Chapter 2. LVM-VDO requirements

VDO on LVM has certain requirements on its placement and your system resources.

2.1. VDO memory requirements

Each VDO volume has two distinct memory requirements:

The VDO module

VDO requires a fixed 38 MB of RAM and several variable amounts:

  • 1.15 MB of RAM for each 1 MB of configured block map cache size. The block map cache requires a minimum of 150MB RAM.
  • 1.6 MB of RAM for each 1 TB of logical space.
  • 268 MB of RAM for each 1 TB of physical storage managed by the volume.
The UDS index

The Universal Deduplication Service (UDS) requires a minimum of 250 MB of RAM, which is also the default amount that deduplication uses. You can configure the value when formatting a VDO volume, because the value also affects the amount of storage that the index needs.

The memory required for the UDS index is determined by the index type and the required size of the deduplication window:

Index typeDeduplication windowNote

Dense

1 TB per 1 GB of RAM

A 1 GB dense index is generally sufficient for up to 4 TB of physical storage.

Sparse

10 TB per 1 GB of RAM

A 1 GB sparse index is generally sufficient for up to 40 TB of physical storage.

Note

The minimal disk usage for a VDO volume using default settings of 2 GB slab size and 0.25 dense index, requires approx 4.7 GB. This provides slightly less than 2 GB of physical data to write at 0% deduplication or compression.

Here, the minimal disk usage is the sum of the default slab size and dense index.

The UDS Sparse Indexing feature is the recommended mode for VDO. It relies on the temporal locality of data and attempts to retain only the most relevant index entries in memory. With the sparse index, UDS can maintain a deduplication window that is ten times larger than with dense, while using the same amount of memory.

Although the sparse index provides the greatest coverage, the dense index provides more deduplication advice. For most workloads, given the same amount of memory, the difference in deduplication rates between dense and sparse indexes is negligible.

Additional resources

2.2. VDO storage space requirements

You can configure a VDO volume to use up to 256 TB of physical storage. Only a certain part of the physical storage is usable to store data. This section provides the calculations to determine the usable size of a VDO-managed volume.

VDO requires storage for two types of VDO metadata and for the UDS index:

  • The first type of VDO metadata uses approximately 1 MB for each 4 GB of physical storage plus an additional 1 MB per slab.
  • The second type of VDO metadata consumes approximately 1.25 MB for each 1 GB of logical storage, rounded up to the nearest slab.
  • The amount of storage required for the UDS index depends on the type of index and the amount of RAM allocated to the index. For each 1 GB of RAM, a dense UDS index uses 17 GB of storage, and a sparse UDS index will use 170 GB of storage.

Additional resources

2.3. Examples of VDO requirements by physical size

The following tables provide approximate system requirements of VDO based on the physical size of the underlying volume. Each table lists requirements appropriate to the intended deployment, such as primary storage or backup storage.

The exact numbers depend on your configuration of the VDO volume.

Primary storage deployment

In the primary storage case, the UDS index is between 0.01% to 25% the size of the physical size.

Table 2.1. Storage and memory requirements for primary storage

Physical sizeRAM usage: UDSRAM usage: VDODisk usageIndex type

10GB–1TB

250MB

472MB

2.5GB

Dense

2–10TB

1GB

3GB

10GB

Dense

250MB

22GB

Sparse

11–50TB

2GB

14GB

170GB

Sparse

51–100TB

3GB

27GB

255GB

Sparse

101–256TB

12GB

69GB

1020GB

Sparse

Backup storage deployment

In the backup storage case, the UDS index covers the size of the backup set but is not bigger than the physical size. If you expect the backup set or the physical size to grow in the future, factor this into the index size.

Table 2.2. Storage and memory requirements for backup storage

Physical sizeRAM usage: UDSRAM usage: VDODisk usageIndex type

10GB–1TB

250MB

472MB

2.5 GB

Dense

2–10TB

2GB

3GB

170GB

Sparse

11–50TB

10GB

14GB

850GB

Sparse

51–100TB

20GB

27GB

1700GB

Sparse

101–256TB

26GB

69GB

3400GB

Sparse

2.4. Placement of LVM-VDO in the storage stack

You must place certain storage layers under a VDO logical volume and others above it.

You can place thick-provisioned layers on top of VDO, but you cannot rely on the guarantees of thick provisioning in that case. Because the VDO layer is thin-provisioned, the effects of thin provisioning apply to all layers above it. If you do not monitor the VDO volume, you might run out of physical space on thick-provisioned volumes above VDO.

The supported placement of the following layers is under VDO. Do not place them above VDO:

  • DM Multipath
  • DM Crypt
  • Software RAID (LVM or MD RAID)

The following configurations are not supported:

  • VDO on top of a loopback device
  • Encrypted volumes on top of VDO
  • Partitions on a VDO volume
  • RAID, such as LVM RAID, MD RAID, or any other type, on top of a VDO volume
  • Deploying Ceph Storage on LVM-VDO

Additional resources