[DRBD-user] Can't shrink backing block device offline

[Adam Goryachev]

On 15/7/21 20:40, Christophorus Reyhan wrote:
> Hello,
>
> I'm trying to shrink LVM backing block device offline because I 
> use internal metadata, and I'm following this guide 
> https://linbit.com/drbd-user-guide/drbd-guide-9_0-en/#s-shrinking-offline 
> <https://linbit.com/drbd-user-guide/drbd-guide-9_0-en/#s-shrinking-offline> 
> . But it failed at create-md step.
>
> *Here's the log for first machine (called ol1):*
> [root at ol1 drbd.d]# e2fsck -f /dev/drbd0
> e2fsck 1.42.9 (28-Dec-2013)
> Pass 1: Checking inodes, blocks, and sizes
> Pass 2: Checking directory structure
> Pass 3: Checking directory connectivity
> Pass 4: Checking reference counts
> Pass 5: Checking group summary information
> /dev/drbd0: 12/51200 files (0.0% non-contiguous), 109774/204732 blocks
> [root at ol1 drbd.d]# drbdadm status
> test role:Primary
>   disk:UpToDate
>   ol2 role:Secondary
>     peer-disk:UpToDate
>
> [root at ol1 drbd.d]# resize2fs /dev/drbd0 180M
So the filesystem inside your DRBD device is going to be 180M
> resize2fs 1.42.9 (28-Dec-2013)
> Resizing the filesystem on /dev/drbd0 to 184320 (1k) blocks.
> The filesystem on /dev/drbd0 is now 184320 blocks long.
>
> [root at ol1 drbd.d]# drbdadm down test
> [root at ol1 drbd.d]# drbdadm dump-md test > /tmp/metadata
> Found meta data is "unclean", please apply-al first
> [root at ol1 drbd.d]# drbdadm apply-al test
> [root at ol1 drbd.d]# drbdadm dump-md test > /tmp/metadata
> [root at ol1 drbd.d]# lvreduce -L 180M /dev/mapper/vgdrbd-vol_drbd
>   WARNING: Reducing active logical volume to 180.00 MiB.
>   THIS MAY DESTROY YOUR DATA (filesystem etc.)
> Do you really want to reduce vgdrbd/vol_drbd? [y/n]: y
>   Size of logical volume vgdrbd/vol_drbd changed from 200.00 MiB (50 
> extents) to 180.00 MiB (45 extents).
Also, the size of the volume containing both your ext filesystem AND 
your DRBD data is going to be 180M....
>   Logical volume vgdrbd/vol_drbd successfully resized.
> [root at ol1 drbd.d]# vi /tmp/metadata
> [root at ol1 drbd.d]# blockdev --getsz /dev/mapper/vgdrbd-vol_drbd
> 368640
> [root at ol1 drbd.d]# vi /tmp/metadata
> [root at ol1 drbd.d]# drbdadm create-md test --max-peers=5
> You want me to create a v09 style flexible-size internal meta data block.
> There appears to be a v09 flexible-size internal meta data block
> already in place on /dev/vgdrbd/vol_drbd at byte offset 188739584
>
> Do you really want to overwrite the existing meta-data?
> [need to type 'yes' to confirm] yes
>
> md_offset 188739584
> al_offset 188706816
> bm_offset 188674048
>
> Found ext3 filesystem
>       184320 kB data area apparently used
>       184252 kB left usable by current configuration

As per the messages, there isn't enough space to preserve both your 
filesystem data AND add the DRBD data. Either resize your filesystem to 
179M, resize the LV to 180M, and then after everything is working again, 
you can resize the filesystem to take all available space, and it should 
increase to 184252kB or 179.9+ MB

I'm pretty sure the DRBD manual has some details on how to calculate 
exactly how much space you will need to leave available for the DRBD 
metadata, but I usually just do the above (make the filesystem smaller 
than my target, and then after DRBD has been resized, increase the FS to 
take up any extra space).

Hope that helps...

Regards,
Adam

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