Re: "not enough space" with "data raid0"

Hugo Mills <hugo@xxxxxxxxxxxxx> · Sat, 17 Mar 2012 14:00:57 +0000

On Sat, Mar 17, 2012 at 01:36:04PM +0000, Alex wrote:
> >    No, a balance isn't going to help here. RAID-0 requires a minimum
> > of 2 chunks in a block group. With two disks, you're only going to be
> > able to fill the smallest one before you run out of space.
> > 
> > > Isn't this going to be a problem for anyone restoring from a backup? Lots of
> > > data heading to btrfs without a pause.
> > 
> >    Again, not the issue here.
> 
> Thank you HH for the explanation.
> 
> What's the 'solution' though to Hugo's situation?
> By 'solution' I mean the highest-utility way of dealing with unequal devices
> problem in a two or more -up setting.

   Use mkfs.btrfs -d single, as I said in another part of this thread.

   We have a bit of a problem right now with adding a second device to
an existing FS with -d single, where the FS "upgrades" the single to
RAID-0. However, there's patches from Ilya that stop that behaviour,
and the restriper can be used to switch back to single after the
"upgrade".

> (I understand that we're violating the expectations of RAID0 and
> have read the
> http://btrfs.ipv5.de/index.php?title=FAQ#How_much_space_do_I_get_with_unequal_devices_in_RAID-1_mode.3F
> about RAID1.)

   I'm working on a comprehensive explanation of the limits on btrfs's
space usage, and a little JavaScript tool to help plan/explain disk
usage.

> Disks are (were) getting cheaper per MB faster than I could run out
> of space! So I have increasing size array of drives - apparently the
> similar situation as Hugo (I have three btrfs-ready drives).

   I don't have this problem personally. I just know something about
the way btrfs allocates chunks. I think you meant Helmut has the
problem. :)

> I'm struggling how to mkfs.btrfs with this new knowledge whilst resolving it
> with the btrfs storage pool concept.
> 
> I stress there is a difficulty with my understanding not btrfs.

   Hugo.

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