Hi Alwin, Thanks for the input. Sounds like "stopped" is the safe way, which is what I'd assumed. It would be good if that was clarified in the Ceph docs (https://docs.ceph.com/en/latest/man/8/rbd/#:~:text=sparsify). I found in the Proxmox forum thread that if I create 1+10 GB in two zero files on a small VM, delete, and trim, "rbd du" USED remained at 37/43 GB for that VM, so that doesn't help. Also same issue on Windows VMs, and we have at least one (a pfSense VM) using ZFS, so it's not just EXT4. Re: the serverfault thread, rebooting a VM results in no change in "rbd du" USED. We do regularly reboot VMs for updates, and migrate too when rebooting PVE nodes. I'm about at the point of ignoring the "rbd du" USED number. It doesn't seem terribly meaningful. -----Original Message----- From: Alwin Antreich via ceph-users <ceph-users@ceph.io> Sent: Wednesday, March 11, 2026 6:27 AM To: Steve Yates <steve@teamits.com> Cc: ceph-users@ceph.io Subject: [ceph-users] Re: Can rbd sparsify be run on a running VM's disk? Hi Steve, I'd advise against using sparsify, especially on any mapped RBD (running VM). We had a support case once where this operation on a live VM caused an OSD lockup, halting the operation for other clients. Sparsify really tries to find 0-filled objects that can be removed. It's best to stick with fstrim (or equivalent) in the guest. rbd du estimates size cheaply by counting the number of objects and multiplying that count by the set object size. Therefore it diverges from the size you see of the guest's filesystem. Some additional points may contribute to the difference: * An OSD has a 4K (SSD) / 64K (HDD) threshold (bluestore_min_alloc_size) for discard operations, meaning any discard smaller than this threshold is ignored. * Disk encryption often doesn't clear block for security. * EXT4 uses an in-memory bitmap that does not trim blocks if they haven't been overwritten before. This safeguards against wear on consumer SSDs but has the disadvantage that a storage move will thick-allocate the disk and an fstrim isn't clearing it. The only option is to write a big file and delete it or reboot. See: https://forum.proxmox.com/threads/help-with-trim-on-virtio-scsi-single.12381... I hope this helps. Cheers, Alwin On Tue, 10 Mar 2026 at 14:59, Steve Yates via ceph-users <ceph-users@ceph.io> wrote:
Question: Can rbd sparsify safely be run on a running VM's disk, in Ceph 19/Squid? Or must the VM be shut down first? I tried looking online but am not finding a direct answer.
More info: I'm looking into why "rbd du" shows usage inconsistent with "ceph df" or Proxmox's "usage" displays (Proxmox thread< https://forum.proxmox.com/threads/ceph-rbd-du-shows-usage-2-4x-higher-than-inside-vm.181348/>). Sparsify on a stopped VM reduces "rbd du" usage, and the number of objects, but doesn't change the amount of space used according to Ceph and PVE. I am wondering if sparsify can be safely scripted. Or if I should just ignore "rbd du" USED amounts.
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