On Fri, Jan 08, 2021 at 09:36:13AM +0100, wrote: > > --- Ursprüngliche Nachricht --- > Von: Andrea Gelmini <andrea.gelm...@gmail.com> > Datum: 08.01.2021 09:16:26 > An: cedric.dew...@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.dew...@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 > > --- > > Take your mailboxes with you. Free, fast and secure Mail & Cloud: > https://www.eclipso.eu - Time to change! > >
