101 lines
5.1 KiB
Markdown
101 lines
5.1 KiB
Markdown
+++
|
|
title = "Internals"
|
|
weight = 20
|
|
+++
|
|
|
|
## Overview
|
|
|
|
TODO: write this section
|
|
|
|
- The Dynamo ring (see [this paper](https://dl.acm.org/doi/abs/10.1145/1323293.1294281) and [that paper](https://www.usenix.org/conference/nsdi16/technical-sessions/presentation/eisenbud))
|
|
|
|
- CRDTs (see [this paper](https://link.springer.com/chapter/10.1007/978-3-642-24550-3_29))
|
|
|
|
- Consistency model of Garage tables
|
|
|
|
In the meantime, you can find some information at the following links:
|
|
|
|
- [this presentation (in French)](https://git.deuxfleurs.fr/Deuxfleurs/garage/src/branch/main/doc/talks/2020-12-02_wide-team/talk.pdf)
|
|
|
|
- [an old design draft](@/documentation/working-documents/design-draft.md)
|
|
|
|
|
|
## Garbage collection
|
|
|
|
A faulty garbage collection procedure has been the cause of
|
|
[critical bug #39](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/39).
|
|
This precise bug was fixed in the code, however there are potentially more
|
|
general issues with the garbage collector being too eager and deleting things
|
|
too early. This has been the subject of
|
|
[PR #135](https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/135).
|
|
This section summarizes the discussions on this topic.
|
|
|
|
Rationale: we want to ensure Garage's safety by making sure things don't get
|
|
deleted from disk if they are still needed. Two aspects are involved in this.
|
|
|
|
### 1. Garbage collection of table entries (in `meta/` directory)
|
|
|
|
The `Entry` trait used for table entries (defined in `tables/schema.rs`)
|
|
defines a function `is_tombstone()` that returns `true` if that entry
|
|
represents an entry that is deleted in the table. CRDT semantics by default
|
|
keep all tombstones, because they are necessary for reconciliation: if node A
|
|
has a tombstone that supersedes a value `x`, and node B has value `x`, A has to
|
|
keep the tombstone in memory so that the value `x` can be properly deleted at
|
|
node `B`. Otherwise, due to the CRDT reconciliation rule, the value `x` from B
|
|
would flow back to A and a deleted item would reappear in the system.
|
|
|
|
Here, we have some control on the nodes involved in storing Garage data.
|
|
Therefore we have a garbage collector that is able to delete tombstones UNDER
|
|
CERTAIN CONDITIONS. This garbage collector is implemented in `table/gc.rs`. To
|
|
delete a tombstone, the following condition has to be met:
|
|
|
|
- All nodes responsible for storing this entry are aware of the existence of
|
|
the tombstone, i.e. they cannot hold another version of the entry that is
|
|
superseeded by the tombstone. This ensures that deleting the tombstone is
|
|
safe and that no deleted value will come back in the system.
|
|
|
|
Garage makes use of Sled's atomic operations (such as compare-and-swap and
|
|
transactions) to ensure that only tombstones that have been correctly
|
|
propagated to other nodes are ever deleted from the local entry tree.
|
|
|
|
This GC is safe in the following sense: no non-tombstone data is ever deleted
|
|
from Garage tables.
|
|
|
|
**However**, there is an issue with the way this interacts with data
|
|
rebalancing in the case when a partition is moving between nodes. If a node has
|
|
some data of a partition for which it is not responsible, it has to offload it.
|
|
However that offload process takes some time. In that interval, the GC does not
|
|
check with that node if it has the tombstone before deleting the tombstone, so
|
|
perhaps it doesn't have it and when the offload finally happens, old data comes
|
|
back in the system.
|
|
|
|
**PR 135 mostly fixes this** by implementing a 24-hour delay before anything is
|
|
garbage collected in a table. This works under the assumption that rebalances
|
|
that follow data shuffling terminate in less than 24 hours.
|
|
|
|
**However**, in distributed systems, it is generally considered a bad practice
|
|
to make assumptions that information propagates in a certain time interval:
|
|
this consists in making a synchrony assumption, meaning that we are basically
|
|
assuming a computing model that has much stronger properties than otherwise. To
|
|
maximize the applicability of Garage, we would like to remove this assumption,
|
|
and implement a system where time does not play a role. To do this, we would
|
|
need to find a way to safely disable the GC when data is being shuffled around,
|
|
and safely detect that the shuffling has terminated and thus the GC can be
|
|
resumed. This introduces some complexity to the protocol and hasn't been
|
|
tackled yet.
|
|
|
|
### 2. Garbage collection of data blocks (in `data/` directory)
|
|
|
|
Blocks in the data directory are reference-counted. In Garage versions before
|
|
PR #135, blocks could get deleted from local disk as soon as their reference
|
|
counter reached zero. We had a mechanism to not trigger this immediately at the
|
|
rc-reaches-zero event, but the cleanup could be triggered by other means (for
|
|
example by a block repair operation...). PR #135 added a safety measure so that
|
|
blocks never get deleted in a 10 minute interval following the time when the RC
|
|
reaches zero. This is a measure to make impossible race conditions such as #39.
|
|
We would have liked to use a larger delay (e.g. 24 hours), but in the case of a
|
|
rebalance of data, this would have led to the disk utilization to explode
|
|
during the rebalancing, only to shrink again after 24 hours. The 10-minute
|
|
delay is a compromise that gives good security while not having this problem of
|
|
disk space explosion on rebalance.
|
|
|