frostfs-node/docs/shard-modes.md
Alexander Chuprov 8fcd0f8f8d [#1121] docs: Change mode of shard components
Signed-off-by: Alexander Chuprov <a.chuprov@yadro.com>
2024-06-05 05:55:24 +00:00

4.8 KiB

Shard modes description

List of modes

Each mode is characterized by two important properties:

  1. Whether modifying operations are allowed.
  2. Whether metabase and write-cache is available. The expected deployment scenario is to place both metabase and write-cache on an SSD drive thus these modes can be approximately described as no-SSD modes.
Mode Description
read-write Default mode, all operations are allowed.
read-only Read-only mode, only read operations are allowed, metabase is available.
degraded Degraded mode in which metabase and write-cache is disabled. It shouldn't be used at all, because metabase can contain important indices, such as LOCK objects info and modifying operation in this mode can lead to unexpected behaviour. The purpose of this mode is to allow PUT/DELETE operations without the metabase if really necessary.
degraded-read-only Same as degraded, but with only read operations allowed. This mode is used during SSD replacement and/or when the metabase error counter exceeds threshold.
disabled Currently used only in config file to temporarily disable a shard.

Shard and Component Status

Shard Mode Metabase Mode Blobstore Mode Writecache Mode Pilorama Mode Blobovnicza Tree Mode FSTree Mode
Read-Write READ_WRITE READ_WRITE READ_WRITE READ_WRITE READ_WRITE READ_WRITE
Read-Only READ_ONLY READ_ONLY READ_ONLY READ_ONLY READ_ONLY READ_ONLY
Degraded-Read-Write CLOSED READ_WRITE CLOSED CLOSED READ_WRITE READ_WRITE
Degraded-Read-Only CLOSED READ_ONLY CLOSED CLOSED READ_ONLY READ_ONLY

Transition order

Because each shard consists of multiple components changing its mode is not an atomic operation. Instead, each component changes its mode independently.

For transitions to read-write mode the order is:

  1. metabase
  2. blobstor
  3. writecache
  4. pilorama

For transitions to all other modes the order is:

  1. writecache
  2. blobstor
  3. metabase
  4. pilorama

The motivation is to avoid transient errors because write-cache can write to both blobstor and metabase. Thus, when we want to stop write operations, write-cache needs to change mode before them. On the other side, when we want to allow them, blobstor and metabase should be writable before write-cache is.

If anything goes wrong in the middle, the mode of some components can be different from the actual mode of a shard. However, all mode changing operations are idempotent.

Automatic mode changes

Shard can automatically switch to a degraded-read-only mode in 3 cases:

  1. If the metabase was not available or couldn't be opened/initialized during shard startup.
  2. If shard error counter exceeds threshold.
  3. If the metabase couldn't be reopened during SIGHUP handling.

Detach shard

To detach a shard use frostfs-cli control shards detach command. This command removes the shards from the storage engine and closes all resources associated with the shards. Limitation: SIGHUP or storage node restart lead to detached shard will be again online.