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Diffstat (limited to 'gitlab/issues_text/target_missing/host_missing/accel_missing/1307')
| -rw-r--r-- | gitlab/issues_text/target_missing/host_missing/accel_missing/1307 | 72 |
1 files changed, 72 insertions, 0 deletions
diff --git a/gitlab/issues_text/target_missing/host_missing/accel_missing/1307 b/gitlab/issues_text/target_missing/host_missing/accel_missing/1307 new file mode 100644 index 000000000..9e90d8ad4 --- /dev/null +++ b/gitlab/issues_text/target_missing/host_missing/accel_missing/1307 @@ -0,0 +1,72 @@ +query-named-block-nodes, without flat=true, is massively slow as number of block nodes increases +Description of problem: +The query-named-block-nodes command is insanely slow with deep backing chains when the flat=true arg is NOT given. + +``` +qemu-img create demo0.qcow2 1g +j=0 +for i in `seq 1 199` +do + qemu-img create -f qcow2 -o backing_file=demo$j.qcow2 -o backing_fmt=qcow2 demo$i.qcow2 + j=$i +done +``` + +Now configure libvirt with + +``` + <disk type='file' device='disk'> + <driver name='qemu' type='qcow2' discard='unmap'/> + <source file='/var/lib/libvirt/images/demo199.qcow2'/> + <target dev='vdb' bus='virtio'/> + <address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/> + </disk> +``` + +This results in `-blockdev` args + +``` +-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/demo0.qcow2","node-name":"libvirt-201-storage","auto-read-only":true,"discard":"unmap"}' \ +-blockdev '{"node-name":"libvirt-201-format","read-only":true,"discard":"unmap","driver":"qcow2","file":"libvirt-201-storage","backing":null}' \ +-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/demo1.qcow2","node-name":"libvirt-200-storage","auto-read-only":true,"discard":"unmap"}' \ +-blockdev '{"node-name":"libvirt-200-format","read-only":true,"discard":"unmap","driver":"qcow2","file":"libvirt-200-storage","backing":"libvirt-201-format"}' \ +-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/demo2.qcow2","node-name":"libvirt-199-storage","auto-read-only":true,"discard":"unmap"}' \ +-blockdev '{"node-name":"libvirt-199-format","read-only":true,"discard":"unmap","driver":"qcow2","file":"libvirt-199-storage","backing":"libvirt-200-format"}' \ +...snip... +-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/demo197.qcow2","node-name":"libvirt-4-storage","auto-read-only":true,"discard":"unmap"}' \ +-blockdev '{"node-name":"libvirt-4-format","read-only":true,"discard":"unmap","driver":"qcow2","file":"libvirt-4-storage","backing":"libvirt-5-format"}' \ +-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/demo198.qcow2","node-name":"libvirt-3-storage","auto-read-only":true,"discard":"unmap"}' \ +-blockdev '{"node-name":"libvirt-3-format","read-only":true,"discard":"unmap","driver":"qcow2","file":"libvirt-3-storage","backing":"libvirt-4-format"}' \ +-blockdev '{"driver":"file","filename":"/var/lib/libvirt/images/demo199.qcow2","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}' \ +-blockdev '{"node-name":"libvirt-1-format","read-only":false,"discard":"unmap","driver":"qcow2","file":"libvirt-1-storage","backing":"libvirt-3-format"}' \ +-device '{"driver":"virtio-blk-pci","bus":"pci.7","addr":"0x0","drive":"libvirt-1-format","id":"virtio-disk1"}' \ +``` + +Now stop libvirt + +``` +systemctl stop libvirtd +``` + +And speak directly to QMP + +``` +$ time socat UNIX:/var/lib/libvirt/qemu/domain-158-fedora38/monitor.sock - > /dev/null +{ "execute": "qmp_capabilities", "arguments": { "enable": ["oob"] } } +{ "execute": "query-named-block-nodes"} +{ "execute": "quit" } + +real 2m19.276s +user 0m0.006s +sys 0m0.014s +``` + +If we save the 'query-named-block-nodes' output instead of sending it to /dev/null, we get a 86 MB file for the QMP response. This will break all known client apps since they limit QMP reply size. + +It appears to have a combinatorial expansion of block nodes in the output. + +Blocking the main event loop for 2 minutes is obviously not good either. + +If we use '"flat": true' parameter to query-named-block-nodes, the command completes in just 15 seconds, and produces a large, but more manageable 2.7 MB + +Since the non-flat query-named-block-nodes output is so incredibly non-scalable, I think we should deprecate non-flat mode, and eventually make flat the mandatory option. |