Re: Recover btrfs volume which can only be mounded in read-only mode

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

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On Thu, Oct 15, 2015 at 04:10:13PM +0200, Dmitry Katsubo wrote:
[snip]
> If I may ask:
> 
> Provided that btrfs allowed to mount a volume in read-only mode – does
> it mean that add data blocks are present (e.g. it has assured that add
> files / directories can be read)?
> 
> Do you have any ideas why "btrfs balance" has pulled all data to two
> drives (and not balanced between three)?

   If you're using a non-striped RAID level (single, 1), btrfs will
start by filling up the largest devices first: balance attempts to
make the free space equal across the devices, not to make the used
space equal.

   If you're using a striped RAID level (0, 5, 6), then the FS will
fill up the devices equally, until one is full, and then switch to
using the remaining devices (until one is full, etc).

> Does btrfs has the following optimization for mirrored data: if drive
> is non-rotational, then prefer reads from it? Or it simply schedules
> the read to the drive that performs faster (irrelative to rotational
> status)?

   No, it'll read arbitrarily from the available devices at the moment.

   Hugo.

-- 
Hugo Mills             | People are too unreliable to be replaced by
hugo@... carfax.org.uk | machines.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                              Nathan Spring, Star Cops

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