# Shard data evacuation

## Overview

Evacuation is the process of transferring data from one shard to another. Evacuation is used in case of problems with the shard in order to save data.

To start the evacuation, it is necessary that the shard is in read-only mode (read more [here](./shard-modes.md)).

First of all, by the evacuation the data is transferred to other shards of the same node; if it is not possible, then the data is transferred to other nodes.

Only one running evacuation process is allowed on the node at a time.

It is not necessary to turn maintenance mode on storage node.

Once evacuation from shard started, it is impossible to read data from it via public API, except the case when evacuation stopped manually or node restarted.

Because it is necessary to prevent removing by policer objects with policy `REP 1 ...` from remote node during evacuation.

`frostfs-cli` utility is used to manage evacuation.

## Commands

`frostfs-cli control shards evacuation start` starts evacuation process for shards specified. To start evacuating all node shards, use the `--all` flag.
By default, objects and trees are evacuated. To limit the evacuation scope, use `--scope` flag (possible values are `all`, `trees`, `objects`).
To evacuate objects only from containers with policy `REP 1` use option `--rep-one-only`.
To adjust resource consumption required for evacuation use options:
 - `--container-worker-count` count of concurrent container evacuation workers
 - `--object-worker-count` count of concurrent object evacuation workers

`frostfs-cli control shards evacuation stop` stops running evacuation process.

`frostfs-cli control shards evacuation status` prints evacuation process status.

`frostfs-cli control shards evacuation reset` resets evacuation process status.

See commands `--help` output for detailed description.

## Examples

### Set shard mode to read only
```bash
frostfs-cli control shards set-mode --mode read-only --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json --id 8kEBwtvKLU3Hva3PaaodUi
Enter password >
Shard mode update request successfully sent.
```

### Start evacuation and get status
```bash
frostfs-cli control shards evacuation start --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json --id 8kEBwtvKLU3Hva3PaaodUi
Enter password >
Shard evacuation has been successfully started.

frostfs-cli control shards evacuation status --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json
Enter password >
Shard IDs: 8kEBwtvKLU3Hva3PaaodUi. Status: running. Evacuated 131 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:13:06Z UTC. Duration: 00:00:03. Estimated time left: 2 minutes.

frostfs-cli control shards evacuation status --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json
Enter password >
Shard IDs: 8kEBwtvKLU3Hva3PaaodUi. Status: running. Evacuated 260 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:13:06Z UTC. Duration: 00:01:05. Estimated time left: 1 minutes.

frostfs-cli control shards evacuation status --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json
Enter password >
Shard IDs: 8kEBwtvKLU3Hva3PaaodUi. Status: completed. Evacuated 618 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 19 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:13:06Z UTC. Duration: 00:02:13.
```

### Stop running evacuation process
```bash
frostfs-cli control shards evacuation start --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json --id 54Y8aot9uc7BSadw2XtYr3
Enter password >
Shard evacuation has been successfully started.

frostfs-cli control shards evacuation status --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json
Enter password >
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Status: running. Evacuated 131 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:15:47Z UTC. Duration: 00:00:03. Estimated time left: 0 minutes.

frostfs-cli control shards evacuation stop --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json
Enter password >
Evacuation stopped.

frostfs-cli control shards evacuation status --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json
Enter password >
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Status: completed. Evacuated 131 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Error: context canceled. Started at: 2023-05-10T10:15:47Z UTC. Duration: 00:00:07.
```

### Start evacuation and await it completes
```bash
frostfs-cli control shards evacuation start --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json --id 54Y8aot9uc7BSadw2XtYr3 --await
Enter password >
Shard evacuation has been successfully started.
Progress will be reported every 5 seconds.
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Status: running. Evacuated 131 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:18:42Z UTC. Duration: 00:00:04.  Estimated time left: 0 minutes.
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Status: running. Evacuated 343 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:18:42Z UTC. Duration: 00:00:09.  Estimated time left: 0 minutes.
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Status: running. Evacuated 545 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 0 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:18:42Z UTC. Duration: 00:00:14.  Estimated time left: 0 minutes.
Shard evacuation has been completed.
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Evacuated 618 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 19 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:18:42Z UTC. Duration: 00:00:14.
```

### Start evacuation and await it completes without progress notifications
```bash
frostfs-cli control shards evacuation start --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json --id 54Y8aot9uc7BSadw2XtYr3 --await --no-progress
Enter password >
Shard evacuation has been successfully started.
Shard evacuation has been completed.
Shard IDs: 54Y8aot9uc7BSadw2XtYr3. Evacuated 618 objects out of 618, failed to evacuate: 0, skipped: 0; evacuated 19 trees out of 19, failed to evacuate: 0. Started at: 2023-05-10T10:20:00Z UTC. Duration: 00:00:14.
```

### Start trees evacuation and await it completes
```bash
frostfs-cli control shards evacuation start --id FxR6QujButNCHn7jjdhxGP --endpoint s01.frostfs.devenv:8081 --wallet ./../frostfs-dev-env/services/storage/wallet01.json --await --scope trees
Enter password >
Shard evacuation has been successfully started.
Progress will be reported every 5 seconds.
Shard evacuation has been completed.
Shard IDs: FxR6QujButNCHn7jjdhxGP. Evacuated 0 objects out of 0, failed to evacuate: 0, skipped: 0; evacuated 2 trees out of 2, failed to evacuate: 0. Started at: 2024-02-08T08:44:17Z UTC. Duration: 00:00:00.
```