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101 lines
4.2 KiB
Markdown
101 lines
4.2 KiB
Markdown
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title = "Multi-HDD support"
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weight = 15
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Since v0.9, Garage natively supports nodes that have several storage drives
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for storing data blocks (not for metadata storage).
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## Initial setup
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To set up a new Garage storage node with multiple HDDs,
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format and mount all your drives in different directories,
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and use a Garage configuration as follows:
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```toml
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data_dir = [
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{ path = "/path/to/hdd1", capacity = "2T" },
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{ path = "/path/to/hdd2", capacity = "4T" },
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]
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```
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Garage will automatically balance all blocks stored by the node
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among the different specified directories, proportionnally to the
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specified capacities.
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## Updating the list of storage locations
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If you add new storage locations to your `data_dir`,
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Garage will not rebalance existing data between storage locations.
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Newly written blocks will be balanced proportionnally to the specified capacities,
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and existing data may be moved between drives to improve balancing,
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but only opportunistically when a data block is re-written (e.g. an object
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is re-uploaded, or an object with a duplicate block is uploaded).
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To understand precisely what is happening, we need to dive in to how Garage
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splits data among the different storage locations.
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First of all, Garage divides the set of all possible block hashes
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in a fixed number of slices (currently 1024), and assigns
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to each slice a primary storage location among the specified data directories.
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The number of slices having their primary location in each data directory
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is proportionnal to the capacity specified in the config file.
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When Garage receives a block to write, it will always write it in the primary
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directory of the slice that contains its hash.
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Now, to be able to not lose existing data blocks when storage locations
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are added, Garage also keeps a list of secondary data directories
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for all of the hash slices. Secondary data directories for a slice indicates
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storage locations that once were primary directories for that slice, i.e. where
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Garage knows that data blocks of that slice might be stored.
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When Garage is requested to read a certain data block,
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it will first look in the primary storage directory of its slice,
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and if it doesn't find it there it goes through all of the secondary storage
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locations until it finds it. This allows Garage to continue operating
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normally when storage locations are added, without having to shuffle
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files between drives to place them in the correct location.
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This relatively simple strategy works well but does not ensure that data
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is correctly balanced among drives according to their capacity.
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To rebalance data, two strategies can be used:
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- Lazy rebalancing: when a block is re-written (e.g. the object is re-uploaded),
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Garage checks whether the existing copy is in the primary directory of the slice
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or in a secondary directory. If the current copy is in a secondary directory,
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Garage re-writes a copy in the primary directory and deletes the one from the
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secondary directory. This might never end up rebalancing everything if there
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are data blocks that are only read and never written.
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- Active rebalancing: an operator of a Garage node can explicitly launch a repair
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procedure that rebalances the data directories, moving all blocks to their
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primary location. Once done, all secondary locations for all hash slices are
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removed so that they won't be checked anymore when looking for a data block.
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## Read-only storage locations
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If you would like to move all data blocks from an existing data directory to one
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or several new data directories, mark the old directory as read-only:
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```toml
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data_dir = [
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{ path = "/path/to/old_data", read_only = true },
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{ path = "/path/to/new_hdd1", capacity = "2T" },
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{ path = "/path/to/new_hdd2", capacity = "4T" },
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]
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```
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Garage will be able to read requested blocks from the read-only directory.
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Garage will also move data out of the read-only directory either progressively
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(lazy rebalancing) or if requested explicitly (active rebalancing).
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Once an active rebalancing has finished, your read-only directory should be empty:
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it might still contain subdirectories, but no data files. You can check that
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it contains no files using:
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```bash
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find -type f /path/to/old_data # should not print anything
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```
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at which point it can be removed from the `data_dir` list in your config file.
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