devmapper: add README and minor fixes

Signed-off-by: Maksym Pavlenko <makpav@amazon.com>

snapshots/devmapper/README.md

@@ -0,0 +1,46 @@
+## Devmapper snapshotter
+
+Devmapper is a `containerd` snapshotter plugin that stores snapshots in ext4-formatted filesystem images
+in a devicemapper thin pool.
+
+## Setup
+
+To make it work you need to prepare `thin-pool` in advance and update containerd's configuration file.
+This file is typically located at `/etc/containerd/config.toml`.
+
+Here's minimal sample entry that can be made in the configuration file:
+
+```
+[plugins]
+  ...
+  [plugins.devmapper]
+    pool_name = "containerd-pool"
+    base_image_size = "128MB"
+  ...
+```
+
+The following configuration flags are supported:
+* `root_path` - a directory where the metadata will be available (if empty
+  default location for `containerd` plugins will be used)
+* `pool_name` - a name to use for the devicemapper thin pool. Pool name
+  should be the same as in `/dev/mapper/` directory
+* `data_device` - path to the data volume that should be used by the thin pool
+* `meta_device` - path to the metadata volume that should be used by the thin-pool
+* `data_block_size` - the size of allocation chunks in data file, between 128
+  sectors (64KB) and and 2097152 sectors (1GB) and a multiple of 128 sectors (64KB)
+* `base_image_size` - defines how much space to allocate when creating the base device
+
+Pool name and base image size are required snapshotter parameters.
+
+## Run
+Give it a try with the following commands:
+
+```bash
+ctr images pull --snapshotter devmapper docker.io/library/hello-world:latest
+ctr run --snapshotter devmapper docker.io/library/hello-world:latest test
+```
+
+## Requirements
+
+The devicemapper snapshotter requires `dmsetup` command line tool to be installed and available on your computer.
+On Ubuntu, it can be installed with `apt-get install dmsetup` command.

snapshots/devmapper/config.go

@@ -65,7 +65,7 @@ type Config struct {
 	BaseImageSizeBytes uint64 `toml:"-"`
 }
 
-// LoadConfig reads devmapper configuration file JSON format from disk
+// LoadConfig reads devmapper configuration file from disk in TOML format
 func LoadConfig(path string) (*Config, error) {
 	if _, err := os.Stat(path); err != nil {
 		if os.IsNotExist(err) {

snapshots/devmapper/snapshotter.go

@@ -39,12 +39,9 @@ import (
 
 func init() {
 	plugin.Register(&plugin.Registration{
-		Type: plugin.SnapshotPlugin,
-		ID:   "devmapper",
-		Config: &Config{
-			PoolName:      "containerd-pool",
-			BaseImageSize: "128Mb",
-		},
+		Type:   plugin.SnapshotPlugin,
+		ID:     "devmapper",
+		Config: &Config{},
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
 			ic.Meta.Platforms = append(ic.Meta.Platforms, ocispec.Platform{
 				OS:           "linux",
@@ -56,6 +53,10 @@ func init() {
 				return nil, errors.New("invalid devmapper configuration")
 			}
 
+			if config.PoolName == "" {
+				return nil, errors.New("devmapper not configured")
+			}
+
 			if config.RootPath == "" {
 				config.RootPath = ic.Root
 			}