Re: multiple device usage

[Chris Samuel <chris@csamuel.org>]

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Hello Yan,

On Mon, 29 Dec 2008 11:33:18 pm Yan Zheng wrote:

> 2008/12/29 Chris Samuel <chris@csamuel.org>:
>
> > The rebalancing code does appear (from a naive read of the code) to be
> > able to rebalance over stripes, but I have no idea if the disk format
> > currently supports changing that on the fly.
>
> The rebalancing moves data/metadata to newly created chunks. If there
> are two devices, the new chunks will be set up as RAID-1 by default.

Very interesting!  I didn't realise that - I was presuming that the 
BTRFS_BLOCK_GROUP_RAID1 ioctl needed to be passed through (as at mkfs time) to 
change it into a RAID1 array.

Could you point me at the code where this change is detected please ?   Just 
so I can try and wrap my mind around it!

> > Again it would appear that you need to have two partitions and that btrfs
> > cannot (at present) keep multiple data stripes on the same partition.
>
> Yes. I think this is due to performance reason.

Understood.

> Changing the code to support data duplication in single spindle configuration
> is easy.

OK, that's encouraging (though for my immediate purposes creating two 
partitions is fine, but it might make life easier for others).

Again thanks so much for the response!

All the best,
Chris
-- 
 Chris Samuel  :  http://www.csamuel.org/  :  Melbourne, VIC

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