Agenda

  • Status Update
  • Upcoming Challenges
  • Structure of a Block Job

What are Block Jobs?

  • Initiated by QMP/HMP Command
    • e.g.:
      { "execute": "block-commit",
          "arguments": { "device": "virtio0" } }
    • Executed via coroutine
      • Asynchronous
    • Perform I/O on images(s)

Current Status (v2.2.0..)

  • Four Block Jobs
    • Backup
      • Point-in-time drive backup
    • Stream
      • Move data in chain into overlays
    • Commit
      • Move data from overlays into backing files
    • Mirror
      • Mirror drive's write to a new destination
  • Block Job infrastructure

Backup

Stream

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup
  • Patches on list
    • Intermediate streaming
      • (Alberto Garcia)

Stream

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup
  • Patches on list
    • Intermediate streaming
      • (Alberto Garcia)

Stream

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup
  • Patches on list
    • Intermediate streaming
      • (Alberto Garcia)

Stream

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup
  • Patches on list
    • Intermediate streaming
      • (Alberto Garcia)

Stream

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup
  • Patches on list
    • Intermediate streaming
      • (Alberto Garcia)

Stream

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup
  • Patches on list
    • Intermediate streaming
      • (Alberto Garcia)

Commit

  • Changes in qemu.git/master
    • Minor bug fixes / cleanup

Mirror

  • Changes in qemu.git/master
    • Bitmap spoiling fix
      • (Vladimir Sementsov-Ogievskiy), 2.2.0+
    • Bitmap scanning speedup
      • (Fam Zheng), 2.4.0+
    • Coroutine re-entrancy fix
      • (Kevin Wolf), 2.5.0+
    • Minor bug fixes / cleanup

Block Job Infrastructure

  • Changes in qemu.git/master
    • Nested Pause
      • (Fam Zheng)
      • v2.4.0+

Future Challenges / Improvements

(v2.5.0+)

  • Op Blockers
    • Finer granularity - more parallelism
    • Safer blocking
  • Safe(r) commit

Safe(r) Commit

Sectors in "active" differ from those in "base"

Safe(r) Commit

Normal commit: push sectors into "base"

Safe(r) Commit

Images "snap-a" and "snap-b" now invalid

Safe(r) Commit

Let's try to be safer

Safe(r) Commit

First, stream affected sectors to overlay image

Safe(r) Commit

Now commit sectors from "active"

Safe(r) Commit

All backing files in the chain are now still valid

Structure of a Block Job

Key Components

  • QMP command definition
    • JSON file (qapi/*.json)
    • qmp_commands.hx
  • Command handler
    • blockdev.c
  • Block Job coroutine
    • e.g. block/stream.c
  • Schedule job completion / cleanup

A sample Block Job:

  • block-null
    • Simple block job
    • Reads all sectors in BDS


Code available at: http://qemu.rocks/kvm2015/git

QMP Command Definition 


                     
{ 'enum': 'BlockJobType',
  'data': ['commit', 'stream', 'mirror', 'backup', 'null'] }

{ 'command': 'block-null',
  'data': { 'device': 'str', '*speed': 'int' } }

QMP Handler

                     
void qmp_block_null(const char *device,
                      bool has_speed, int64_t speed,
                      Error **errp)
{
    BlockBackend *blk;
    BlockDriverState *bs;
    AioContext *aio_context;
    Error *local_err = NULL;

    if (!has_speed) {
        speed = 0;
    }

    blk = blk_by_name(device);
    if (!blk) {
        error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
                  "device '%s' not found", device);
        return;
    }
    bs = blk_bs(blk);

    aio_context = bdrv_get_aio_context(bs);
    aio_context_acquire(aio_context);

    if (local_err != NULL) {
        error_propagate(errp, local_err);
        goto out;
    }

    null_start(bs, speed, block_job_cb, bs, &local_err);

out:
    aio_context_release(aio_context);
}

Block Job Coroutine

Block Job Events

  • BLOCK_JOB_COMPLETED
  • BLOCK_JOB_CANCELLED
  • BLOCK_JOB_ERROR
  • BLOCK_JOB_READY

Block Job Control

  • block-job-set-speed
    • Set maximum speed of block job
  • block-job-cancel
    • QEMU will emit BLOCK_JOB_CANCELLED
  • block-job-pause
  • block-job-resume
  • block-job-complete
    • Send after BLOCK_JOB_READY event

Questions?

THE END

Jeff Cody | Red Hat, Inc.

Presentation available at: http://qemu.rocks/jtc-kvm2015








Videos from wedistill.io: Robert Cole (title slide), Tyler Finck (end slide)
Videos licensed under Creative Commons Zero License