da989b473d
The program is called ro*files* and ostree creates physical copies of directories, so changing them is fine. I hit this when trying to do a copy checkout onto an rofiles-fuse mount. Closes: #368 Approved by: jlebon |
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README.md
rofiles-fuse
Create a mountpoint that represents an underlying directory hierarchy, but where non-directory inodes cannot have content or xattrs changed. Files can still be unlinked, and new ones created.
This filesystem is designed for OSTree and other systems that create "hardlink farms", i.e. filesystem trees deduplicated via hardlinks.
Normally with hard links, if you change one, you change them all.
There are two approaches to dealing with that:
- Copy on write: implemented by BTRFS, overlayfs, and http://linux-vserver.org/util-vserver:Vhashify
- Make them read-only: what this FUSE mount does
Usage
Let's say that you have immutable data in /srv/backups/20150410, and
you want to update it with a new version, storing the result in
/srv/backups/20150411. Further assume that all software operating
on the directory does the "create tempfile and rename()" dance
rather than in-place edits.
$ mkdir -p /srv/backups/mnt # Ensure we have a mount point
$ cp -al /srv/backups/20150410 /srv/backups/20150411
$ rofiles-fuse /srv/backups/20150411 /srv/backups/mnt
Now we have a "rofiles" mount at /srv/backups/mnt. If we try this:
$ echo new doc content > /srv/backups/mnt/document
bash: /srv/backups/mnt/document: Read-only file system
It failed because the > redirection operator will try to truncate
the existing file. If instead we create document.tmp and then
rename it atomically over the old one, it will work:
$ echo new doc content > /srv/backups/mnt/document.tmp
$ mv /srv/backups/mnt/document.tmp /srv/backups/mnt/document
Let's unmount:
$ fusermount -u /srv/backups/mnt
Now we have two directories /srv/backups/20150410
/srv/backups/20150411 which share all file storage except for the
new document.