# VDO - Compression and deduplication.

Currently device stacking looks like this:

    Physical x [multipath] x [partition] x [mdadm] x [LUKS] x [LVS] x [LUKS] x [FS|Database|...]

Adding VDO:

    Physical x [multipath] x [partition] x [mdadm] x [LUKS] x [LVS] x [LUKS] x VDO x [LVS] x [FS|Database|...]

## Where VDO fits (and where it does not):

### Backing devices for VDO volumes:

1. Physical x [multipath] x [partition] x [mdadm],
2. LUKS over (1) - full disk encryption.
3. LVs (raids|mirror|stripe|linear) x [cache] over (1).
4. LUKS over (3) - especially when using raids.

Usual limitations apply:

- Never layer LUKS over another LUKS - it makes no sense.
- LUKS is better over the raids, than under.

### Using VDO as a PV:

1. under tpool
    - The best fit - it will deduplicate additional redundancies among all
      snapshots and will reduce the footprint.
    - Risks: Resize! dmevent will not be able to handle resizing of tpool ATM.
2. under corig
    - Cache fits better under VDO device - it will reduce amount of data, and
      deduplicate, so there should be more hits.
    - This is useful to keep the most frequently used data in cache
      uncompressed (if that happens to be a bottleneck.)
3. under (multiple) linear LVs - e.g. used for VMs.

### And where VDO does not fit:

- *never* use VDO under LUKS volumes
    - these are random data and do not compress nor deduplicate well,
- *never* use VDO under cmeta and tmeta LVs
    - these are random data and do not compress nor deduplicate well,
- under raids
    - raid{4,5,6} scrambles data, so they do not deduplicate well,
    - raid{1,4,5,6,10} also causes amount of data grow, so more (duplicit in
      case of raid{1,10}) work has to be done in order to find less duplicates.

### And where it could be useful:

- under snapshot CoW device - when there are multiple of those it could deduplicate

### Things to decide

- under integrity devices - it should work - mostly for data
    - hash is not compressible and unique - it makes sense to have separate imeta and idata volumes for integrity devices

### Future Integration of VDO into LVM:

One issue is using both LUKS and RAID under VDO. We have two options:

- use mdadm x LUKS x VDO+LV
- use LV RAID x LUKS x VDO+LV - still requiring recursive LVs.

Another issue is duality of VDO - it is a top level LV but it can be seen as a "pool" for multiple devices.

- This is one usecase which could not be handled by LVM at the moment.
- Size of the VDO is its physical size and virtual size - just like tpool.
      - same problems with virtual vs physical size - it can get full, without exposing it fo a FS

Another possible RFE is to split data and metadata:

- e.g. keep data on HDD and metadata on SSD

## Issues / Testing

- fstrim/discard pass down - does it work with VDO?
- VDO can run in synchronous vs. asynchronous mode
    - synchronous for devices where write is safe after it is confirmed. Some devices are lying.
    - asynchronous for devices requiring flush
- multiple devices under VDO - need to find common options
- pvmove - changing characteristics of underlying device
- autoactivation during boot
    - Q: can we use VDO for RootFS?