Re: unexpected raid1 behavior?

Hugo Mills
2009-11-27T21:04:42Z

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yOn Fri, Nov 27, 2009 at 03:41:17PM -0500, Jean-Philippe Robichaud wrote:
> Now I have 2 partitions (on 2 different sata disks) that are free for me to
> play with, each about 375 gb in size. I wanted to create a "raid1" volume
> using these two partitions, so I did:
>
> # mkfs.btrfs -d raid1 /dev/sda5 /dev/sdb5
> # mount /dev/sda5 /btrfs
>
> and everything seems fine.
>
> Now what I find strange is that everything looks like a raid0 was created, not
> a raid1:
>
> $ df -h /btrfs/
> Filesystem Size Used Avail Use% Mounted on
> /dev/sda5 684G 72G 612G 11% /btrfs
>
> What am I doing (or understanding) wrong?

It's effectively showing you the number of unallocated blocks, so
(with a RAID-1, single-redundancy filesystem), files will appear to
take twice as much of your free space as you think they should: write
a 2GiB file to that filesystem, and free space will drop by 4GiB.

I think that the reasoning behind this is that if you're using
per-object mirroring/striping, it's impossible to give a precise count
of the free space remaining on the volume in any meaningful way: write
a 1GiB striped file, and you'll take 1GiB of space; write the same
file mirrored, and you'll take 2GiB of space.

Hugo.

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