Re: [PATCH 1/2] btrfs: create degraded-RAID1 chunks

From: David Sterba <dsterba@suse.cz>
To: Anand Jain <anand.jain@oracle.com>
Cc: linux-btrfs@vger.kernel.org, clm@fb.com
Subject: Re: [PATCH 1/2] btrfs: create degraded-RAID1 chunks
Date: Fri, 29 Apr 2016 18:42:34 +0200	[thread overview]
Message-ID: <20160429164234.GE29353@suse.cz> (raw)
In-Reply-To: <1461812780-538-2-git-send-email-anand.jain@oracle.com>

On Thu, Apr 28, 2016 at 11:06:19AM +0800, Anand Jain wrote:
> When RAID1 is degraded, newer chunks should be degraded-RAID1
> chunks instead of single chunks.
> 
> The bug is because the devs_min for raid1 was wrong it should
> be 1, instead of 2.
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>  fs/btrfs/volumes.c | 38 +++++++++++++++++++++++++++++++++-----
>  1 file changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index e2b54d546b7c..8b87ed6eb381 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -56,7 +56,7 @@ const struct btrfs_raid_attr btrfs_raid_array[BTRFS_NR_RAID_TYPES] = {
>  		.sub_stripes	= 1,
>  		.dev_stripes	= 1,
>  		.devs_max	= 2,
> -		.devs_min	= 2,
> +		.devs_min	= 1,

I think we should introduce another way how to determine the lower limit
for the degraded mounts. We need the proper raidX constraints and use
the degraded limits only if in case of the degraded mount.

>  		.tolerated_failures = 1,

Which is exactly the tolerated_failures:

  degraded_devs_min == devs_min - tolerated_failures

which works for all raid levels with redundancy.

>  		.devs_increment	= 2,
>  		.ncopies	= 2,