New replication modes and their documentation

This commit is contained in:
Alex 2022-03-28 16:20:15 +02:00
parent 8f9cf3a5d1
commit 0091002ef2
Signed by: lx
GPG key ID: 0E496D15096376BE
2 changed files with 81 additions and 19 deletions
doc/book/reference-manual
src/table/replication

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@ -48,7 +48,6 @@ root_domain = ".web.garage"
[admin]
api_bind_addr = "0.0.0.0:3903"
trace_sink = "http://localhost:4317"
```
The following gives details about each available configuration option.
@ -89,20 +88,47 @@ might use more storage space that is optimally possible.
Garage supports the following replication modes:
- `none` or `1`: data stored on Garage is stored on a single node. There is no redundancy,
and data will be unavailable as soon as one node fails or its network is disconnected.
Do not use this for anything else than test deployments.
- `none` or `1`: data stored on Garage is stored on a single node. There is no
redundancy, and data will be unavailable as soon as one node fails or its
network is disconnected. Do not use this for anything else than test
deployments.
- `2`: data stored on Garage will be stored on two different nodes, if possible in different
zones. Garage tolerates one node failure before losing data. Data should be available
read-only when one node is down, but write operations will fail.
Use this only if you really have to.
- `2`: data stored on Garage will be stored on two different nodes, if possible
in different zones. Garage tolerates one node failure, or several nodes
failing but all in a single zone (in a deployment with at least two zones),
before losing data. Data remains available in read-only mode when one node is
down, but write operations will fail.
- `3`: data stored on Garage will be stored on three different nodes, if possible each in
a different zones.
Garage tolerates two node failure before losing data. Data should be available
read-only when two nodes are down, and writes should be possible if only a single node
is down.
- `2-dangerous`: a variant of mode `2`, where written objects are written to
the second replica asynchronously. This means that Garage will return `200
OK` to a PutObject request before the second copy is fully written (or even
before it even starts being written). This means that data can more easily
be lost if the node crashes before a second copy can be completed. This
also means that written objects might not be visible immediately in read
operations. In other words, this mode severely breaks the consistency and
durability guarantees of standard Garage cluster operation. Benefits of
this mode: you can still write to your cluster when one node is
unavailable.
- `3`: data stored on Garage will be stored on three different nodes, if
possible each in a different zones. Garage tolerates two node failure, or
several node failures but in no more than two zones (in a deployment with at
least three zones), before losing data. As long as only a single node fails,
or node failures are only in a single zone, reading and writing data to
Garage can continue normally.
- `3-degraded`: a variant of replication mode `3`, that lowers the read
quorum to `1`, to allow you to read data from your cluster when several
nodes (or nodes in several zones) are unavailable. In this mode, Garage
does not provide read-after-write consistency anymore. The write quorum is
still 2, ensuring that data successfully written to Garage is stored on at
least two nodes.
- `3-dangerous`: a variant of replication mode `3` that lowers both the read
and write quorums to `1`, to allow you to both read and write to your
cluster when several nodes (or nodes in several zones) are unavailable. It
is the least consistent mode of operation proposed by Garage, and also one
that should probably never be used.
Note that in modes `2` and `3`,
if at least the same number of zones are available, an arbitrary number of failures in
@ -111,8 +137,35 @@ any given zone is tolerated as copies of data will be spread over several zones.
**Make sure `replication_mode` is the same in the configuration files of all nodes.
Never run a Garage cluster where that is not the case.**
Changing the `replication_mode` of a cluster might work (make sure to shut down all nodes
and changing it everywhere at the time), but is not officially supported.
The quorums associated with each replication mode are described below:
| `replication_mode` | Number of replicas | Write quorum | Read quorum | Read-after-write consistency? |
| ------------------ | ------------------ | ------------ | ----------- | ----------------------------- |
| `none` or `1` | 1 | 1 | 1 | yes |
| `2` | 2 | 2 | 1 | yes |
| `2-dangerous` | 2 | 1 | 1 | NO |
| `3` | 3 | 2 | 2 | yes |
| `3-degraded` | 3 | 2 | 1 | NO |
| `3-dangerous` | 3 | 1 | 1 | NO |
Changing the `replication_mode` between modes with the same number of replicas
(e.g. from `3` to `3-degraded`, or from `2-dangerous` to `2`), can be done easily by
just changing the `replication_mode` parameter in your config files and restarting all your
Garage nodes.
It is also technically possible to change the replication mode to a mode with a
different numbers of replicas, although it's a dangerous operation that is not
officially supported. This requires you to delete the existing cluster layout
and create a new layout from scratch, meaning that a full rebalancing of your
cluster's data will be needed. To do it, shut down your cluster entirely,
delete the `custer_layout` files in the meta directories of all your nodes,
update all your configuration files with the new `replication_mode` parameter,
restart your cluster, and then create a new layout with all the nodes you want
to keep. Rebalancing data will take some time, and data might temporarily
appear unavailable to your users. It is recommended to shut down public access
to the cluster while rebalancing is in progress. In theory, no data should be
lost as rebalancing is a routine operation for Garage, although we cannot
guarantee you that everything will go right in such an extreme scenario.
### `compression_level`

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@ -1,7 +1,10 @@
pub enum ReplicationMode {
None,
TwoWay,
TwoWayDangerous,
ThreeWay,
ThreeWayDegraded,
ThreeWayDangerous,
}
impl ReplicationMode {
@ -9,7 +12,10 @@ impl ReplicationMode {
match v {
"none" | "1" => Some(Self::None),
"2" => Some(Self::TwoWay),
"2-dangerous" => Some(Self::TwoWayDangerous),
"3" => Some(Self::ThreeWay),
"3-degraded" => Some(Self::ThreeWayDegraded),
"3-dangerous" => Some(Self::ThreeWayDangerous),
_ => None,
}
}
@ -24,16 +30,17 @@ impl ReplicationMode {
pub fn replication_factor(&self) -> usize {
match self {
Self::None => 1,
Self::TwoWay => 2,
Self::ThreeWay => 3,
Self::TwoWay | Self::TwoWayDangerous => 2,
Self::ThreeWay | Self::ThreeWayDegraded | Self::ThreeWayDangerous => 3,
}
}
pub fn read_quorum(&self) -> usize {
match self {
Self::None => 1,
Self::TwoWay => 1,
Self::TwoWay | Self::TwoWayDangerous => 1,
Self::ThreeWay => 2,
Self::ThreeWayDegraded | Self::ThreeWayDangerous => 1,
}
}
@ -41,7 +48,9 @@ impl ReplicationMode {
match self {
Self::None => 1,
Self::TwoWay => 2,
Self::ThreeWay => 2,
Self::TwoWayDangerous => 1,
Self::ThreeWay | Self::ThreeWayDegraded => 2,
Self::ThreeWayDangerous => 1,
}
}
}