Re: unexpected raid1 behavior?

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

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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.

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