Feature pages are design documents that developers have created while collaborating on oVirt.

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This feature enables the user to run virt-sparsify utility on the virtual machine disks. virt-sparsify removes unused space from a disk image and returns it to storage.


  • Name: Shmuel Melamud
  • Email:

Detailed Design


Previously VDSM supported virt-sparsify in a simple form that created a temporary disk. With the new approach, a temporary disk, location, and extra disk space are not needed, thanks to in-place operation mode of virt-sparsify.

The new SDM.sparsify_volume verb was created that runs virt-sparsify asynchronously using storage jobs mechanism. Storage jobs allow the operation to be performed on any host.

Since virt-sparsify tool cannot leave the volume in corrupted state, there is no need to mark the volume as ILLEGAL before running the tool. Otherwise, if the storage domain becomes unavailable during the operation, the volume will be left in ILLEGAL state and its futher usage will be impossible.


Added SparsifyImageCommand to sparsify the given disk image. This command checks all prerequisites, locks the disk and invokes SparsifyImageVDSCommand. SparsifyImageVDSCommand uses SDM.sparsify_inplace VDSM API call to perform the actual work.

In the REST API, added the sparsify method to DiskService that invokes SparsifyImageCommand on the disk.


Added “Sparsify” button to Disks subtab of Virtual Machines main tab. This button opens a confirmation dialog to approve sparsification operation and, after the operation is confirmed, invokes SparsifyImageCommand for the disk selected.


  • Only the leaf volume of the image (the active snapshot) can be sparsified.
  • If some file existed in previous snapshots and was deleted in the latest snapshot, its space cannot be freed and returned to the storage. In such situations the virtual disk may even become slightly bigger (maximum 0.01% of its size), because sparsification may need more space in the disk’s metadata. To sparsify effectively, merge all snapshots before sparsification.
  • The disk cannot have derived disks.
  • If the disk is attached to a VM, it must be down.
  • Direct LUNs cannot be sparsified.
  • Cinder disks cannot be sparsified.
  • Pre-allocated disks cannot be sparsified. virt-sparsify works on pre-allocated disks, but its usage is not justified for this case. User chooses pre-allocated option if she needs better performance, because no additional allocation and no fragmentation will occur when the disk is used. Deallocating some clusters as result of sparsification will overturn this advantage. If user wants to save space at cost of some performance decrease, she can select thin-provisioned option.
  • Disks on NFS storage may be sparsified only if NFS version >= 4.2. Note nfs-utils package in RHEL 7.4 contains bug 1489262 that prevents NFS storage to be mounted as NFS 4.2. That means that virt-sparsify will not work on NFS partitions if RHEL 7.4 is installed on the host. This bug is fixed in nfs-utils-1.3.0-0.49.el7 that is planned to be released with RHEL 7.5. A backport to RHEL 7.4.z is requested also.
  • For an iSCSI volume to be sparsified, support of both host kernel and the actual backing store is required.

    You can verify this support using various SCSI flags reported by the device. These are contained in the SCSI “VPD” data. Run sg_inq and sg_vpd (from sg3_utils) on the device:

     $ sudo sg_inq -p 0xb0 /dev/sda
     $ sudo sg_vpd -p 0xb2 /dev/sda

    The LBPRZ and various unmap settings are all relevant.

    See also

Future Development

  • Display the actual size of disks (currently we display only the virtual size).
  • Add indication that disk needs to be sparsified, if amount of unused space passes a defined threshold.
  • Added a user-configurable periodic job that sparsifies disks when needed.

Current Status

  • VDSM: Released
  • Engine: Released
  • UI: Released