Re: [PATCH] btrfs-progs: add RAID5/6 support to btrfs fi us

[Goffredo Baroncelli <kreijack@libero.it>]

On 5/25/20 3:27 PM, David Sterba wrote:
> On Wed, Mar 18, 2020 at 10:11:56PM +0100, Goffredo Baroncelli wrote:
>> this patch adds support for the raid5/6 profiles in the command
>> 'btrfs filesystem usage'.
>>
>> Until now the problem was that the value r_{data,metadata}_used is not
>> easy to get for a RAID5/6, because it depends by the number of disks.
>> And in a filesystem it is possible to have several raid5/6 chunks with a
>> different number of disks.
> 
> I'd like to get the raid56 'fi du' fixed but the way you implement it
> seems to be a too big leap. I've tried to review this patch several
> times but always got the impression that reworking the calculations to
> make it work for some profiles will most likely break something else. It
> has happened in the past.

I understand your fear. Frankly speaking this code is quite complex.
More than it should be (even without the raid56 support).

I am looking for a solution less intrusive. Let me few days and I will
update the patch.

Then we can discuss its validity.

  
> So, let's start with the case where the filesystem does not have
> multiple profiles per block group type, eg. just raid5 for data and
> calculate that.
> 
> If this also covers the raid56 case with different stripe counts, then
> good but as this is special case I won't mind addressing it separately.
> 
> The general case of multiple profiles per type is probably an
> intermediate state of converting profiles, we can return something sane
> if possible or warn as what we have now.

Another possibility is when a drive is added and a balance is not performed.

However this should be "safe" because it would underestimate the free space.

> 
> I'm fine if you say you're not going to implement that.
> 

I want to work on that.


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