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

[Zygo Blaxell <ce3g8jdj@umail.furryterror.org>]

On Fri, Jan 08, 2021 at 09:36:13AM +0100,   wrote:
> 
> --- 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?

mdadm doesn't handle data corruption, and (except for a /sys counter),
reads from mirror devices interchangeably, and silently propagates
data between devices during resync, so the array will almost certainly
be destroyed.

> 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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