NAME
lvmvdo — Support for Virtual Data Optimizer in LVM
DESCRIPTION
VDO is software that provides inline block-level deduplication, compression, and thin provisioning capabilities for primary storage.
Deduplication is a technique for reducing the consumption of storage resources by eliminating multiple copies of duplicate blocks. Compression takes the individual unique blocks and shrinks them. These reduced blocks are then efficiently packed together into physical blocks. Thin provisioning manages the mapping from logical blocks presented by VDO to where the data has actually been physically stored, and also eliminates any blocks of all zeroes.
With deduplication, instead of writing the same data more than once, VDO detects and records each duplicate block as a reference to the original block. VDO maintains a mapping from Logical Block Addresses (LBA) (used by the storage layer above VDO) to physical block addresses (used by the storage layer under VDO). After deduplication, multiple logical block addresses may be mapped to the same physical block address; these are called shared blocks and are reference-counted by the software.
With compression, VDO compresses multiple blocks (or shared blocks) with the fast LZ4 algorithm, and bins them together where possible so that multiple compressed blocks fit within a 4 KB block on the underlying storage. Mapping from LBA is to a physical block address and index within it for the desired compressed data. All compressed blocks are individually reference counted for correctness.
Block sharing and block compression are invisible to applications using the storage, which read and write blocks as they would if VDO were not present. When a shared block is overwritten, a new physical block is allocated for storing the new block data to ensure that other logical block addresses that are mapped to the shared physical block are not modified.
To use VDO with lvm(8), you must install the standard VDO user-space tools vdoformat(8) and the currently non-standard kernel VDO module "kvdo".
The "kvdo" module implements fine-grained storage virtualization, thin provisioning, block sharing, and compression. The "uds" module provides memory-efficient duplicate identification. The user-space tools include vdostats(8) for extracting statistics from VDO volumes.
VDO TERMS
VDODataLV
VDO data LV
A large hidden LV with the _vdata suffix. It is created in a
VG
used by the VDO kernel target to store all data and metadata
blocks.
VDOPoolLV
VDO pool LV
A pool for virtual VDOLV(s), which are the size of used
VDODataLV.
Only a single VDOLV is currently supported.
VDOLV |
VDO LV |
Created from VDOPoolLV.
Appears blank after creation.
VDO USAGE
The primary methods for using VDO with lvm2:
1. Create a
VDOPoolLV and a VDOLV
Create a VDOPoolLV that will hold VDO data, and a virtual
size VDOLV that the user can use. If you do not specify the
virtual size, then the VDOLV is created with the maximum
size that always fits into data volume even if no
deduplication or compression can happen (i.e. it can hold
the incompressible content of /dev/urandom). If you do not
specify the name of VDOPoolLV, it is taken from the sequence
of vpool0, vpool1 ...
Note: The performance of TRIM/Discard operations is slow for large volumes of VDO type. Please try to avoid sending discard requests unless necessary because it might take considerable amount of time to finish the discard operation.
lvcreate
--type vdo -n VDOLV -L DataSize -V LargeVirtualSize
VG/VDOPoolLV
lvcreate --vdo -L DataSize VG
Example
# lvcreate --type vdo -n vdo0 -L 10G -V 100G vg/vdopool0
# mkfs.ext4 -E nodiscard /dev/vg/vdo0
2. Convert
an existing LV into VDOPoolLV
Convert an already created or existing LV into a VDOPoolLV,
which is a volume that can hold data and metadata. You will
be prompted to confirm such conversion because it
IRREVERSIBLY DESTROYS the content of such volume and
the volume is immediately formatted by vdoformat(8)
as a VDO pool data volume. You can specify the virtual size
of the VDOLV associated with this VDOPoolLV. If you do not
specify the virtual size, it will be set to the maximum size
that can keep 100% incompressible data there.
lvconvert
--type vdo-pool -n VDOLV -V VirtualSize VG/VDOPoolLV
lvconvert --vdopool VG/VDOPoolLV
Example
# lvconvert --type vdo-pool -n vdo0 -V10G vg/ExistingLV
3. Change
the compression and deduplication of a VDOPoolLV
Disable or enable the compression and deduplication for
VDOPoolLV (the volume that maintains all VDO LV(s)
associated with it).
lvchange --compression y|n --deduplication y|n VG/VDOPoolLV
Example
# lvchange --compression n vg/vdopool0
# lvchange --deduplication y vg/vdopool1
4. Change
the default settings used for creating a VDOPoolLV
VDO allows to set a large variety of options. Lots of these
settings can be specified in lvm.conf or profile settings.
You can prepare a number of different profiles in the
/etc/lvm/profile directory and just specify the
profile file name. Check the output of lvmconfig --type
default --withcomments for a detailed description of all
individual VDO settings.
Example
# cat <<EOF > /etc/lvm/profile/vdo_create.profile
allocation {
vdo_use_compression=1
vdo_use_deduplication=1
vdo_use_metadata_hints=1
vdo_minimum_io_size=4096
vdo_block_map_cache_size_mb=128
vdo_block_map_period=16380
vdo_use_sparse_index=0
vdo_index_memory_size_mb=256
vdo_slab_size_mb=2048
vdo_ack_threads=1
vdo_bio_threads=1
vdo_bio_rotation=64
vdo_cpu_threads=2
vdo_hash_zone_threads=1
vdo_logical_threads=1
vdo_physical_threads=1
vdo_write_policy="auto"
vdo_max_discard=1
}
EOF
# lvcreate
--vdo -L10G --metadataprofile vdo_create vg/vdopool0
# lvcreate --vdo -L10G --config
’allocation/vdo_cpu_threads=4’ vg/vdopool1
5. Set or
change VDO settings with option --vdosettings
Use the form ’option=value’ or
’option1=value option2=value’, or repeat
--vdosettings for each option being set. Options are listed
in the Example section above, for the full description see
lvm.conf(5). Options can omit ’vdo_’ and
’vdo_use_’ prefixes and all its underscores. So
i.e. vdo_use_metadata_hints=1 and metadatahints=1 are
equivalent. To change the option for an already existing
VDOPoolLV use lvchange(8) command. However not all
option can be changed. Only compression and deduplication
options can be also changed for an active VDO LV. Lowest
priority options are specified with configuration file, then
with --vdosettings and highest are expliction option
--compression and --deduplication.
Example
# lvcreate
--vdo -L10G --vdosettings ’ack_threads=1
hash_zone_threads=2’ vg/vdopool0
# lvchange --vdosettings ’bio_threads=2
deduplication=1’ vg/vdopool0
6. Checking
the usage of VDOPoolLV
To quickly check how much data on a VDOPoolLV is already
consumed, use lvs(8). The Data% field reports how
much data is occupied in the content of the virtual data for
the VDOLV and how much space is already consumed with all
the data and metadata blocks in the VDOPoolLV. For a
detailed description, use the vdostats(8)
command.
Note: vdostats(8) currently understands only /dev/mapper device names.
Example
# lvcreate --type vdo -L10G -V20G -n vdo0 vg/vdopool0
# mkfs.ext4 -E nodiscard /dev/vg/vdo0
# lvs -a vg
LV VG Attr
LSize Pool Origin Data%
vdo0 vg vwi-a-v--- 20.00g vdopool0 0.01
vdopool0 vg dwi-ao---- 10.00g 30.16
[vdopool0_vdata] vg Dwi-ao---- 10.00g
# vdostats
--all /dev/mapper/vg-vdopool0-vpool
/dev/mapper/vg-vdopool0 :
version : 30
release version : 133524
data blocks used : 79
...
7. Extending
the VDOPoolLV size
You can add more space to hold VDO data and metadata by
extending the VDODataLV using the commands
lvresize(8) and lvextend(8). The extension
needs to add at least one new VDO slab. You can configure
the slab size with the allocation/vdo_slab_size_mb
setting.
You can also enable automatic size extension of a monitored VDOPoolLV with the activation/vdo_pool_autoextend_percent and activation/ vdo_pool_autoextend_threshold settings.
Note: You cannot reduce the size of a VDOPoolLV.
lvextend -L+AddingSize VG/VDOPoolLV
Example
# lvextend -L+50G vg/vdopool0
# lvresize -L300G vg/vdopool1
8. Extending
or reducing the VDOLV size
You can extend or reduce a virtual VDO LV as a standard LV
with the lvresize(8), lvextend(8), and
lvreduce(8) commands.
Note: The reduction needs to process TRIM for reduced disk area to unmap used data blocks from the VDOPoolLV, which might take a long time.
lvextend
-L+AddingSize VG/VDOLV
lvreduce -L-ReducingSize VG/VDOLV
Example
# lvextend -L+50G vg/vdo0
# lvreduce -L-50G vg/vdo1
# lvresize -L200G vg/vdo2
9. Component
activation of a VDODataLV
You can activate a VDODataLV separately as a component LV
for examination purposes. The activation of the VDODataLV
activates the data LV in read-only mode, and the data LV
cannot be modified. If the VDODataLV is active as a
component, any upper LV using this volume CANNOT be
activated. You have to deactivate the VDODataLV first to
continue to use the VDOPoolLV.
Example
# lvchange -ay vg/vpool0_vdata
# lvchange -an vg/vpool0_vdata
VDO TOPICS
1. Stacking
VDO
You can convert or stack a VDOPooLV with these currently
supported volume types: linear, stripe, raid and cache with
cachepool.
1. Using
multiple volumes using same VDOPoolLV
You can convert existing VDO LV into a thin volume. After
this conversion you can create a thin snapshot or you can
add more thin volumes with thin-pool named after orignal LV
name LV_tpool0.
Example
# lvcreate --type vdo -L 5G -V 10G -n vdo1 vg/vdopool
# lvconvert --type thin vg/vdo1
# lvcreate -V20 vg/vdo1_tpool0
2. VDOPoolLV
on top of raid
Using a raid type LV for a VDODataLV.
Example
# lvcreate --type raid1 -L 5G -n vdopool vg
# lvconvert --type vdo-pool -V 10G vg/vdopool
3. Caching a
VDOPoolLV
VDOPoolLV (accepts also VDODataLV volume name) caching
provides a mechanism to accelerate reads and writes of
already compressed and deduplicated data blocks together
with VDO metadata.
Example
# lvcreate --type vdo -L 5G -V 10G -n vdo1 vg/vdopool
# lvcreate --type cache-pool -L 1G -n cachepool vg
# lvconvert --cache --cachepool vg/cachepool vg/vdopool
# lvconvert --uncache vg/vdopool
4. Caching a
VDOLV
VDO LV cache allow you to ’cache’ a device for
better performance before it hits the processing of the VDO
Pool LV layer.
Example
# lvcreate --type vdo -L 5G -V 10G -n vdo1 vg/vdopool
# lvcreate --type cache-pool -L 1G -n cachepool vg
# lvconvert --cache --cachepool vg/cachepool vg/vdo1
# lvconvert --uncache vg/vdo1
5. Usage of
Discard/TRIM with a VDOLV
You can discard data on a VDO LV and reduce used blocks on a
VDOPoolLV. However, the current performance of discard
operations is still not optimal and takes a considerable
amount of time and CPU. Unless you really need it, you
should avoid using discard.
When a block device is going to be rewritten, its blocks will be automatically reused for new data. Discard is useful in situations when user knows that the given portion of a VDO LV is not going to be used and the discarded space can be used for block provisioning in other regions of the VDO LV. For the same reason, you should avoid using mkfs with discard for a freshly created VDO LV to save a lot of time that this operation would take otherwise as device is already expected to be empty.
6. Memory
usage
The VDO target requires 38 MiB of RAM and several
variable amounts:
• |
1.15 MiB of RAM for each 1 MiB of configured block map cache size. The block map cache requires a minimum of 150 MiB RAM. | ||
• |
1.6 MiB of RAM for each 1 TiB of logical space. | ||
• |
268 MiB of RAM for each 1 TiB of physical storage managed by the volume. |
UDS requires a minimum of 250 MiB of RAM, which is also the default amount that deduplication uses.
The memory required for the UDS index is determined by the index type and the required size of the deduplication window and is controlled by the allocation/vdo_use_sparse_index setting.
With enabled UDS sparse indexing, it relies on the temporal locality of data and attempts to retain only the most relevant index entries in memory and 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.
A dense index with 1 GiB of RAM maintains a 1 TiB deduplication window, while a sparse index with 1 GiB of RAM maintains a 10 TiB deduplication window. In general, 1 GiB is sufficient for 4 TiB of physical space with a dense index and 40 TiB with a sparse index.
7. Storage
space requirements
You can configure a VDOPoolLV to use up to 256 TiB 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.
The VDO target requires storage for two types of VDO metadata and for the UDS index:
• |
The first type of VDO metadata uses approximately 1 MiB for each 4 GiB of physical storage plus an additional 1 MiB per slab. | ||
• |
The second type of VDO metadata consumes approximately 1.25 MiB for each 1 GiB 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 GiB of RAM, a dense UDS index uses 17 GiB of storage and a sparse UDS index will use 170 GiB of storage. |
SEE ALSO
lvm(8), lvm.conf(5), lvmconfig(8), lvcreate(8), lvconvert(8), lvchange(8), lvextend(8), lvreduce(8), lvresize(8), lvremove(8), lvs(8),