From: " " <Cedric.dewijs@eclipso.eu>
To: "Andrea Gelmini" <andrea.gelmini@gmail.com>
Cc: <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, 08 Jan 2021 09:36:13 +0100 [thread overview]
Message-ID: <eb0f5e05a563009af95439f446659cf3@mail.eclipso.de> (raw)
In-Reply-To: <CAK-xaQZS+ANoD+QbPTHwL-ErapA-7PDZe_z=OOWq_axAyR1KfA@mail.gmail.com>
--- Ursprüngliche Nachricht ---
Von: Andrea Gelmini <andrea.gelmini@gmail.com>
Datum: 08.01.2021 09:16:26
An: Cedric.dewijs@eclipso.eu
Betreff: Re: Raid1 of a slow hdd and a fast(er) SSD, howto to prioritize the SSD?
Il giorno mar 5 gen 2021 alle ore 07:44 <Cedric.dewijs@eclipso.eu>
ha scritto:
>
> Is there a way to tell btrfs to leave the slow hdd alone, and to prioritize
the SSD?
You can use mdadm to do this (I'm using this feature since years in
setup where I have to fallback on USB disks for any reason).
From manpage:
-W, --write-mostly
subsequent devices listed in a --build, --create, or
--add command will be flagged as 'write-mostly'. This is valid for
RAID1 only and means that the 'md' driver will avoid
reading from these devices if at all possible. This can be useful if
mirroring over a slow link.
--write-behind=
Specify that write-behind mode should be enabled
(valid for RAID1 only). If an argument is specified, it will set the
maximum number of outstanding writes allowed. The
default value is 256. A write-intent bitmap is required in order
to
use write-behind mode, and write-behind is only
attempted on drives marked as write-mostly.
So you can do this:
(be carefull, this wipe your data)
mdadm --create --verbose --assume-clean /dev/md0 --level=1
--raid-devices=2 /dev/sda1 --write-mostly /dev/sdb1
Then you use BTRFS on top of /dev/md0, after mkfs.btrfs, of course.
Ciao,
Gelma
Thanks Gelma.
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?
When I do this test on a plain btrfs raid 1 with 2 drives, all the data comes out OK (while generating a lot of messages about correcting data in dmesg -w)
Cheers,
Cedric
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next prev parent reply other threads:[~2021-01-08 8:37 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-05 6:39 Raid1 of a slow hdd and a fast(er) SSD, howto to prioritize the SSD?
2021-01-05 6:53 ` Qu Wenruo
2021-01-05 18:19 `
2021-01-07 22:11 ` Zygo Blaxell
2021-01-05 19:19 ` Stéphane Lesimple
2021-01-06 2:55 ` Anand Jain
2021-01-08 8:16 ` Andrea Gelmini
2021-01-08 8:36 ` [this message]
2021-01-08 14:00 ` Zygo Blaxell
2021-01-08 19:29 ` Andrea Gelmini
2021-01-09 21:40 ` Zygo Blaxell
2021-01-10 9:00 ` Andrea Gelmini
2021-01-16 1:04 ` Zygo Blaxell
2021-01-16 15:27 `
2021-01-18 0:45 ` Zygo Blaxell
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