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Snapshots FAQ
Amir Goldstein edited this page Nov 14, 2018
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- Overlayfs snapshots feature is a POC
- There is a testsuite that verifies snapshots consistency (see Snapshots testing).
- There is an known issue regarding files that are memory mapped during snapshot take.
- Any local file system that can be used as overlayfs upper layer:
- Not network file systems
- Not FUSE file systems
- Not another overlay file system
- The local file system must support NFS export operations.
- Without reflink/clone support of local file system the performance overhead of overlayfs snapshots for opening a large file for write can be significant.
- The reflink support requirement makes XFS with reflink support the best candidate for local file system for overlayfs snapshot.
- Btrfs could also be used as local file system for overlayfs snapshots, but it already has built in snapshots support.
- Ext4 and XFS without reflink support can be used with overlayfs snapshots as long as long delays on opening large files for write can be tolerated.
- Overlayfs snapshots work on top of another local file system, btrfs included. Btrfs snapshots are built in.
- Overlayfs snapshots are on directory sub-tree level. Btrfs snapshot are on [sub]volume level.
- Overlayfs shares the inode cache of unmodified files among all snapshots. Btrfs uses a separate inode cache entry per snapshot.
- Btrfs snapshots are writable and may be nested. Overlayfs snapshots are read-only.
- Overlayfs snapshots are on directory sub-tree level. LVM thin snapshot are on volume level.
- Overlayfs snapshots use available local file system free space. LVM uses thinp pool blocks.
- Overlayfs snapshots use local file system block allocators, which are aware of underlying disk geometry.
- Overlayfs shares the inode cache of unmodified files among all snapshots. LVM uses a separate inode cache entry per snapshot.
- LVM thin volumes mangle the underlying block layout, which may lead to fragmentation and performance degradation over time.
- Overlayfs snapshot should have negligible performance overhead on file system operations.
- LVM thin volumes incur copy on write cost per block write, because they do not work with native file system block size.
- Yes. You can.
- See Snapshots HOWTO.
- More convenient installer for standalone module will be provided further down the road.