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RobM
Asked: 2015-11-25 07:30:51 +0800 CST

LVM2: Can I change the LV size (not COW-table size) of a snapshot?

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I'm curious if it is possible to create a snapshot of (e.g.) 10G volume, and then resize the snapshot to be 20G, along with the file-system within?

Using lvresize or lvextend only alters the COW-table size, the LV Size remains at 10G, and subsequently attempting to resize the file-system fails. E.g.

# lvextend --size 20G --resizefs /dev/vg_data/bigsize_test
fsck from util-linux 2.20.1
e2fsck 1.42 (29-Nov-2011)
/dev/mapper/vg_data-bigsize_test: clean, 27/655360 files, 85571/2621440 blocks
  Extending logical volume bigsize_test to 20.00 GiB
  Logical volume bigsize_test successfully resized
resize2fs 1.42 (29-Nov-2011)
The containing partition (or device) is only 2621440 (4k) blocks.
You requested a new size of 5242880 blocks.

fsadm: Resize ext4 failed
  fsadm failed: 1

This leaves the snapshot with a big COW-table size, but not a big LV Size which is what I was aiming for:

  # lvdisplay /dev/vg_data/bigsize_test
  --- Logical volume ---
  LV Name                /dev/vg_data/bigsize_test
  VG Name                vg_data
  LV UUID                77uCVJ-Sdml-KlQS-xGkS-mPiT-ITzU-0i7Zwi
  LV Write Access        read/write
  LV snapshot status     active destination for /dev/vg_data/origin
  LV Status              available
  # open                 0
  LV Size                10.00 GiB
  Current LE             2560
  COW-table size         20.00 GiB
  COW-table LE           5120
  Allocated to snapshot  0.00%
  Snapshot chunk size    4.00 KiB
  Segments               2
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           252:30

Is it possible to make a snapshot bigger than its origin?

lvm

1 Answers

  1. Best Answer

    Am LVM2 snapshot isn't a clone, it's just keeps track of the state of the snapshotted partition at the time. If it was 10GB, you can't just change that. If you want to create a larger copy of the content of a partition at a certain time, just make a new 20GB volume, and copy the snapshot over. It is good to be clear (to yourself and others) what you want to achieve.

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