ToDo/Block

This page contains block layer and storage features that have been proposed. These features may not be in active development and questions about them should be addressed to the QEMU mailing list at qemu-devel@nongnu.org.

op blockers

• Mutual exclusion of operations/background jobs
  • Streaming in two different parts of the backing chain - allowed? (Benoît though that not, but does anything break?)
  • Does streaming only require that streamed images stay read-only (i.e. backing chain segment on which the operation is performed)
  • Live commit in the opposite direction at the same time?
  • Action:
    • Draw up matrix of operations (mirror, stream, resize, etc)
    • Make op blocker mechanism use matrix as data instead of code (define an array)
    • Enforce that new QMP/QAPI commands and block jobs add themselves to the matrix
• node-name allows starting operations in the middle of the chain; we need to protect against incompatible concurrent operations
  • In fact, we even used paths before node-name (e.g. for live commit), so this has existed for a while
• bs->backing_blocker already forbids almost everything on backing files
  • Except live commit, which needs to be forbidden only when another job runs on the same chain
• Plan for 2.1 was to block all nodes recursively
  • bdrv_swap() during block job completion turns out to be nasty, especially for live commit of active layer:
    • Need to clean up blockers on the removed subchain
    • Which blockers should the newly swapped in node have?
• Alternative plan for 2.1:
  • Keep checking blockers on the requested node (for bs->backing_blockers to be effective)
  • But also check in the active layer because this is where block jobs do their blocking
    • bottommost node might work as well
      • As Kevin pointed out on IRC, in the current code blockers exist on backing files that don't exist on the active layer
• Long term (2.2+): Block categories of operations
  • Some thoughts, in the form of code example for block-commit: http://fpaste.org/113005/14036975/
• blockdev-add probably shouldn't be able to reference a node that has blockers

Basic infrastructure for blockdev-add [Kevin, Markus]

• Allow libvirt to override backing file options (filename, format, cache mode)
  • Convert bdrv_open() flags to QDict options (or handle in drive_init)
  • Implemented: filename, format, driver-specific options
  • Missing: Cache mode, AIO mode, discard mode, copy on read (basically everything that is contained in bs->open_flags)
  • Investigating possible QemuOpts misuse killing drive_del [Markus]
• Convert remaining drivers to make use of "QDict options" argument
• bdrv_reopen() needs to use "QDict options" instead of only flags

blockdev-add + blockdev-del QMP interface

• By default, return an error for blockdev-del if reference count > 1
• But have a force option that closes the image file, even if it breaks the remaining users (e.g. uncooperative guest that doesn't release its PCI device)
• Prevent mixing blockdev-add with drive_del and vice versa
  • Ideally drive_add BDSes are exactly those with a DriveInfo

Split BlockBackend from BlockDriverState [armbru]

• Need to start somewhere to get anywhere, so let's split it with a big axe:
  • clone the bdrv interface to blk, implemented as trivial wrapper around bdrv
  • convert appropriate users from bdrv to blk
  • clean up the silly wrappers as we go
  • first batch of patches posted
• Separate lifecycles for BlockDriverStates and guest devices (e.g. keep block job running after removing the guest device)
  • Need QMP command to disconnect BDS from guest device (already have blockjob-cancel)
  • blockdev-del only works with refcount = 1
• s/bs->device_name/bs->id/ after splitting out BlockBackend fields
  • BDS is no longer associated with a DeviceState, don't call it "device_name"
  • Careful, device_name[0] == '\0' has special meaning: internal BDS chain node
    • BDSes with a name are in bdrv_states and can be accessed e.g. by monitor commands
  • instead, lift the name from BDS up into BB
• Make block driver private embedded in BlockDriverState instead of opaque pointer
  • bdrv_new+bdrv_open and bdrv_close+bdrv_unref should be same, eliminate ENOMEDIUM semantics
• BlockBackend
  • Device emulation uses only BlockBackend, no direct BDS reference
  • Builtin NBD server uses BB (?)
  • Fields to move:
    • inserted
    • dev_*
    • enable_write_cache (?)
      • Separate field/flag for block jobs
    • on_read/write_error
    • BDRV_O_UNMAP (?)
  • To be moved to BlockFilters later (stay in BDS for now; BlockFilters implemented as BlockDriver):
    • bps_limits
    • copy_on_read

BlockFilter and dynamic reconfiguration of the BDS graph

• Add/remove (e.g. filter) BDSes at runtime
• Ability to implement light-weight block drivers that play together with snapshots (e.g. block debug, active-mirroring, copy-on-read, I/O throttling, multipath, etc)
  • Converting current I/O throttling code to a block filter should be simple, mostly a mechanical task.
• Requires BlockBackend split
  • Keep filters on top even after taking snapshots
• filters implement ops normally, and call out to their child BDS explicitly, no before- or after-ops-magic
• Benoît's customer may want I/O throttling in arbitrary places in the graph
• Be careful to never add cycles to the graph!

New design for better encryption support [armbru]

• Requires BlockFilter
• Getting rid of qcow2 encryption, make it modular using luks or something, make it stackable (a block filter)
• Why? It's amateur encryption, should not be used for anything! Besides, usability is horrible.
• Deprecate, except for qemu-img convert?

Block jobs

• node-name support for block-commit, block-stream
  • will be needed by libvirt for mixed local name / relative protocol (e.g. gluster) pathname support
• Live streaming of intermediate layers (using block-stream) [benoit]
  • Ask Eric if libvirt will use this functionality [jeff]
  • Benoît needs this

Remove bs->job field and allow multiple jobs on a BDS [mreitz]

• allows more than one blockjob at a block device at a time
• infra-structure, refactoring work
• Careful not to break QMP API, need wrapper
• Allow non-block jobs (long-running operations outside block layer)
• Alternatively, move to BB for now

Test Infrastructure

• Desires?
• support for testing AIO requests
  • No design yet, but we need some way to label I/O requests in blkdebug
  • right now we sleep, which is stupid
  • related to SCSI req tags
• QMP support in qtest
  • events still missing

Tests for -drive discard=

• Currently the discard feature is not well-tested in qemu-iotests

Block device model tests

• insufficient
  • WIP AHCI testing
  • rest is very basic

Active mirroring: just like mirroring, but live, on the fly, skip the bitmap

• similar to drive-backup
• security (virus-scan or some sort of inspection)
• should be implemented as a block filter
• apparently DTorrentGuy (OFTC#kvm) working on it

qcow2

• Cluster allocation performance: qcow2 delayed COW [kevin]
• qcow2 backing file validation (parent modification invalidates children) [jeff]
  • similar to vmdk and vhdx
• qcow2 internal snapshot read-only BlockDriverState
  • Allows accessing snapshots while guest accesses disk image
  • Tricky, insufficient prio
• subclusters
  • allocate larger chunks, cow smallers ones, for perf
• Implement refcount_bits for smaller metadata

Image fleecing [jsnow]

• Image creation with existing BlockDriverState as backing file (BlockDriverState ref count)
• Patches: http://lists.gnu.org/archive/html/qemu-devel/2013-11/msg03692.html
  • still not merged
• Writable backing file

Incremental backup [jsnow]

• Backup applications need a dirty block bitmap so they can read only blocks that changed
• Two approaches discussed:
  • Dirty bitmap file
  • Write changed data through NBD:
    • See also: http://lists.gnu.org/archive/html/qemu-devel/2013-11/msg03035.html
  • Merkle tree - hash tree allows efficient syncing of image files between hosts

virtio-blk data plane [stefanha]

• Protect BlockDriverState:
  • A few monitor commands still need work:
    • transaction
    • block-passwd

virtio-scsi data plane [fam]

• Look done right now

virtio data plane [stefan]

• thread-safe dirty bitmap for migration

Multiqueue block layer

• Depends on: virtio-blk dataplane
• Single BlockDriverState, multiple independent threads accessing in parallel
• Allows us to extend Linux multiqueue block layer up into guest
• For maximum SMP scalability and performance with high IOPS SSDs

Make qemu-img use QMP command implementations internally (e.g. use mirroring for qemu-img convert) [Max]

• Ensures that live operations provide the same functionality as we have offline
• First round of patches posted, not merged yet

AHCI emulation [jsnow]

• Enable migration
• qtests
  • In progress
• rerror=/werror=
  • queued I/O migration needed

Block job I/O throttling

• Reuse Benoît's throttling implementation
• Handle large buffer sizes used by block jobs
  • block jobs like to work in bulk for efficiency but throttling doesn't like big, bursty requests (note from Benoît: I think the solution would be to make the clock used for the throttling computation coarser)

I/O accounting (for query-blockstats)

• Driving it was made device model's responsibility (commit a597e79 in 2011)
  • Most of them still don't
  • The ones that do are inconsistent
  • Consequently, query-blockstats is better treated as wild guess, not data
• Need to take a step back
  • Benoît got us use cases, discussed on list
    • measuring in the device model is good for billing
    • some metrics are missing
    • It would be good to collect the same data everywhere in the BDS graph for telemetry purpose (seeking hidden costs)
    • having a dedicated JSON socket to output accounting data would be good
    • so we can keep analysis out of qemu
• Working on revamping the I/O accounting infrastructure (Benoît)
  • Preliminary patches merged
  • Averaging module under review
  • More tocme

iotests.py - Python module for writing qemu-iotests [stefan]

• Extract qemu.py generic QEMU interaction code
• Document qemu.py and iotests.py so it meets standard Python module conventions
• Port live migration qemu-iotest to Python to see if it's preferrable to the shell version

I/O throttling groups

• Action:
  • Add group=<name> property to -drive [Benoit]
  • Patches written an on the list for review

Dynamic graph reconfiguration (e.g. adding block filters, taking snapshots, etc.)

• Where does the new node get inserted and how to specify how it is linked up with the existing nodes?
  • On a given "arrow" between two nodes (only works with 1 child, 1 parent)
  • On a given set of arrows (possibly more complex than what is really needed?)
• How does removing a node work with more than one child of the deleted node?
• Keep using the existing QMP command for I/O throttling for now, until we understand the general problem reasonably well
• Action:
  • Figure out the general problem
  • Split I/O throttling off into own BDS [Benoît]
    • Requires some care with snapshots etc.

Proper specification for blockdev-add [Kevin, Max]

• What does -drive add?
• Filename parsing and protocol detection
• Format probing
• Desugaring -drive
• What does BDRV_O_PROTOCOL mean?
  • Disable format probing
  • Parse protocol name from filename (but not from options QDict)
    • Put filename then into options QDict
  • Set bs->growable
  • Disable adding of a bs->file layer
  • Ignore BDRV_O_SNAPSHOT
  • Which callers need which of these properties?
• Action:
  • Convert network block drivers to QDict options (keep legacy filename parsing for compatibility)
  • Add network block drivers to blockdev-add
  • Translate bdrv_open() arguments into options qdict, if appropriate [Kevin]
    • Translate legacy "filename" to qdict
  • Specify bdrv_open() behavior (especially magic) [Kevin]

BDS graph rules and manipulating arbitrary nodes

• A proper design: iterate children, safely manipulate graph
  • Action:
    • Get rid of bdrv_swap() and update child/parent pointers instead (depends on BlockBackend) [Markus with help]
      • Add notifier list to BDS so users can get updated when pointer changes
        • bdrv_register_bs_pointer(bs, &mystruct->bs) /* automatically refresh pointer */
    • Add parents and child list to BlockDriverState (could be realloc array or just a function interface that operates on ->file/->backing_hd) [nice to have]
      • Audit drivers
        • Especially VMDK and quorum
        • Make them use generic child interface
        • Use child list where generic block layer currently hardcodes ->backing_hd and ->file
• Arbitrary nodes
  • Accept node-name where we now have other means to id BDS
    • drive-mirror of arbitrary node [Benoît]
    • block-stream of arbitrary node [Benoît]
  • Action:
    • Add base-nodename argument to block-stream command [Jeff]
    • Add top-nodename argument to block-stream command [Benoit]
      • If command can modify part of a backing chain, need to add option to update the parent's backing filename field on disk! [Jeff]
      • Add optional backing-filename argument (since libvirt may use fd passing and QEMU's filename is useless) [Jeff]
        • Done: block-commit, block-stream, change-backing-file
        • Might need more
    • Deprecate filename references in QMP commands in favour of node names (e.g. streaming base) [Jeff?]

Dataplane programming model recap

• What do block drivers need to be careful of?
• Any comments on new docs/dataplane.txt documentation?
• Action:
  • AioContext assertions to prevent callbacks in wrong event loop [Stefan]

geometry and block size passthrough for host devices

• See the patches from Ekaterina Tumanova. https://lists.nongnu.org/archive/html/qemu-devel/2014-07/msg03791.html
• On s390 we have a stricter requirement of passing through geometry and block size for DASD disk devices as this geometry is more than a imaginary number triplet. It is required for proper partition detection and is actually a property of the real (or better emulated on storage servers like DS8x000) disk.

Adding QMP to qemu-nbd

Patches are on the list

Adding TLS to NBD

NBD does not have support encryption. Something similar to VNC TLS is needed for NBD since the network may not be trusted. Both qemu-nbd and the run-time NBD server must support TLS. The NBD client also needs TLS support.

Export QEMU volumes as ISCSI or FCOE

• Andy Grover is working on implementing a preliminary tcmu-runner plugin using the block layer
• a QMP socket would be needed to make this usable in a cloud context

Dependency graph

(paste on http://yuml.me/diagram/scruffy/class/draw) [Image fleecing] [Incremental backup] [qcow2 improvements] [Make qemu-img use QMP command implementations internally] [Recursive Op Blocker]->[Category Op Blocker] [Category Op Blocker]->[intermediate layers live streaming] [Category Op Blocker]->[Basic infrastructure for blockdev-add] [Category Op Blocker]->[drive-mirror (block-mirror) of arbitrary node] [Category Op Blocker]->[Jeff Cody's block-commit of arbitrary node] [Proper specification for blockdev-add]->[Basic infrastructure for blockdev-add] [Basic infrastructure for blockdev-add]->[blockdev-add + blockdev-del QMP interface] [Image formats] [virtio-blk data plane]->[Multiqueue block layer] [virtio-scsi data plane] [Split BlockBackend from BlockDriverState]->[Get rid of bdrv_swap] [Split BlockBackend from BlockDriverState]->[BlockFilter and dynamic reconfiguration of the BDS graph] [BlockFilter and dynamic reconfiguration of the BDS graph]->[New design for better encryption support] [BlockFilter and dynamic reconfiguration of the BDS graph]->[Active mirroring] [BlockFilter and dynamic reconfiguration of the BDS graph]->[Throttle as a filter ?] [Remove bs->job field and allow multiple jobs on a BDS] [iotests.py - Python module for writing qemu-iotests]->[Test Infrastructure] [Tests for -drive discard] [Adding TLS to NBD] [AHCI emulation] [Block job I/O throttling] [I/O accounting] [Add guard page to bottom of coroutine stack] [I/O throttling groups] [QMP added to qemu-nbd] -> [Export QEMU volumes as ISCSI or FCOE with QMP] (result: http://yuml.me/c2e9edb8)