11:55 a.m.
Hi Eric,
I wonder if you'd know what the difference is here:
$ ./nbdkit -U - --filter=cow --filter=xz \
curl
https://download.fedoraproject.org/pub/fedora/linux/releases/34/Cloud/x86...
\
--run '
guestfish add "" format:raw protocol:nbd server:unix:$unixsocket
cachemode:unsafe discard:enable copyonread:true : run : mount /dev/sda1 / : fstrim /
&&
nbdinfo --map $uri
'
0 5368709120 0 data
(trimming didn't happen)
Changing copyonread:true to copyonread:false and keeping everything
else identical, the output changes to:
0 38010880 0 data
38010880 917504 3 hole,zero
38928384 6881280 0 data
45809664 89456640 3 hole,zero
135266304 8781824 0 data
[...]
(trimming happened)
There's a more detailed analysis below, but at first glance it appears
as if the copy-on-read filter in qemu doesn't pass trim requests. In
the nbdkit verbose output in the first case I don't see trim at all.
In the second (working) case, I see trim requests.
What's weird is that this case comes from a problem with modular
virt-v2v, yet old virt-v2v worked in the same way, except we're using
the qemu nbd client here but old virt-v2v would have used a qcow2 file.
This is with qemu-6.0.0-10.fc35.x86_64. I checked the qemu sources
and as far as I can tell discards (cor_co_pdiscard?) get passed
through.
======================================================================
MORE DETAIL: What's happening is that guestfish uses libvirt to set up
these disks.
With copyonread:true:
<disk device="disk" type="network">
<source protocol="nbd">
<host transport="unix"
socket="/tmp/nbdkit4IH9fV/socket"/>
</source>
<target dev="sda" bus="scsi"/>
<driver name="qemu" type="raw" cache="unsafe"
discard="unmap" copy_on_read="on"/>
<address type="drive" controller="0" bus="0"
target="0" unit="0"/>
</disk>
<disk type="file" device="disk">
with copyonread:false:
<disk device="disk" type="network">
<source protocol="nbd">
<host transport="unix"
socket="/tmp/nbdkitxmjFrS/socket"/>
</source>
<target dev="sda" bus="scsi"/>
<driver name="qemu" type="raw" cache="unsafe"
discard="unmap"/>
<address type="drive" controller="0" bus="0"
target="0" unit="0"/>
</disk>
These translate to the following qemu command lines (copyonread:true):
-blockdev
'{"driver":"nbd","server":{"type":"unix","path":"/tmp/nbdkit4IH9fV/socket"},"node-name":"libvirt-2-storage","cache":{"direct":false,"no-flush":true},"auto-read-only":true,"discard":"unmap"}'
\
-blockdev
'{"node-name":"libvirt-2-format","read-only":false,"discard":"unmap","cache":{"direct":false,"no-flush":true},"driver":"raw","file":"libvirt-2-storage"}'
\
-blockdev
'{"driver":"copy-on-read","node-name":"libvirt-CoR-sda","file":"libvirt-2-format"}'
\
-device
scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,device_id=drive-scsi0-0-0-0,drive=libvirt-CoR-sda,id=scsi0-0-0-0,bootindex=1,write-cache=on
\
or (copyonread:false):
-blockdev
'{"driver":"nbd","server":{"type":"unix","path":"/tmp/nbdkitxmjFrS/socket"},"node-name":"libvirt-2-storage","cache":{"direct":false,"no-flush":true},"auto-read-only":true,"discard":"unmap"}'
\
-blockdev
'{"node-name":"libvirt-2-format","read-only":false,"discard":"unmap","cache":{"direct":false,"no-flush":true},"driver":"raw","file":"libvirt-2-storage"}'
\
-device
scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,device_id=drive-scsi0-0-0-0,drive=libvirt-2-format,id=scsi0-0-0-0,bootindex=1,write-cache=on
\
These seem very similar to me, except there's an extra copy-on-read
layer in the first (non-working) case.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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