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# 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?
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