1c0ba930b8
This PR should be merged after the new website is deployed. - [x] Rename files - [x] Add front matter section to all `.md` files in the book (necessary for Zola) - [x] Change all internal links to use Zola's linking system that checks broken links - [x] Some updates to documentation contents and organization Co-authored-by: Alex Auvolat <alex@adnab.me> Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/213 Co-authored-by: Alex <alex@adnab.me> Co-committed-by: Alex <alex@adnab.me>
166 lines
8.3 KiB
Markdown
166 lines
8.3 KiB
Markdown
+++
|
|
title = "Design draft"
|
|
weight = 25
|
|
+++
|
|
|
|
**WARNING: this documentation is a design draft which was written before Garage's actual implementation.
|
|
The general principle are similar, but details have not been updated.**
|
|
|
|
|
|
#### Modules
|
|
|
|
- `membership/`: configuration, membership management (gossip of node's presence and status), ring generation --> what about Serf (used by Consul/Nomad) : https://www.serf.io/? Seems a huge library with many features so maybe overkill/hard to integrate
|
|
- `metadata/`: metadata management
|
|
- `blocks/`: block management, writing, GC and rebalancing
|
|
- `internal/`: server to server communication (HTTP server and client that reuses connections, TLS if we want, etc)
|
|
- `api/`: S3 API
|
|
- `web/`: web management interface
|
|
|
|
#### Metadata tables
|
|
|
|
**Objects:**
|
|
|
|
- *Hash key:* Bucket name (string)
|
|
- *Sort key:* Object key (string)
|
|
- *Sort key:* Version timestamp (int)
|
|
- *Sort key:* Version UUID (string)
|
|
- Complete: bool
|
|
- Inline: bool, true for objects < threshold (say 1024)
|
|
- Object size (int)
|
|
- Mime type (string)
|
|
- Data for inlined objects (blob)
|
|
- Hash of first block otherwise (string)
|
|
|
|
*Having only a hash key on the bucket name will lead to storing all file entries of this table for a specific bucket on a single node. At the same time, it is the only way I see to rapidly being able to list all bucket entries...*
|
|
|
|
**Blocks:**
|
|
|
|
- *Hash key:* Version UUID (string)
|
|
- *Sort key:* Offset of block in total file (int)
|
|
- Hash of data block (string)
|
|
|
|
A version is defined by the existence of at least one entry in the blocks table for a certain version UUID.
|
|
We must keep the following invariant: if a version exists in the blocks table, it has to be referenced in the objects table.
|
|
We explicitly manage concurrent versions of an object: the version timestamp and version UUID columns are index columns, thus we may have several concurrent versions of an object.
|
|
Important: before deleting an older version from the objects table, we must make sure that we did a successfull delete of the blocks of that version from the blocks table.
|
|
|
|
Thus, the workflow for reading an object is as follows:
|
|
|
|
1. Check permissions (LDAP)
|
|
2. Read entry in object table. If data is inline, we have its data, stop here.
|
|
-> if several versions, take newest one and launch deletion of old ones in background
|
|
3. Read first block from cluster. If size <= 1 block, stop here.
|
|
4. Simultaneously with previous step, if size > 1 block: query the Blocks table for the IDs of the next blocks
|
|
5. Read subsequent blocks from cluster
|
|
|
|
Workflow for PUT:
|
|
|
|
1. Check write permission (LDAP)
|
|
2. Select a new version UUID
|
|
3. Write a preliminary entry for the new version in the objects table with complete = false
|
|
4. Send blocks to cluster and write entries in the blocks table
|
|
5. Update the version with complete = true and all of the accurate information (size, etc)
|
|
6. Return success to the user
|
|
7. Launch a background job to check and delete older versions
|
|
|
|
Workflow for DELETE:
|
|
|
|
1. Check write permission (LDAP)
|
|
2. Get current version (or versions) in object table
|
|
3. Do the deletion of those versions NOT IN A BACKGROUND JOB THIS TIME
|
|
4. Return succes to the user if we were able to delete blocks from the blocks table and entries from the object table
|
|
|
|
To delete a version:
|
|
|
|
1. List the blocks from Cassandra
|
|
2. For each block, delete it from cluster. Don't care if some deletions fail, we can do GC.
|
|
3. Delete all of the blocks from the blocks table
|
|
4. Finally, delete the version from the objects table
|
|
|
|
Known issue: if someone is reading from a version that we want to delete and the object is big, the read might be interrupted. I think it is ok to leave it like this, we just cut the connection if data disappears during a read.
|
|
|
|
("Soit P un problème, on s'en fout est une solution à ce problème")
|
|
|
|
#### Block storage on disk
|
|
|
|
**Blocks themselves:**
|
|
|
|
- file path = /blobs/(first 3 hex digits of hash)/(rest of hash)
|
|
|
|
**Reverse index for GC & other block-level metadata:**
|
|
|
|
- file path = /meta/(first 3 hex digits of hash)/(rest of hash)
|
|
- map block hash -> set of version UUIDs where it is referenced
|
|
|
|
Usefull metadata:
|
|
|
|
- list of versions that reference this block in the Casandra table, so that we can do GC by checking in Cassandra that the lines still exist
|
|
- list of other nodes that we know have acknowledged a write of this block, usefull in the rebalancing algorithm
|
|
|
|
Write strategy: have a single thread that does all write IO so that it is serialized (or have several threads that manage independent parts of the hash space). When writing a blob, write it to a temporary file, close, then rename so that a concurrent read gets a consistent result (either not found or found with whole content).
|
|
|
|
Read strategy: the only read operation is get(hash) that returns either the data or not found (can do a corruption check as well and return corrupted state if it is the case). Can be done concurrently with writes.
|
|
|
|
**Internal API:**
|
|
|
|
- get(block hash) -> ok+data/not found/corrupted
|
|
- put(block hash & data, version uuid + offset) -> ok/error
|
|
- put with no data(block hash, version uuid + offset) -> ok/not found plz send data/error
|
|
- delete(block hash, version uuid + offset) -> ok/error
|
|
|
|
GC: when last ref is deleted, delete block.
|
|
Long GC procedure: check in Cassandra that version UUIDs still exist and references this block.
|
|
|
|
Rebalancing: takes as argument the list of newly added nodes.
|
|
|
|
- List all blocks that we have. For each block:
|
|
- If it hits a newly introduced node, send it to them.
|
|
Use put with no data first to check if it has to be sent to them already or not.
|
|
Use a random listing order to avoid race conditions (they do no harm but we might have two nodes sending the same thing at the same time thus wasting time).
|
|
- If it doesn't hit us anymore, delete it and its reference list.
|
|
|
|
Only one balancing can be running at a same time. It can be restarted at the beginning with new parameters.
|
|
|
|
#### Membership management
|
|
|
|
Two sets of nodes:
|
|
|
|
- set of nodes from which a ping was recently received, with status: number of stored blocks, request counters, error counters, GC%, rebalancing%
|
|
(eviction from this set after say 30 seconds without ping)
|
|
- set of nodes that are part of the system, explicitly modified by the operator using the web UI (persisted to disk),
|
|
is a CRDT using a version number for the value of the whole set
|
|
|
|
Thus, three states for nodes:
|
|
|
|
- healthy: in both sets
|
|
- missing: not pingable but part of desired cluster
|
|
- unused/draining: currently present but not part of the desired cluster, empty = if contains nothing, draining = if still contains some blocks
|
|
|
|
Membership messages between nodes:
|
|
|
|
- ping with current state + hash of current membership info -> reply with same info
|
|
- send&get back membership info (the ids of nodes that are in the two sets): used when no local membership change in a long time and membership info hash discrepancy detected with first message (passive membership fixing with full CRDT gossip)
|
|
- inform of newly pingable node(s) -> no result, when receive new info repeat to all (reliable broadcast)
|
|
- inform of operator membership change -> no result, when receive new info repeat to all (reliable broadcast)
|
|
|
|
Ring: generated from the desired set of nodes, however when doing read/writes on the ring, skip nodes that are known to be not pingable.
|
|
The tokens are generated in a deterministic fashion from node IDs (hash of node id + token number from 1 to K).
|
|
Number K of tokens per node: decided by the operator & stored in the operator's list of nodes CRDT. Default value proposal: with node status information also broadcast disk total size and free space, and propose a default number of tokens equal to 80%Free space / 10Gb. (this is all user interface)
|
|
|
|
|
|
#### Constants
|
|
|
|
- Block size: around 1MB ? --> Exoscale use 16MB chunks
|
|
- Number of tokens in the hash ring: one every 10Gb of allocated storage
|
|
- Threshold for storing data directly in Cassandra objects table: 1kb bytes (maybe up to 4kb?)
|
|
- Ping timeout (time after which a node is registered as unresponsive/missing): 30 seconds
|
|
- Ping interval: 10 seconds
|
|
- ??
|
|
|
|
#### Links
|
|
|
|
- CDC: <https://www.usenix.org/system/files/conference/atc16/atc16-paper-xia.pdf>
|
|
- Erasure coding: <http://web.eecs.utk.edu/~jplank/plank/papers/CS-08-627.html>
|
|
- [Openstack Storage Concepts](https://docs.openstack.org/arch-design/design-storage/design-storage-concepts.html)
|
|
- [RADOS](https://doi.org/10.1145/1374596.1374606) [[pdf](https://ceph.com/assets/pdfs/weil-rados-pdsw07.pdf)]
|