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doc: move replication_mode to top of configuration page reference
This commit is contained in:
Alex Auvolat
parent
939d1f2e17
commit
cfa5550cb2
1 changed files with 85 additions and 85 deletions
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@ -8,6 +8,8 @@ weight = 20
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Here is an example `garage.toml` configuration file that illustrates all of the possible options:
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```toml
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replication_mode = "3"
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metadata_dir = "/var/lib/garage/meta"
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data_dir = "/var/lib/garage/data"
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metadata_fsync = true
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@ -21,8 +23,6 @@ sled_cache_capacity = "128MiB"
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sled_flush_every_ms = 2000
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lmdb_map_size = "1T"
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replication_mode = "3"
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compression_level = 1
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rpc_secret = "4425f5c26c5e11581d3223904324dcb5b5d5dfb14e5e7f35e38c595424f5f1e6"
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@ -134,6 +134,89 @@ The `[admin]` section:
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### Top-level configuration options
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#### `replication_mode` {#replication_mode}
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Garage supports the following replication modes:
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- `none` or `1`: data stored on Garage is stored on a single node. There is no
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redundancy, and data will be unavailable as soon as one node fails or its
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network is disconnected. Do not use this for anything else than test
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deployments.
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- `2`: data stored on Garage will be stored on two different nodes, if possible
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in different zones. Garage tolerates one node failure, or several nodes
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failing but all in a single zone (in a deployment with at least two zones),
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before losing data. Data remains available in read-only mode when one node is
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down, but write operations will fail.
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- `2-dangerous`: a variant of mode `2`, where written objects are written to
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the second replica asynchronously. This means that Garage will return `200
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OK` to a PutObject request before the second copy is fully written (or even
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before it even starts being written). This means that data can more easily
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be lost if the node crashes before a second copy can be completed. This
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also means that written objects might not be visible immediately in read
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operations. In other words, this mode severely breaks the consistency and
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durability guarantees of standard Garage cluster operation. Benefits of
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this mode: you can still write to your cluster when one node is
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unavailable.
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- `3`: data stored on Garage will be stored on three different nodes, if
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possible each in a different zones. Garage tolerates two node failure, or
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several node failures but in no more than two zones (in a deployment with at
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least three zones), before losing data. As long as only a single node fails,
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or node failures are only in a single zone, reading and writing data to
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Garage can continue normally.
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- `3-degraded`: a variant of replication mode `3`, that lowers the read
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quorum to `1`, to allow you to read data from your cluster when several
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nodes (or nodes in several zones) are unavailable. In this mode, Garage
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does not provide read-after-write consistency anymore. The write quorum is
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still 2, ensuring that data successfully written to Garage is stored on at
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least two nodes.
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- `3-dangerous`: a variant of replication mode `3` that lowers both the read
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and write quorums to `1`, to allow you to both read and write to your
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cluster when several nodes (or nodes in several zones) are unavailable. It
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is the least consistent mode of operation proposed by Garage, and also one
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that should probably never be used.
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Note that in modes `2` and `3`,
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if at least the same number of zones are available, an arbitrary number of failures in
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any given zone is tolerated as copies of data will be spread over several zones.
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**Make sure `replication_mode` is the same in the configuration files of all nodes.
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Never run a Garage cluster where that is not the case.**
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The quorums associated with each replication mode are described below:
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| `replication_mode` | Number of replicas | Write quorum | Read quorum | Read-after-write consistency? |
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| ------------------ | ------------------ | ------------ | ----------- | ----------------------------- |
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| `none` or `1` | 1 | 1 | 1 | yes |
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| `2` | 2 | 2 | 1 | yes |
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| `2-dangerous` | 2 | 1 | 1 | NO |
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| `3` | 3 | 2 | 2 | yes |
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| `3-degraded` | 3 | 2 | 1 | NO |
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| `3-dangerous` | 3 | 1 | 1 | NO |
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Changing the `replication_mode` between modes with the same number of replicas
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(e.g. from `3` to `3-degraded`, or from `2-dangerous` to `2`), can be done easily by
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just changing the `replication_mode` parameter in your config files and restarting all your
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Garage nodes.
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It is also technically possible to change the replication mode to a mode with a
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different numbers of replicas, although it's a dangerous operation that is not
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officially supported. This requires you to delete the existing cluster layout
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and create a new layout from scratch, meaning that a full rebalancing of your
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cluster's data will be needed. To do it, shut down your cluster entirely,
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delete the `custer_layout` files in the meta directories of all your nodes,
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update all your configuration files with the new `replication_mode` parameter,
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restart your cluster, and then create a new layout with all the nodes you want
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to keep. Rebalancing data will take some time, and data might temporarily
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appear unavailable to your users. It is recommended to shut down public access
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to the cluster while rebalancing is in progress. In theory, no data should be
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lost as rebalancing is a routine operation for Garage, although we cannot
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guarantee you that everything will go right in such an extreme scenario.
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#### `metadata_dir` {#metadata_dir}
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The directory in which Garage will store its metadata. This contains the node identifier,
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@ -287,89 +370,6 @@ The value of this parameter is the maximum size the metadata database can take.
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This value is not bound by the physical RAM size of the machine running Garage.
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If not specified, it defaults to 1GiB on 32-bit machines and 1TiB on 64-bit machines.
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#### `replication_mode` {#replication_mode}
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Garage supports the following replication modes:
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- `none` or `1`: data stored on Garage is stored on a single node. There is no
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redundancy, and data will be unavailable as soon as one node fails or its
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network is disconnected. Do not use this for anything else than test
|
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deployments.
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- `2`: data stored on Garage will be stored on two different nodes, if possible
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in different zones. Garage tolerates one node failure, or several nodes
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failing but all in a single zone (in a deployment with at least two zones),
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before losing data. Data remains available in read-only mode when one node is
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down, but write operations will fail.
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- `2-dangerous`: a variant of mode `2`, where written objects are written to
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the second replica asynchronously. This means that Garage will return `200
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OK` to a PutObject request before the second copy is fully written (or even
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before it even starts being written). This means that data can more easily
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be lost if the node crashes before a second copy can be completed. This
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also means that written objects might not be visible immediately in read
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operations. In other words, this mode severely breaks the consistency and
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durability guarantees of standard Garage cluster operation. Benefits of
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this mode: you can still write to your cluster when one node is
|
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unavailable.
|
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- `3`: data stored on Garage will be stored on three different nodes, if
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possible each in a different zones. Garage tolerates two node failure, or
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several node failures but in no more than two zones (in a deployment with at
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least three zones), before losing data. As long as only a single node fails,
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or node failures are only in a single zone, reading and writing data to
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Garage can continue normally.
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- `3-degraded`: a variant of replication mode `3`, that lowers the read
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quorum to `1`, to allow you to read data from your cluster when several
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nodes (or nodes in several zones) are unavailable. In this mode, Garage
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does not provide read-after-write consistency anymore. The write quorum is
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still 2, ensuring that data successfully written to Garage is stored on at
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least two nodes.
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- `3-dangerous`: a variant of replication mode `3` that lowers both the read
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and write quorums to `1`, to allow you to both read and write to your
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cluster when several nodes (or nodes in several zones) are unavailable. It
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is the least consistent mode of operation proposed by Garage, and also one
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that should probably never be used.
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Note that in modes `2` and `3`,
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if at least the same number of zones are available, an arbitrary number of failures in
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any given zone is tolerated as copies of data will be spread over several zones.
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**Make sure `replication_mode` is the same in the configuration files of all nodes.
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Never run a Garage cluster where that is not the case.**
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The quorums associated with each replication mode are described below:
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| `replication_mode` | Number of replicas | Write quorum | Read quorum | Read-after-write consistency? |
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| ------------------ | ------------------ | ------------ | ----------- | ----------------------------- |
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| `none` or `1` | 1 | 1 | 1 | yes |
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| `2` | 2 | 2 | 1 | yes |
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| `2-dangerous` | 2 | 1 | 1 | NO |
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| `3` | 3 | 2 | 2 | yes |
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| `3-degraded` | 3 | 2 | 1 | NO |
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| `3-dangerous` | 3 | 1 | 1 | NO |
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Changing the `replication_mode` between modes with the same number of replicas
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(e.g. from `3` to `3-degraded`, or from `2-dangerous` to `2`), can be done easily by
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just changing the `replication_mode` parameter in your config files and restarting all your
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Garage nodes.
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It is also technically possible to change the replication mode to a mode with a
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different numbers of replicas, although it's a dangerous operation that is not
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officially supported. This requires you to delete the existing cluster layout
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and create a new layout from scratch, meaning that a full rebalancing of your
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cluster's data will be needed. To do it, shut down your cluster entirely,
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delete the `custer_layout` files in the meta directories of all your nodes,
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update all your configuration files with the new `replication_mode` parameter,
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restart your cluster, and then create a new layout with all the nodes you want
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to keep. Rebalancing data will take some time, and data might temporarily
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appear unavailable to your users. It is recommended to shut down public access
|
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to the cluster while rebalancing is in progress. In theory, no data should be
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lost as rebalancing is a routine operation for Garage, although we cannot
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guarantee you that everything will go right in such an extreme scenario.
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#### `compression_level` {#compression_level}
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Zstd compression level to use for storing blocks.
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