Re: Help me understand what is going on with my RAID1 FS

[FLJ]

Thanks everyone for the helpful and detailed responses.
Now that you confirmed that everything is fine with my FS, I'm all
relaxed because I can for sure live with the output of df.



On Mon, Sep 11, 2017 at 5:29 AM, Andrei Borzenkov  wrote:
> 10.09.2017 23:17, Dmitrii Tcvetkov пишет:
 Drive1  Drive2Drive3
 X   X
 X X
 X X

 Where X is a chunk of raid1 block group.
>>>
>>> But this table clearly shows that adding third drive increases free
>>> space by 50%. You need to reallocate data to actually make use of it,
>>> but it was done in this case.
>>
>> It increases it but I don't see how this space is in any way useful
>> unless data is in single profile. After full balance chunks will be
>> spread over 3 devices, how it helps in raid1 data profile case?
>>
> A1 A2  => A1 A2 - => A1 A2 B1 => A1 A2 B1
> B1 B2 B1 B2 --  B2  -C1 B2 C2
>
> It is raid1 profile on three disks fully utilizing them (assuming equal
> sizes of course). Where "raid1" means - each data block has two copies
> on different devices.
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Help me understand what is going on with my RAID1 FS

[FLJ]

Hello all,

I have a BTRFS RAID1 volume running for the past year. I avoided all
pitfalls known to me that would mess up this volume. I never
experimented with quotas, no-COW, snapshots, defrag, nothing really.
The volume is a RAID1 from day 1 and is working reliably until now.

Until yesterday it consisted of two 3 TB drives, something along the lines:

Label: 'BigVault'  uuid: a37ad5f5-a21b-41c7-970b-13b6c4db33db
Total devices 2 FS bytes used 2.47TiB
devid1 size 2.73TiB used 2.47TiB path /dev/sdb
devid2 size 2.73TiB used 2.47TiB path /dev/sdc

Yesterday I've added a new drive to the FS and did a full rebalance
(without filters) over night, which went through without any issues.

Now I have:
 Label: 'BigVault'  uuid: a37ad5f5-a21b-41c7-970b-13b6c4db33db
Total devices 3 FS bytes used 2.47TiB
devid1 size 2.73TiB used 1.24TiB path /dev/sdb
devid2 size 2.73TiB used 1.24TiB path /dev/sdc
devid3 size 7.28TiB used 2.48TiB path /dev/sda

# btrfs fi df /mnt/BigVault/
Data, RAID1: total=2.47TiB, used=2.47TiB
System, RAID1: total=32.00MiB, used=384.00KiB
Metadata, RAID1: total=4.00GiB, used=2.74GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

But still df -h is giving me:
Filesystem   Size  Used Avail Use% Mounted on
/dev/sdb 6.4T  2.5T  1.5T  63% /mnt/BigVault

Although I've heard and read about the difficulty in reporting free
space due to the flexibility of BTRFS, snapshots and subvolumes, etc.,
but I only have a single volume, no subvolumes, no snapshots, no
quotas and both data and metadata are RAID1.

My expectation would've been that in case of BigVault Size == Used + Avail.

Actually based on http://carfax.org.uk/btrfs-usage/index.html I
would've expected 6 TB of usable space. Here I get 6.4 which is odd,
but that only 1.5 TB is available is even stranger.

Could anyone explain what I did wrong or why my expectations are wrong?

Thank you in advance
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