Refactor and comment table GC logic

This commit is contained in:
Alex 2021-10-28 12:49:37 +02:00
parent 8e25a37f0e
commit cc255d46cd
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GPG key ID: EDABF9711E244EB1
2 changed files with 130 additions and 23 deletions

View file

@ -12,6 +12,7 @@ use garage_util::error::*;
use garage_rpc::system::System;
use crate::crdt::Crdt;
use crate::gc::GcTodoEntry;
use crate::replication::*;
use crate::schema::*;
@ -176,7 +177,7 @@ where
let pk_hash = Hash::try_from(&tree_key[..32]).unwrap();
let nodes = self.replication.write_nodes(&pk_hash);
if nodes.first() == Some(&self.system.id) {
self.gc_todo.insert(&tree_key, new_bytes_hash.as_slice())?;
GcTodoEntry::new(tree_key, new_bytes_hash).save(&self.gc_todo)?;
}
}
}

View file

@ -12,7 +12,7 @@ use futures_util::future::*;
use tokio::sync::watch;
use garage_util::data::*;
use garage_util::error::Error;
use garage_util::error::*;
use garage_rpc::system::System;
use garage_rpc::*;
@ -24,7 +24,7 @@ use crate::schema::*;
const TABLE_GC_BATCH_SIZE: usize = 1024;
const TABLE_GC_RPC_TIMEOUT: Duration = Duration::from_secs(30);
pub struct TableGc<F: TableSchema + 'static, R: TableReplication + 'static> {
pub(crate) struct TableGc<F: TableSchema + 'static, R: TableReplication + 'static> {
system: Arc<System>,
data: Arc<TableData<F, R>>,
@ -94,31 +94,45 @@ where
let mut entries = vec![];
let mut excluded = vec![];
for item in self.data.gc_todo.iter() {
let (k, vhash) = item?;
// List entries in the GC todo list
// These entries are put there when a tombstone is inserted in the table
// This is detected and done in data.rs in update_entry
for entry_kv in self.data.gc_todo.iter() {
let (k, vhash) = entry_kv?;
let mut todo_entry = GcTodoEntry::parse(&k, &vhash);
let vhash = Hash::try_from(&vhash[..]).unwrap();
let v_opt = self
// Check if the tombstone is still the current value of the entry.
// If not, we don't actually want to GC it, and we will remove it
// from the gc_todo table later (below).
todo_entry.value = self
.data
.store
.get(&k[..])?
.filter(|v| blake2sum(&v[..]) == vhash);
.filter(|v| blake2sum(&v[..]) == vhash)
.map(|v| v.to_vec());
if let Some(v) = v_opt {
entries.push((ByteBuf::from(k.to_vec()), vhash, ByteBuf::from(v.to_vec())));
if todo_entry.value.is_some() {
entries.push(todo_entry);
if entries.len() >= TABLE_GC_BATCH_SIZE {
break;
}
} else {
excluded.push((k, vhash));
excluded.push(todo_entry);
}
}
for (k, vhash) in excluded {
self.todo_remove_if_equal(&k[..], vhash)?;
// Remove from gc_todo entries for tombstones where we have
// detected that the current value has changed and
// is no longer a tombstone.
for entry in excluded {
entry.remove_if_equal(&self.data.gc_todo)?;
}
// Remaining in `entries` is the list of entries we want to GC,
// and for which they are still currently tombstones in the table.
if entries.is_empty() {
// Nothing to do in this iteration
return Ok(false);
@ -126,9 +140,17 @@ where
debug!("({}) GC: doing {} items", self.data.name, entries.len());
// Split entries to GC by the set of nodes on which they are stored.
// Here we call them partitions but they are not exactly
// the same as partitions as defined in the ring: those partitions
// are defined by the first 8 bits of the hash, but two of these
// partitions can be stored on the same set of nodes.
// Here we detect when entries are stored on the same set of nodes:
// even if they are not in the same 8-bit partition, we can still
// handle them together.
let mut partitions = HashMap::new();
for (k, vhash, v) in entries {
let pkh = Hash::try_from(&k[..32]).unwrap();
for entry in entries {
let pkh = Hash::try_from(&entry.key[..32]).unwrap();
let mut nodes = self.data.replication.write_nodes(&pkh);
nodes.retain(|x| *x != self.system.id);
nodes.sort();
@ -136,9 +158,12 @@ where
if !partitions.contains_key(&nodes) {
partitions.insert(nodes.clone(), vec![]);
}
partitions.get_mut(&nodes).unwrap().push((k, vhash, v));
partitions.get_mut(&nodes).unwrap().push(entry);
}
// For each set of nodes that contains some items,
// ensure they are aware of the tombstone status, and once they
// are, instruct them to delete the entries.
let resps = join_all(
partitions
.into_iter()
@ -146,6 +171,8 @@ where
)
.await;
// Collect errors and return a single error value even if several
// errors occurred.
let mut errs = vec![];
for resp in resps {
if let Err(e) = resp {
@ -162,23 +189,40 @@ where
.collect::<Vec<_>>()
.join(", "),
))
.err_context("in try_send_and_delete:")
}
}
async fn try_send_and_delete(
&self,
nodes: Vec<Uuid>,
items: Vec<(ByteBuf, Hash, ByteBuf)>,
items: Vec<GcTodoEntry>,
) -> Result<(), Error> {
let n_items = items.len();
// Strategy: we first send all of the values to the remote nodes,
// to ensure that they are aware of the tombstone state.
// (if they have a newer state that overrides the tombstone, that's fine).
// Second, once everyone is at least at the tombstone state,
// we instruct everyone to delete the tombstone IF that is still their current state.
// If they are now at a different state, it means that that state overrides the
// tombstone in the CRDT lattice, and it will be propagated back to us at some point
// (either just a regular update that hasn't reached us yet, or later when the
// table is synced).
// Here, we store in updates all of the tombstones to send for step 1,
// and in deletes the list of keys and hashes of value for step 2.
let mut updates = vec![];
let mut deletes = vec![];
for (k, vhash, v) in items {
updates.push(v);
deletes.push((k, vhash));
for item in items {
updates.push(ByteBuf::from(item.value.unwrap()));
deletes.push((ByteBuf::from(item.key), item.value_hash));
}
// Step 1: ensure everyone is at least at tombstone in CRDT lattice
// Here the quorum is nodes.len(): we cannot tolerate even a single failure,
// otherwise old values before the tombstone might come back in the data.
// GC'ing is not a critical function of the system, so it's not a big
// deal if we can't do it right now.
self.system
.rpc
.try_call_many(
@ -189,13 +233,18 @@ where
.with_quorum(nodes.len())
.with_timeout(TABLE_GC_RPC_TIMEOUT),
)
.await?;
.await
.err_context("GC: send tombstones")?;
info!(
"({}) GC: {} items successfully pushed, will try to delete.",
self.data.name, n_items
);
// Step 2: delete tombstones everywhere.
// Here we also fail if even a single node returns a failure:
// it means that the garbage collection wasn't completed and has
// to be retried later.
self.system
.rpc
.try_call_many(
@ -206,11 +255,17 @@ where
.with_quorum(nodes.len())
.with_timeout(TABLE_GC_RPC_TIMEOUT),
)
.await?;
.await
.err_context("GC: remote delete tombstones")?;
// GC has been successfull for all of these entries.
// We now remove them all from our local table and from the GC todo list.
for (k, vhash) in deletes {
self.data.delete_if_equal_hash(&k[..], vhash)?;
self.todo_remove_if_equal(&k[..], vhash)?;
self.data
.delete_if_equal_hash(&k[..], vhash)
.err_context("GC: local delete tombstones")?;
self.todo_remove_if_equal(&k[..], vhash)
.err_context("GC: remove from todo list after successfull GC")?;
}
Ok(())
@ -248,3 +303,54 @@ where
}
}
}
/// An entry stored in the gc_todo Sled tree associated with the table
/// Contains helper function for parsing, saving, and removing
/// such entry in Sled
pub(crate) struct GcTodoEntry {
key: Vec<u8>,
value_hash: Hash,
value: Option<Vec<u8>>,
}
impl GcTodoEntry {
/// Creates a new GcTodoEntry (not saved in Sled) from its components:
/// the key of an entry in the table, and the hash of the associated
/// serialized value
pub(crate) fn new(key: Vec<u8>, value_hash: Hash) -> Self {
Self {
key,
value_hash,
value: None,
}
}
/// Parses a GcTodoEntry from a (k, v) pair stored in the gc_todo tree
pub(crate) fn parse(sled_k: &[u8], sled_v: &[u8]) -> Self {
Self {
key: sled_k.to_vec(),
value_hash: Hash::try_from(sled_v).unwrap(),
value: None,
}
}
/// Saves the GcTodoEntry in the gc_todo tree
pub(crate) fn save(&self, gc_todo_tree: &sled::Tree) -> Result<(), Error> {
gc_todo_tree.insert(&self.key[..], self.value_hash.as_slice())?;
Ok(())
}
/// Removes the GcTodoEntry from the gc_todo tree if the
/// hash of the serialized value is the same here as in the tree.
/// This is usefull to remove a todo entry only under the condition
/// that it has not changed since the time it was read, i.e.
/// what we have to do is still the same
pub(crate) fn remove_if_equal(&self, gc_todo_tree: &sled::Tree) -> Result<(), Error> {
let _ = gc_todo_tree.compare_and_swap::<_, _, Vec<u8>>(
&self.key[..],
Some(self.value_hash),
None,
)?;
Ok(())
}
}