Re: raid0, raid1, raid5, what to choose?

[Hugo Mills <hugo@carfax.org.uk>]

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On Thu, Jun 13, 2013 at 11:09:00PM +0200, Hendrik Friedel wrote:
> Hello,
> 
> I'd appreciate your recommendation on this:
> 
> I have three hdd with 3TB each. I intend to use them as raid5 eventually.
> currently I use them like this:
> 
> # mount|grep sd
> /dev/sda1 on /mnt/Datenplatte type ext4
> /dev/sdb1 on /mnt/BTRFS/Video type btrfs
> /dev/sdb1 on /mnt/BTRFS/rsnapshot type btrfs
> 
> #df -h
> /dev/sda1       2,7T  1,3T  1,3T  51% /mnt/Datenplatte
> /dev/sdb1       5,5T  5,4T   93G  99% /mnt/BTRFS/Video
> /dev/sdb1       5,5T  5,4T   93G  99% /mnt/BTRFS/rsnapshot
> 
> Now, what surprises me, and here I lack memory- is that sdb appears
> twice.. I think, I created a raid1, but how can I find out?

   Appearing twice in that list is more an indication that you have
multiple subvolumes -- check the subvol= options in /etc/fstab

> #/usr/local/smarthome# ~/btrfs/btrfs-progs/btrfs fi show /dev/sdb1
> Label: none  uuid: 989306aa-d291-4752-8477-0baf94f8c42f
>         Total devices 2 FS bytes used 2.68TB
>         devid    2 size 2.73TB used 2.73TB path /dev/sdc1
>         devid    1 size 2.73TB used 2.73TB path /dev/sdb1
> 
> Now, I wanted to convert it to raid0, because I lack space and
> redundancy is not important for the Videos and the Backup, but this
> fails:
> ~/btrfs/btrfs-progs/btrfs fi balance start -dconvert=raid0  /mnt/BTRFS/
> ERROR: error during balancing '/mnt/BTRFS/' - Inappropriate ioctl for device

   /mnt/BTRFS isn't a btrfs subvol, according to what you have listed
above. It's a subdirectory in /mnt which is contains two subdirs
(Video and rsnapshot) which are used as mountpoints for subvolumes.

   Try running the above command with /mnt/BTRFS/Video instead (or
rsnapshot -- it doesn't matter which).

> dmesg does not help here.
> 
> Anyway: This gave me some time to think about this. In fact, as soon
> as raid5 is stable, I want to have all three as a raid5. Will this
> be possible with a balance command? If so: will this be possible as
> soon as raid5 is stable, or will I have to wait longer?

   Yes, it's possible to convert to RAID-5 right now -- although the
code's not settled down into its final form quite yet. Note that
RAID-5 over two devices won't give you any space benefits over RAID-1
over two devices. (Or any reliability benefits either).

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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