Hi administrator,

I am a cloud compute developer. I need some help from you about libvirt.

I have a work to modify a image file which is saved by virDomainSave() or virDomainSaveFlags(). So virDomainSaveImageGetXMLDesc() and virDomainSaveImageDefineXML() are APIs I choosed to do.

Because I found a sentence: A save file can be inspected or modified slightly with virDomainSaveImageGetXMLDesc() and virDomainSaveImageDefineXML().

But an error is happened when I do like that.  libvirt: QEMU Driver error: operation failed: new xml too large to fit in file.

I found that if I increase strlen(xml_old) to strlen(xml_new).

if (strlen(xml_new) - strlen(xml_old) <= 29) {

this is right;

}

but if (strlen(xml_new) - strlen(xml_old) >= 50) {

this is error;

}

But I don't choose to find an accurate number.  I think this value will be affected by some factors.

For example: memory alignment, range safety or other rules.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+                                                           strlen(xml_old)                                                  +  free space  +

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

+                                                             strlen(xml_new)                                                                     +

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

I really want to know how long free space I can use. Can you convert slightly to a precise number?

Thank you for taking so long to read my Email.

Looking forward to your reply.

ps:

OS: CentOS7.4

libvirt: 4.5.0

hypervisor: KVM

Sincerely, 

Vincent Wu