Re: [RFC][PATCH V3] btrfs: ssd_metadata: storing metadata on SSD

[Goffredo Baroncelli <kreijack@libero.it>]

On 4/6/20 4:24 AM, Zygo Blaxell wrote:
>>> Of course btrfs is slower than ext4 when a lot of sync/flush are involved. Using
>>> apt on a rotational was a dramatic experience. And IMHO  this should be replaced
>>> by using the btrfs snapshot capabilities. But this is another (not easy) story.
> flushoncommit and eatmydata work reasonably well...once you patch out the
> noise warnings from fs-writeback.
> 

You wrote flushoncommit, but did you mean "noflushoncommit" ?

Regarding eatmydata, I used it too. However I was never happy. Below my script:
----------------------------------
ghigo@venice:/etc/apt/apt.conf.d$ cat 10btrfs.conf

DPkg::Pre-Invoke {"bash /var/btrfs/btrfs-apt.sh snapshot";};
DPkg::Post-Invoke {"bash /var/btrfs/btrfs-apt.sh clean";};
Dpkg::options {"--force-unsafe-io";};
---------------------------------
ghigo@venice:/etc/apt/apt.conf.d$ cat /var/btrfs/btrfs-apt.sh

btrfsroot=/var/btrfs/debian
btrfsrollback=/var/btrfs/debian-rollback


do_snapshot() {
	if [ -d "$btrfsrollback" ]; then
		btrfs subvolume delete "$btrfsrollback"
	fi

	i=20
	while [ $i -gt 0 -a -d "$btrfsrollback" ]; do
		i=$(( $i + 1 ))
		sleep 0.1
	done
	if [ $i -eq 0 ]; then
		exit 100
	fi

	btrfs subvolume snapshot "$btrfsroot" "$btrfsrollback"
	
}

do_removerollback() {
	if [ -d "$btrfsrollback" ]; then
		btrfs subvolume delete "$btrfsrollback"
	fi
}

if [ "$1" = "snapshot" ]; then
	do_snapshot
elif [ "$1" = "clean" ]; then
	do_removerollback
else
	echo "usage: $0  snapshot|clean"
fi
--------------------------------------------------------------

Suggestion are welcome how detect automatically where is mount the btrfs root (subvolume=/) and  my root subvolume name (debian in my case). So I will avoid to wrote directly in my script.

BR
G.Baroncelli
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