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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 type Deduplication window Note 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.
NoteThe 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 size RAM usage: UDS RAM usage: VDO Disk usage Index 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 size RAM usage: UDS RAM usage: VDO Disk usage Index 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
