Re: [PATCH] Btrfs: Check metadata redundancy on balance

[sam tygier <samtygier@yahoo.co.uk>]

On 16/09/15 11:15, Zhao Lei wrote:
> Hi, sam tygier
> 
>> -----Original Message-----
>> From: linux-btrfs-owner@vger.kernel.org
>> [mailto:linux-btrfs-owner@vger.kernel.org] On Behalf Of sam tygier
>> Sent: Wednesday, September 16, 2015 4:42 PM
>> To: linux-btrfs@vger.kernel.org
>> Subject: [PATCH] Btrfs: Check metadata redundancy on balance
>>
>> It was recommended that I resend after the merge window. No changes since
>> last version.
>>
>> 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.
>>
> 
> Can we check and show warning of balance operation in btrfs-progs,
> just like above patch?
> 

I was not completely sure if this was better suited in btrfs-progs or the kernel.
The existing checks against reducing redundancy are on the kernel side. Currently
it looks like btrfs-progs does look at the current levels, only passes the arguments
through to the kernel. If this was put on the btrfs-progs side, would it be an
issue that some other tool might call the kernel directly, and bypass the check. 


>> 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.
>>
>> Signed-off-by: Sam Tygier <samtygier@yahoo.co.uk>
>>
>> From: Sam Tygier <samtygier@yahoo.co.uk>
>> Date: Sat, 13 Jun 2015 18:13:06 +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 | 24 ++++++++++++++++++++++++
>>   1 file changed, 24 insertions(+)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index fbe7c10..a0ce1f7
>> 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -3454,6 +3454,24 @@ 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;
>> +	}
>> +}
>> +
> 
> Maybe btrfs_get_num_tolerated_disk_barrier_failures()
> fits above request, better to use existence function if possible.

Sorry, I missed that. If it stays kernel side, I can switch to this. If it moves
to btrfs-progs I'll use the existing group_profile_max_safe_loss()

> Thanks
> Zhaolei
> 
> 
>>   /*
>>    * Should be called with both balance and volume mutexes held
>>    */
>> @@ -3572,6 +3590,12 @@ 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;
>> --
>> 2.4.3