[libvirt] Regarding guest-file-write
Hi,
From host, I wrote 26 alphabets in guest file (/tmp/testqga) using guest-file-write guest agent command (logs pasted below). I faced 2 issues when doing that.
1a. It could wrote only 18bytes! Why could it not write all 26 characters? Are we supposed to track how much data is written and need to resend the remaining one? 1b. What is the limit of data, I can send in one guest-file-write command? 2. In the guest, file data seems to be different. Am I doing something wrong here? Host (file write) [root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-open", "arguments":{"path":"/tmp/testqga","mode":"w +"}}' {"return":1000} [root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-write", "arguments":{"handle":1000,"buf-b64":"abcdefghijklmnopqrstuvwxyz"}}' {"return":{"count":18,"eof":false}} [root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-close", "arguments":{"handle":1000}}' {"return":{}} Host (file read) [root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-open", "arguments":{"path":"/tmp/testqga","mode":"r"}}' {"return":1001} [root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-read", "arguments":{"handle":1001,"count":18}}' {"return":{"count":18,"buf-b64":"abcdefghijklmnopqrstuvwx","eof":false}} [root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-close", "arguments":{"handle":1001}}' {"return":{}} Guest [root@vm04 qga]# ll /tmp/testqga -rw-rw-rw-. 1 root root 18 Jun 24 15:06 /tmp/testqga [root@vm04 qga]# cat /tmp/testqga i�^]y�!�9%�z)��-��1[root@vm04 qga]# [root@vm04 qga]# od -x /tmp/testqga 0000000 b769 791d 21f8 398a 9a25 297a bbaa ba2d 0000020 31fc 0000022 [root@vm04 qga]# strings /tmp/testqga [root@vm04 qga]# hexdump /tmp/testqga 0000000 b769 791d 21f8 398a 9a25 297a bbaa ba2d 0000010 31fc 0000012 [root@vm04 qga]# hexdump -c /tmp/testqga 0000000 i 267 035 y 370 ! 212 9 % 232 z ) 252 273 - 272 0000010 374 1 0000012 [root@vm04 qga]# Regards, ~Puneet
On Tue, Jun 24, 2014 at 06:20:16PM +0530, Puneet Bakshi wrote:
Hi,
From host, I wrote 26 alphabets in guest file (/tmp/testqga) using guest-file-write guest agent command (logs pasted below). I faced 2 issues when doing that.
1a. It could wrote only 18bytes! Why could it not write all 26 characters? Are we supposed to track how much data is written and need to resend the remaining one?
1b. What is the limit of data, I can send in one guest-file-write command?
2. In the guest, file data seems to be different. Am I doing something wrong here?
Yes, the command does not take raw data, it takes base-64 encoded data. So those 26 characters you specified were base64 decoded which gives you 18 bytes of raw data. The clue is in the variable name 'buf-b64' - a shorthand for base64:
[root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-write", "arguments":{"handle":1000,"buf-b64":"abcdefghijklmnopqrstuvwxyz"}}'
{"return":{"count":18,"eof":false}}
Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
1. In that case, I hope it allow sending binary data (non-ASCII) also. 2. If I encode my data in base-64 form, is there a limit on size of the data I can pass (or I can pass arbitrarily large amount of data). Regards, ~Puneet On Tue, Jun 24, 2014 at 6:27 PM, Daniel P. Berrange <berrange@redhat.com> wrote:
On Tue, Jun 24, 2014 at 06:20:16PM +0530, Puneet Bakshi wrote:
Hi,
From host, I wrote 26 alphabets in guest file (/tmp/testqga) using guest-file-write guest agent command (logs pasted below). I faced 2 issues when doing that.
1a. It could wrote only 18bytes! Why could it not write all 26 characters? Are we supposed to track how much data is written and need to resend the remaining one?
1b. What is the limit of data, I can send in one guest-file-write command?
2. In the guest, file data seems to be different. Am I doing something wrong here?
Yes, the command does not take raw data, it takes base-64 encoded data. So those 26 characters you specified were base64 decoded which gives you 18 bytes of raw data.
The clue is in the variable name 'buf-b64' - a shorthand for base64:
[root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-write", "arguments":{"handle":1000,"buf-b64":"abcdefghijklmnopqrstuvwxyz"}}'
{"return":{"count":18,"eof":false}}
Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
On 06/24/2014 07:10 AM, Puneet Bakshi wrote: [please don't top-post on technical lists]
1. In that case, I hope it allow sending binary data (non-ASCII) also.
Yes, the reason guest-file-write takes base64 encoded data is PRECISELY so that it can cope with binary data while still using only ascii representation over the wire.
2. If I encode my data in base-64 form, is there a limit on size of the data I can pass (or I can pass arbitrarily large amount of data).
At this point, it may be faster for you to read the qemu source code. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
On 06/24/2014 06:50 AM, Puneet Bakshi wrote:
Hi,
From host, I wrote 26 alphabets in guest file (/tmp/testqga) using guest-file-write guest agent command (logs pasted below). I faced 2 issues when doing that.
1a. It could wrote only 18bytes! Why could it not write all 26 characters? Are we supposed to track how much data is written and need to resend the remaining one?
You aren't using the command correctly. The qemu-guest-agent interface for guest-file-write expects data to be base64-encoded.
1b. What is the limit of data, I can send in one guest-file-write command?
That's a question for the qemu list, as qemu-guest-agent is maintained there, not here.
2. In the guest, file data seems to be different. Am I doing something wrong here?
Yes, you aren't encoding your data properly.
Host (file write)
[root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-open", "arguments":{"path":"/tmp/testqga","mode":"w +"}}'
Remember, the use of qemu-agent-command is EXPLICITLY unsupported by libvirt. It's there as a debugging and development aid, and NOT something you should be using in your production environment. I *highly* recommend that you figure out how to set up shared file systems (NFS, gluster, plan9, MTP, ...) so that you can have the guest read a file already exported by the host through existing shared filesystem code, rather than slogging through trying to write the file through arcane invocations of the qemu-guest-agent.
[root@sdsr720-14 ~]# virsh qemu-agent-command vm_04 '{" execute":"guest-file-write", "arguments":{"handle":1000,"buf-b64":"abcdefghijklmnopqrstuvwxyz"}}'
{"return":{"count":18,"eof":false}}
That is NOT a valid base64 encoded buffer. $ printf abcdefghijklmnopqrstuvwxyz | base64 -d | od -tx1z base64: invalid input 0000000 69 b7 1d 79 f8 21 8a 39 25 9a 7a 29 aa bb 2d ba >i..y.!.9%.z)..-.< 0000020 fc 31 cb >.1.< 0000023
[root@vm04 qga]# cat /tmp/testqga
i�^]y�!�9%�z)��-��1[root@vm04 qga]#
But those contents match the base64 decoding of (the valid portion of) the buffer that you passed in. So the only bug here is in your usage, not in qemu-guest-agent or in libvirt. - Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
participants (3)
-
Daniel P. Berrange -
Eric Blake -
Puneet Bakshi