domain: how long is new xml in saved file

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().&nbsp;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()&nbsp;and virDomainSaveImageDefineXML(). But an error is happened when I do like that. &nbsp;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) &gt;= 50) { this is error; } But I don't choose to find an accurate number. &nbsp;I think this value will be affected by some factors. For example: memory alignment, range safety or other rules. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp;&nbsp;strlen(xml_old) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+ &nbsp;free space &nbsp;+ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;strlen(xml_new) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; + ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 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,&nbsp; Vincent Wu