Re: Re: Raid1 of a slow hdd and a fast(er) SSD, howto to prioritize the SSD?

From: Andrea Gelmini <andrea.gelmini@gmail.com>
To: Cedric.dewijs@eclipso.eu
Cc: Linux BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: Re: Raid1 of a slow hdd and a fast(er) SSD, howto to prioritize the SSD?
Date: Fri, 8 Jan 2021 20:29:45 +0100	[thread overview]
Message-ID: <CAK-xaQbQPSS7=cH1qmb9S51CL34VRfyE_=eNwb-GhSL1b8Yz2g@mail.gmail.com> (raw)
In-Reply-To: <eb0f5e05a563009af95439f446659cf3@mail.eclipso.de>

Il giorno ven 8 gen 2021 alle ore 09:36 <Cedric.dewijs@eclipso.eu> ha scritto:
> What happens when I poison one of the drives in the mdadm array using this command? Will all data come out OK?
> dd if=/dev/urandom of=/dev/dev/sdb1 bs=1M count = 100?

<smiling>
Well, (happens) the same thing when your laptop is stolen or you read
"open_ctree failed"...You restore backup...
</smiling>

I have a few idea, but it's much more quicker to try it. Let's see:

truncate -s 5G dev1
truncate -s 5G dev2
losetup /dev/loop31 dev1
losetup /dev/loop32 dev2
mdadm --create --verbose --assume-clean /dev/md0 --level=1
--raid-devices=2 /dev/loop31 --write-mostly /dev/loop32
mkfs.btrfs /dev/md0
mount -o compress=lzo /dev/md0 /mnt/sg10/
cd /mnt/sg10/
cp -af /home/gelma/dev/kernel/ .
root@glet:/mnt/sg10# dmesg -T
[Fri Jan  8 19:51:33 2021] md/raid1:md0: active with 2 out of 2 mirrors
[Fri Jan  8 19:51:33 2021] md0: detected capacity change from 0 to 5363466240
[Fri Jan  8 19:51:53 2021] BTRFS: device fsid
2fe43610-20e5-48de-873d-d1a6c2db2a6a devid 1 transid 5 /dev/md0
scanned by mkfs.btrfs (512004)
[Fri Jan  8 19:51:53 2021] md: data-check of RAID array md0
[Fri Jan  8 19:52:19 2021] md: md0: data-check done.
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): setting incompat
feature flag for COMPRESS_LZO (0x8)
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): use lzo compression, level 0
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): disk space caching
is enabled
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): has skinny extents
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): flagging fs with
big metadata feature
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): enabling ssd optimizations
[Fri Jan  8 19:53:13 2021] BTRFS info (device md0): checking UUID tree

root@glet:/mnt/sg10# btrfs scrub start -B .
scrub done for 2fe43610-20e5-48de-873d-d1a6c2db2a6a
Scrub started:    Fri Jan  8 20:01:59 2021
Status:           finished
Duration:         0:00:04
Total to scrub:   4.99GiB
Rate:             1.23GiB/s
Error summary:    no errors found

We check the array is in sync:

root@glet:/mnt/sg10# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
[raid4] [raid10]
md0 : active raid1 loop32[1](W) loop31[0]
     5237760 blocks super 1.2 [2/2] [UU]

unused devices: <none>

Now we wipe the storage;
root@glet:/mnt/sg10# dd if=/dev/urandom of=/dev/loop32 bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 0.919025 s, 114 MB/s

sync

echo 3 > /proc/sys/vm/drop_caches

I do rm to force write i/o:

root@glet:/mnt/sg10# rm kernel/v5.11/ -rf

root@glet:/mnt/sg10# btrfs scrub start -B .
scrub done for 2fe43610-20e5-48de-873d-d1a6c2db2a6a
Scrub started:    Fri Jan  8 20:11:21 2021
Status:           finished
Duration:         0:00:03
Total to scrub:   4.77GiB
Rate:             1.54GiB/s
Error summary:    no errors found

Now, I stop the array and re-assembly:
mdadm -Ss

root@glet:/# mdadm --assemble /dev/md0 /dev/loop31 /dev/loop32
mdadm: /dev/md0 has been started with 2 drives.

root@glet:/# mount /dev/md0 /mnt/sg10/
root@glet:/# btrfs scrub start -B  /mnt/sg10/
scrub done for 2fe43610-20e5-48de-873d-d1a6c2db2a6a
Scrub started:    Fri Jan  8 20:15:16 2021
Status:           finished
Duration:         0:00:03
Total to scrub:   4.77GiB
Rate:             1.54GiB/s
Error summary:    no errors found

Ciao,
Gelma