[RFC pve-storage/qemu-server 00/10] introduce thin provisioned drives to thick LVM storage

[Tiago Sousa]

As discussed with Alexandre Derumier, I’m sharing a prototype daemon
that monitors an extend queue and performs live VM disk resizes to
enable thin provisioning on LVM storage.

The idea is to create disks smaller than their qcow2 virtual size,
currently hardcoded to 2 GiB. This applies when a VM disk
has a snapshot. A write threshold is set on the file blockdev node so
that it triggers only when the actual filesystem reaches it.

To handle this, a notion of underlay is introduced. For LVM,the
underlay controls the underlying LV of the disk. For example, a
200 GiB qcow2 disk starts with a 2 GiB LV and grows incrementally as
needed. Qcow2 preallocation must be fully off, which has performance
implications.

The block write threshold is calculated from two storage config
variables, chunksize and chunk-percentage, using:
underlay_size - chunksize * chunk-percentage.
For example, if chunk-percentage = 0.3, the event fires when 30% of
chunksize remains.

When qmeventd receives an event, it appends it to /etc/pve/extend-queue
as vmid:blockdev_nodename. In a cluster, all nodes write to the same
queue.

pvestord (PVE Storage Daemon) runs a 1-second cycle checking the
queue. If a request is found and the VM is local, the entry is dequeued
to avoid conflicts. It then queries the QMP socket for the VM’s
blockstats, identifies the disk, and extends the LV.

Flow: qemu-vm -> qmeventd -> /etc/pve/extend-queue <- pvestord

So far test have been done manually.

Some problems and questions:
- Thin provisioning is currently hardcoded for drives that have a parent
  snapshot.
- The thin variable that is introduced in the drive config needs
  review before wider implementation.
- Consider making thin optional via snapshot prompt checkbox.
- Could eventually offer the option for all qcow2 disks on LVM.
- Re-evaluate blockdev name generation: sha256 vs reversible
  encoding (like base64) to identify drives and allow offline extends.
- Since all extend requests share the same queue, drives on different
  LVM storages must wait for their turn, even though actions on
  separate storages could run concurrently.
- qmeventd writes to the queue aren’t cluster-safe. I couldn’t find any
  primitives in the C code to lock the file via pmxcfs (like
  cfs_lock_file in Perl). Is there any function that handles this?

pve-storage:

Tiago Sousa (4):
  pvestord: setup new pvestord daemon
  storage: add extend queue handling
  lvmplugin: add thin volume support for LVM external snapshots
  plugin: lvmplugin: add underlay functions

 src/Makefile                  |   1 +
 src/PVE/Makefile              |   1 +
 src/PVE/Service/Makefile      |  10 ++
 src/PVE/Service/pvestord.pm   | 193 ++++++++++++++++++++++++++++++++++
 src/PVE/Storage.pm            | 100 +++++++++++++++++-
 src/PVE/Storage/Common.pm     |   4 +-
 src/PVE/Storage/LVMPlugin.pm  |  84 ++++++++++++---
 src/PVE/Storage/Plugin.pm     |  29 ++++-
 src/bin/Makefile              |   3 +
 src/bin/pvestord              |  24 +++++
 src/services/Makefile         |  14 +++
 src/services/pvestord.service |  15 +++
 12 files changed, 462 insertions(+), 16 deletions(-)
 create mode 100644 src/PVE/Service/Makefile
 create mode 100644 src/PVE/Service/pvestord.pm
 create mode 100755 src/bin/pvestord
 create mode 100644 src/services/Makefile
 create mode 100644 src/services/pvestord.service

qemu-server:

Tiago Sousa (4):
  qmeventd: add block write threshold event handling
  blockdev: add set write threshold
  blockdev: add query-blockstats qmp command
  blockdev: add underlay resize

 src/PVE/QemuServer.pm          | 22 ++++++++++
 src/PVE/QemuServer/Blockdev.pm | 80 ++++++++++++++++++++++++++++++++++
 src/PVE/QemuServer/Drive.pm    |  7 +++
 src/qmeventd/qmeventd.c        | 21 ++++++++-
 4 files changed, 129 insertions(+), 1 deletion(-)

pve-manager:

Tiago Sousa (1):
  services: add pvestord service

 PVE/API2/Services.pm | 1 +
 1 file changed, 1 insertion(+)

pve-cluster:

Tiago Sousa (1):
  observe extend queue

 src/PVE/Cluster.pm | 1 +
 1 file changed, 1 insertion(+)

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