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



On Sat, Mar 17, 2012 at 02:46:00PM +0100, Helmut Hullen wrote:
> Hallo, Hugo,
> 
> Du meintest am 17.03.12:
> 
> [no space left on device ...]
> 
> >>> Where is the problem, how can I use the full space?
> 
> >> Effectively it's missing the trigger to rebalance when the 'primary'
> >> device starts to get full, or just to randomly spread the data
> >> between the devices.
> 
> >    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.
> 
> Ok - it happens only with 2 disks/Partitions?

   Not quite. With the current RAID-0 implementation, you will lose
the difference between the largest disk and the second-largest.
There's a proposal from November that may help with that, but I
haven't had a chance to look at it yet to see what the implications
are.

> I've continued playing; added a 3rd partition/device and then balanced:
> 
> 
> # btrfs device add /dev/sdd1 /mnt/btr
> ## 73 + 146 + 146 GByte

   OK, so in this case the largest and second-largest devices are the
same, so you will lose no space.

[snip]

> Looks as desired, the 3-disks-system contains more than 3 times the  
> smallest disk.
> 
> Balancing hasn't redistributed the contents - no problem.
> 
> By the way: you should name the prefixes in the NIST way for powers of  
> 2: KiB, MiB, GiB. Or change to decimal prefixes.

   I know. It's something I submitted patches for a long time ago, and
they never got taken up. It's on my list of things to fix.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
        --- UNIX: Spanish manufacturer of fire extinguishers. ---        

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