FAQ

The intention of this page is to hold answers to frequently (and no so frequently) asked questions as well as some random data spread among developers' heads ;-) Note, that a big portion of FAQ-s are about why C/R fails.

• Q: Why CRIU dumps parts of read-only mappings that map the code section of a binary? For instance, there is a mapping at, say 0x400000, that maps the code of /usr/bin/something. After dump there will be at least a page at 0x400000 in the pagemap
• A: The code section may have been COWed, for instance during dynamic load of the shared libraries.

• Q: Why my test cannot do privileged operation, though I run zdtm.py as root?
• A: That's because zdtm.py runs all sub-tests from non-existing non-root user. If you need root prio in your sub-test add 'flags': 'suid' into test's .desc file.

• Q: Is it possible to do live migration from one server to another with changing the IP address?
• A: Quick answer is -- if breaking the connections is OK, then yes. In details it's described in this article.

• Q: Why does dump fail with "Cannot dump half of a stream unix connection" message in logs?
• A: There are configurations when C/R fails. This particular error is about external UNIX socket.

• Q: Why does restore fail with "... pid mismatch ..." error in logs?
• A: The PID of the process of thread CRIU tries to restore is already busy. To overcome this, try CR in namespace.

• Q: I've built CRIU, how can I make sure it works?
• A: There are two minimal things to do. First is to check the kernel and the second is run the ZDTM test suite.

Docker

• Q: Cannot restore from images onto freshly created container (E.g. this gihub issue)
• A: CRIU doesn't checkpoint filesystem, so once you've checkpointed the container, the images contain path to files, that sit in the modified root tree. So you cannot restore from images on another root tree, you need to get the same tree back. You need to say docker commit after the checkpoint and restore only on this particular image.