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Recreating a bcache Backing Device with a New Block Size (Cache Device Not Used)

#cli #linux #performance #tutorial

This article describes how to change the bcache block size when you are operating without a cache device (the cache device has already been detached, and you will not add one during re-creation). Because bcache block size is defined at creation time, the practical approach is to recreate the bcache backing device with the desired parameters.

Important Notes (Read First)

  • Changing the bcache block size requires recreating bcache metadata. Treat this as a destructive operation unless you have verified recovery procedures and have a current backup.
  • bcache stores data beginning at a data offset on the backing device. You must capture the current offset and reuse it during re-creation to keep layout alignment consistent.

  • This procedure assumes:

    • The cache device is already detached.
    • You will create only the backing device (-B) and will not create or attach a cache device (-C / attach).

Prerequisites

  • Root privileges.
  • bcache-tools installed (provides bcache-super-show and make-bcache).
  • The backing device identified (example: /dev/sdb1).
  • Any filesystems/LVM/dm-crypt/MD layers above /dev/bcache* will be stopped cleanly.

Step 1 — Record the Current Data Offset

Read the existing bcache superblock on the backing device and record the data offset (commonly shown as dev.data.first_sector or an equivalent field):

sudo bcache-super-show /dev/sdb1
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Make a note of the value that corresponds to the data start offset. You will reuse it as <OFFSET> in Step 4.

Step 2 — Stop Using the bcache Device Cleanly

2.1 Unmount (and stop upper layers)

If /dev/bcache0 (or similar) is mounted, unmount it:

mount | grep bcache
sudo umount /dev/bcache0
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If you use LVM, dm-crypt, MD RAID, etc. on top of /dev/bcache0, stop those layers first.

2.2 Stop the bcache device

Stop the bcache device via sysfs (example for /dev/bcache0):

echo 1 | sudo tee /sys/block/bcache0/bcache/stop
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If this fails, an upper layer is still holding the device open. Re-check mounts and dependent mappings.

Step 3 — Remove Existing bcache Signatures from the Backing Device

Wipe the bcache signatures from the backing device:

sudo wipefs -a /dev/sdb1
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This removes filesystem/RAID/bcache signatures that would otherwise conflict with re-creation.

Step 4 — Recreate bcache on the Backing Device with a New Block Size

Now recreate the backing device bcache metadata, specifying:

  • Your new block size (example: 4k)
  • The previously recorded data offset (<OFFSET>)
  • Backing mode only (-B) 
sudo make-bcache --block 4k --data-offset <OFFSET> -B /dev/sdb1
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Notes:

  • Replace 4k with your target block size.
  • Replace <OFFSET> with the value captured in Step 1.
  • Because you are not using a cache device, do not run make-bcache -C ... and do not attach a cache set.

Step 5 — Verify the New bcache Device

Confirm that /dev/bcache* has been created:

ls -l /dev/bcache*
lsblk -f | grep -E 'bcache|sdb1'
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Check state (example for /dev/bcache0):

cat /sys/block/bcache0/bcache/state
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At this point, you can create a filesystem (if appropriate) and mount it, or reintroduce your higher layers (LVM, dm-crypt, etc.) as required by your environment.

Operational Consideration (Running Without a Cache Device)

If you do not add a cache device, bcache effectively behaves as a pass-through block device wrapper. This can still be useful if you plan to attach cache later or want consistent device naming/management, but you should be aware that you are not gaining caching benefits until a cache set is attached.

Common Failure Modes and How to Avoid Them

  • “Device or resource busy” when stopping bcache An upper layer (mount, LVM, dm-crypt, MD, etc.) is still open. Stop/unmount from the top down.
  • Wrong offset used during re-creation Using an incorrect <OFFSET> can cause data layout mismatch and potential data loss. Always record the offset directly from bcache-super-show.

Summary

To change bcache block size without using a cache device, you:

  1. Record the data offset from bcache-super-show.
  2. Cleanly stop usage and stop the bcache device.
  3. Remove signatures with wipefs -a.
  4. Recreate bcache backing only using make-bcache --block <new> --data-offset <old> -B.
  5. Verify /dev/bcache* and proceed with your storage stack.

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