refactor(xapi/VDI_exportContent): can export from NBD

[fbeauchamp]

Goal

use NBD wherever it's possible . Today, NBD is only used for delta backup on vhd block storage. In this case, the blocks are read in parallel, but this reading process is completly separated from other reading neeeding for other remotes or for continuous replication.

Impacts

NBD is used for

• export vhd
• export vmdk
• export ova
• export raw disk
• delta backup, vhd file or blocks , full disk backup or delta disk backup
• continuous replication
• no more constrained by the 24 maximum task duration, especially when doing a full disk export

It is not used for

• xva export
• full VM backup
• disaster recovery

NBD block reading is done sequentially. It reads block only once even with multiple target of a backup job.

NBD is enabled as a xapi option : if a NBD network is present and is accessible between the VM and XO, it will be used.

NBD server is choosen at random between the available one. That means that if the VM use a shared storage, all the host of the pool can be used.

No more visible task during export , need to implement the new task system + UX

futures

• implement retry when reading a block fail ( thus allowing us to not fail a backup when there is a short network outage between host and XO )
• implement resume / global retry of a backup (for example at the end of a backup job, retry all the failed VMs before cleanVM)
• create and handle task manually

review by commit

Check list

Check if done, if not relevant leave unchecked.

• PR reference the relevant issue (e.g. Fixes #007 or See xoa-support#42)
• if UI changes, a screenshot has been added to the PR
• documentation updated
• CHANGELOG.unreleased.md:
  • enhancement/bug fix entry added
  • list of packages to release updated (${name} v${new version})
• I have tested added/updated features (and impacted code)
  • unit tests (e.g. cron/parse.spec.js)
  • if xo-server API changes, the corresponding test has been added to/updated on xo-server-test
  • at least manual testing

Process

1. create a PR as soon as possible
2. mark it as WiP: (Work in Progress) if not ready to be merged
3. when you want a review, add a reviewer (and only one)
4. if necessary, update your PR, and re- add a reviewer

From the Four Agreements:

1. Be impeccable with your word.
2. Don't take anything personally.
3. Don't make assumptions.
4. Always do your best.