Check metadata redundancy on balance

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.

From: Sam Tygier <[email protected]>
Date: Sat, 13 Jun 2015 18:13:06 +0100
Subject: [PATCH] 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 | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

index 174f5e1..875d608 100644
@@ -3336,6 +3336,24 @@ static void __cancel_balance(struct btrfs_fs_info *fs_info)
 	atomic_set(&fs_info->mutually_exclusive_operation_running, 0);
 }
 
+static int group_profile_max_safe_loss(u64 flag)
+{
+	switch (flag & BTRFS_BLOCK_GROUP_PROFILE_MASK) {
+	case 0: /* single */
+	case BTRFS_BLOCK_GROUP_DUP:
+	case BTRFS_BLOCK_GROUP_RAID0:
+		return 0;
+	case BTRFS_BLOCK_GROUP_RAID1:
+	case BTRFS_BLOCK_GROUP_RAID5:
+	case BTRFS_BLOCK_GROUP_RAID10:
+		return 1;
+	case BTRFS_BLOCK_GROUP_RAID6:
+		return 2;
+	default:
+		return -1;
+	}
+}
+
 /*
  * Should be called with both balance and volume mutexes held
  */

@@ -3454,6 +3472,12 @@ int btrfs_balance(struct btrfs_balance_control *bctl,
 		}
 	} while (read_seqretry(&fs_info->profiles_lock, seq));
 
+	if (group_profile_max_safe_loss(bctl->meta.target) <
+		group_profile_max_safe_loss(bctl->data.target)) {
+		btrfs_info(fs_info,
+			"Warning: metatdata has lower redundancy than data\n");
+	}
+
 	if (bctl->sys.flags & BTRFS_BALANCE_ARGS_CONVERT) {
 		int num_tolerated_disk_barrier_failures;
 		u64 target = bctl->sys.target;

2.4.2