[Qemu-devel] Block layer roadmap

[Stefan Hajnoczi]

Hi,
Here is a list of block layer and storage changes that have been discussed.  It
is useful to have a roadmap of changes in order to avoid duplication, allow
more developers to contribute, and to communicate the direction of storage in
QEMU.

I suggest we first do a braindump of all changes that have been discussed.
Later we can discuss specific changes and if/when they fit into the roadmap -
please don't jump into discussion about specific changes yet.

Kevin: I hope this a useful starting point.  Here are all the items
that I am aware of:

=Material for next QEMU release=

Coroutines in the block layer [Kevin]
 * Programming model to simplify block drivers without blocking QEMU threads

Generic copy-on-read [Stefan]
 * Populate image file to avoid fetching same block from backing file
again later

Generic image streaming [Stefan]
 * Make block_stream commands available for all image formats that
support backing files

Live block copy [Marcelo/Kevin/Stefan?]
 * Copy the contents of an image file while a guest is using it

In-place qcow2 <-> qed conversion [Devin, GSoC 2011]:
 * Fast conversion between qcow2 and qed image formats without copy all data

VMDK enhancements [Fam, GSoC 2011]
 * Implement latest VMDK specs to support modern image files

Block I/O limits [Zhi Yong]
 * Resource control for guest I/O bandwidth/iops consumption

=Changes where I am not aware of firm plans=

Cow overlay [Dong Xu "Robert"]
 * Allow live block copy and image streaming to raw destination files

snapshot_blkdev and Backup API [Jes, Jagane]
 * Support for consistent disk snapshots and dirty block tracking
 * Allow backup software to integrate with QEMU

-blockdev [Markus?]
 * Explicit user control over block device trees

QCOW3
 * Extend qcow2 format to address current and future image format challenges

iSCSI/NBD/Remote block device integration
 * Enable remote access to disk images for live migration and other tasks

Pre/post block copy
 * Working block migration

Avoid blocking QEMU threads
 * Today loss of NFS connectivity can hang guests
 * It's critical never to block the vcpu thread
 * The iothread should also not block while the qemu mutex is held
 * All blocking operations must be done asynchronously or in a worker thread

virtio-scsi [Paolo/Stefan]
 * The next step after virtio-blk, full SCSI command set and appears
as SCSI HBA in guest

tcm_vhost [Stefan]
 * Directly connect virtio-scsi with Linux in-kernel SCSI target
 * Pass-through of host SCSI devices