Overlayfs and containers
Amir Goldstein edited this page Aug 23, 2020
·
31 revisions
- shiftfs/fsid mapping
- Userns root mount (FS_USERNS_MOUNT) (coming up)
- "volatile" overlay (coming up)
- Containerized syncfs(2) (RFC)
- Because it is the default
- Share page cache
- Fast startup time
- Native diff - faster image build
- Non-standard behavior: inconsistent file data, inconsistent object identifier, broken hardlinks, failure to rename directory
- Userspace mitigations: yum-plugin-ovl, recursive mv
- Rename directories (redirect_dir=on) (v4.10)
- Preserve hardlinks (index=on) (v4.13)
- NFS export (nfs_export=on) (v4.16)
- Unique object identifier (xino=auto) (v4.12..v4.17)
- Consistent file data (excluding mmap) (v4.19)
- Metadata only copy up (metacopy=on) (v4.19)
- Backward compatibility with old kernels
- Enabling xino only impacts 32bit compat st_ino
- Impact some container use cases
- Stays on one container host and storage driver (overlay2)
- Can safely enable: redirect_dir and metacopy
- Enabling index and nfs_export exposes mount leak bugs
- You've got nothing to lose!
- Migrate instance between container hosts with same native storage driver (overlay2)
- Include trusted.overlay xattrs in image layers
- No need to convert opaque xattr and whiteout chardev to portable .wh. files
- Can safely enable: redirect_dir and metacopy
- The index and nfs_export features are NOT "natively migratable"
- Enabling redirect_dir and metacopy requires more work
- vnd.oci.image.layer.v1 conforms to decade old overlay/aufs whiteouts and opaque directories
- Is it time for vnd.oci.image.layer.v2 with metacopy/redirect support?
- Downgrade to vnd.oci.image.layer.v1 format with overlayfs tools
- IF is_metacopy_file_or_redirect_dir(upperdir/$path):
- rm -rf imagedir/$path
- cp -a mountdir/$path imagedir/$path
- IF is_metacopy_file_or_redirect_dir(upperdir/$path):
- Enable redirect_dir,metacopy for new instances
- Allow user to configure NFS exportable instances (index,nfs_export)
- Disable index before instance migration
- Downgrade redirect_dir,metacopy during image creation