Shrinking a Google Compute disk

27 Oct 2019   •google computeOSunix

Problem: How to shrink a disk (partitionless) as well as the filesystem on it?

Situation: We have a disk that is much larger than what we need. Specifically, we had a 200 GB regional standard persistent disk on Google Compute, but we only needed a fraction of that. In addition, we cannot resize the disk itself, because regional standard persistent disks have a 200-GB minimum. ¹

Solution: Shrink the filesystem with resize2fs -M (here /dev/sdc) so that it can fit on the new smaller disk (here /dev/sdd), than use partclone.[fstype] -b to copy to a new disk.

• However,

  • before we can run resize2fs, we need to run e2fsck first.

    ray@instance-1:/tmp$ sudo e2fsck -f /dev/sdc
    e2fsck 1.44.5 (15-Dec-2018)
    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/sdc: 434/434176 files (0.7% non-contiguous), 1257420/1732511 blocks
    

• Now, we can run resize2fs. The -M option tells it to shrink the filesystem to the minimum size. ²

  ray@instance-1:/tmp$ sudo resize2fs -M /dev/sdc
  resize2fs 1.44.5 (15-Dec-2018)
  Resizing the filesystem on /dev/sdc to 1732504 (4k) blocks.
  The filesystem on /dev/sdc is now 1732504 (4k) blocks long.
  

• Clone the filesystem to a new disk. We can use partclone.[fstype]. In our case, we are using partclone.ext4. We specify the -b to select the device-to-device cloning mode. ³

  ray@instance-1:/tmp$ sudo partclone.ext4 -b -s /dev/sdc -o /dev/sdd
  Partclone v0.3.11 http://partclone.org
  Starting to back up device(/dev/sdc) to device(/dev/sdd)
  Elapsed: 00:00:01, Remaining: 00:00:00, Completed: 100.00%                      
  Total Time: 00:00:01, 100.00% completed!
  done!
  File system:  EXTFS
  Device size:    7.1 GB = 1732504 Blocks
  Space in use:   5.2 GB = 1257420 Blocks
  Free Space:     1.9 GB = 475084 Blocks
  Block size:   4096 Byte
  Elapsed: 00:02:16, Remaining: 00:00:00, Completed: 100.00%, Rate:   2.27GB/min, 
  current block:    1606658, total block:    1732504, Complete: 100.00%           
  Total Time: 00:02:16, Ave. Rate:    2.3GB/min, 100.00% completed!
  Syncing... OK!
  Partclone successfully cloned the device (/dev/sdc) to the device (/dev/sdd)
  Cloned successfully.
  

  • Based on the examples section of partclone, one might think we can just use -c for cloning (which fails). You might then try to overcome the error by forcing the output with -O instead of just -o (which also fails). Same with throwing in a -b option since both are devices. This won’t work because -c writes to the output a “special image format”.

• We had to shrink the filesystem to fit the new smaller disk. It might be the case that the filesystem is not taking up the entire capacity of the new disk. To resize the filesystem to fit the entirety of the new disk, just run resize2fs without any options on the disk: sudo resize2fs /dev/sdd.