mnt/file1
> >    sync
> >    umount $mnt
> >
> >    btrfs dev scan -u $dev2
> >    mount -o degraded $dev1 $mnt
> >    #xfs_io -f -c "pwrite -S 0xff 0 128M" $mnt/file2
> >    mkdir -p $mnt/branch1; /bin/cp -R /usr/bin $mnt/branch1 #complex than xfs_io
> >    umount $mnt
> >
> >    btrfs dev scan
> >    btrfs dev scan -u $dev1
> >    mount -o degraded $dev2 $mnt
> 
> Your case is the full split brain case.
> 
> Not possible to solve.
> 
> In fact, if you don't do the degraded mount on dev2, btrfs is completely
> fine to resilve the fs without any problem.

step1: we mark btrfs/RAID1 with degraded write as not-clean-RAID1.
step2: in that state, we default try to read copy 0 of RAID1
	current pid based i/o patch select policy
           preferred_mirror = first + (current->pid % num_stripes);

this idea seem to work?

degraded RAID1 write is almost the same as full split brain?

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2022/06/08