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I could reproduce this. Note that overlayfs is still in TECH PREVIEW state, but RH is actively fixing bugs. Pinning down the problematic part is hard and I haven't suceeded, but I found a following upstream kernel-4.7 commit: commit 76bc8e2843b66f8205026365966b49ec6da39ae7 Author: Miklos Szeredi <mszeredi.redhat.com> Date: Fri Jul 29 12:05:24 2016 +0200 ovl: disallow overlayfs as upperdir This does not work and does not make sense. So instead of fixing it (probably not hard) just disallow. This commit disallows anything overlayfs as upper=, so the reproducing example disintegrates into # mount -t overlay overlay -olowerdir=merged/lower2,upperdir=merged/upper2,workdir=merged/work2 merged/merged2/ overlayfs: filesystem on 'merged/upper2' not supported as upperdir So, using overlayfs as upperdir will be unsupported in future kernel. RHE7's fs/overlayfs/ seems to track kernel-4.x rather than 3.x series, so RH may or may not 'fix' this by disallowing overlayfs as upper. |
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Using non-overlayfs as upper2 and work2 works as expected: sudo make test2 [sudo] password for kabe: rm -fr upper lower work merged mkdir upper lower work merged mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merged/ mkdir upper2 work2 mkdir merged/lower2 merged/merged2 mount -t overlay overlay -olowerdir=merged/lower2,upperdir=upper2,workdir=work2 merged/merged2/ echo hello > merged/merged2/test.txt ls -li merged/merged2 total 4 536920 -rw-r--r--. 1 root root 6 Mar 21 14:26 test.txt I know there are some corner usage cases when you can't prepare a non-overlayfs directory to mount from. |
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Release notes for RHEL-7.3 says: >> OverlayFS is only supported for use as a Docker graph driver. >> Its use can only be supported for container COW content, >> not for persistent storage. Any persistent storage must be placed >> on non-OverlayFS volumes to be supported. ~~~~~~~~~~~~~ >> Only default Docker configuration can be used; that is, >> one level of overlay, one lowerdir, and both lower and upper levels ~~~~~~~~~~~~~~~~~~~~ >> are on the same file system. I don't think they will extend the support for layered overlayfs since kernel upstream decided to not. |
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Hi kabe, thank you very much for all the information! I already thought that they won't support stacked overlay any further but I couldn't find any proof for that, but the commit you provided (https://github.com/torvalds/linux/commit/76bc8e2843b66f8205026365966b49ec6da39ae7) clearly confirms that. I think we will have to look for another solution. |
- Anonymous
