Btrfs: Check metadata redundancy on balance

Resending as previous comments did not need any changes.

Currently BTRFS allows you to make bad choices of data and metadata levels. For example -d raid1 -m raid0 means you can only use half your total disk space, but will loose everything if 1 disk fails. It should give a warning in these cases.

This patch is a follow up to
[PATCH v2] btrfs-progs: check metadata redundancy
in order to cover the case of using balance to convert to such a set of raid levels.

A simple example to hit this is to create a single device fs, which will default to single:dup, then to add a second device and attempt to convert to raid1 with the command
btrfs balance start -dconvert=raid1 /mnt
this will result in a filesystem with raid1:dup, which will not survive the loss of one drive. I personally don't see why the tools should allow this, but in the previous thread a warning was considered sufficient.

Changes in v2
Use btrfs_get_num_tolerated_disk_barrier_failures()

From: Sam Tygier <[email protected]>
Date: Sat, 3 Oct 2015 16:43:48 +0100
Subject: [PATCH] Btrfs: Check metadata redundancy on balance

When converting a filesystem via balance check that metadata mode is at least as redundant as the data mode. For example give warning when:
-dconvert=raid1 -mconvert=single

---
 fs/btrfs/volumes.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 6fc73586..40247e9 100644
@@ -3584,6 +3584,12 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 		}
 	} while (read_seqretry(&fs_info->profiles_lock, seq));
 
+	if (btrfs_get_num_tolerated_disk_barrier_failures(bctl->meta.target) <
+		btrfs_get_num_tolerated_disk_barrier_failures(bctl->data.target)) {
+		btrfs_info(fs_info,
+			"Warning: metatdata has lower redundancy than data\n");
+	}
+
 	if (bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) {
 		fs_info->num_tolerated_disk_barrier_failures = min(
 			btrfs_calc_num_tolerated_disk_barrier_failures(fs_info),

2.4.3