domain: how long is new xml in saved file

[Vincent Wu]

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) &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.


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&nbsp;&nbsp;&nbsp;&nbsp;strlen(xml_old) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
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&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;+ &nbsp;free 
space &nbsp;+
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&nbsp;&nbsp;strlen(xml_new) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
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&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