Refactor and comment table GC logic
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8e25a37f0e
commit
cc255d46cd
2 changed files with 130 additions and 23 deletions
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@ -12,6 +12,7 @@ use garage_util::error::*;
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use garage_rpc::system::System;
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use crate::crdt::Crdt;
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use crate::gc::GcTodoEntry;
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use crate::replication::*;
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use crate::schema::*;
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@ -176,7 +177,7 @@ where
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let pk_hash = Hash::try_from(&tree_key[..32]).unwrap();
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let nodes = self.replication.write_nodes(&pk_hash);
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if nodes.first() == Some(&self.system.id) {
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self.gc_todo.insert(&tree_key, new_bytes_hash.as_slice())?;
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GcTodoEntry::new(tree_key, new_bytes_hash).save(&self.gc_todo)?;
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}
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}
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}
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150
src/table/gc.rs
150
src/table/gc.rs
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@ -12,7 +12,7 @@ use futures_util::future::*;
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use tokio::sync::watch;
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use garage_util::data::*;
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use garage_util::error::Error;
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use garage_util::error::*;
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use garage_rpc::system::System;
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use garage_rpc::*;
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@ -24,7 +24,7 @@ use crate::schema::*;
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const TABLE_GC_BATCH_SIZE: usize = 1024;
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const TABLE_GC_RPC_TIMEOUT: Duration = Duration::from_secs(30);
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pub struct TableGc<F: TableSchema + 'static, R: TableReplication + 'static> {
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pub(crate) struct TableGc<F: TableSchema + 'static, R: TableReplication + 'static> {
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system: Arc<System>,
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data: Arc<TableData<F, R>>,
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@ -94,31 +94,45 @@ where
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let mut entries = vec![];
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let mut excluded = vec![];
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for item in self.data.gc_todo.iter() {
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let (k, vhash) = item?;
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// List entries in the GC todo list
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// These entries are put there when a tombstone is inserted in the table
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// This is detected and done in data.rs in update_entry
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for entry_kv in self.data.gc_todo.iter() {
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let (k, vhash) = entry_kv?;
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let mut todo_entry = GcTodoEntry::parse(&k, &vhash);
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let vhash = Hash::try_from(&vhash[..]).unwrap();
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let v_opt = self
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// Check if the tombstone is still the current value of the entry.
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// If not, we don't actually want to GC it, and we will remove it
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// from the gc_todo table later (below).
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todo_entry.value = self
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.data
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.store
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.get(&k[..])?
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.filter(|v| blake2sum(&v[..]) == vhash);
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.filter(|v| blake2sum(&v[..]) == vhash)
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.map(|v| v.to_vec());
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if let Some(v) = v_opt {
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entries.push((ByteBuf::from(k.to_vec()), vhash, ByteBuf::from(v.to_vec())));
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if todo_entry.value.is_some() {
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entries.push(todo_entry);
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if entries.len() >= TABLE_GC_BATCH_SIZE {
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break;
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}
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} else {
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excluded.push((k, vhash));
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excluded.push(todo_entry);
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}
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}
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for (k, vhash) in excluded {
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self.todo_remove_if_equal(&k[..], vhash)?;
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// Remove from gc_todo entries for tombstones where we have
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// detected that the current value has changed and
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// is no longer a tombstone.
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for entry in excluded {
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entry.remove_if_equal(&self.data.gc_todo)?;
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}
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// Remaining in `entries` is the list of entries we want to GC,
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// and for which they are still currently tombstones in the table.
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if entries.is_empty() {
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// Nothing to do in this iteration
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return Ok(false);
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@ -126,9 +140,17 @@ where
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debug!("({}) GC: doing {} items", self.data.name, entries.len());
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// Split entries to GC by the set of nodes on which they are stored.
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// Here we call them partitions but they are not exactly
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// the same as partitions as defined in the ring: those partitions
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// are defined by the first 8 bits of the hash, but two of these
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// partitions can be stored on the same set of nodes.
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// Here we detect when entries are stored on the same set of nodes:
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// even if they are not in the same 8-bit partition, we can still
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// handle them together.
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let mut partitions = HashMap::new();
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for (k, vhash, v) in entries {
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let pkh = Hash::try_from(&k[..32]).unwrap();
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for entry in entries {
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let pkh = Hash::try_from(&entry.key[..32]).unwrap();
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let mut nodes = self.data.replication.write_nodes(&pkh);
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nodes.retain(|x| *x != self.system.id);
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nodes.sort();
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@ -136,9 +158,12 @@ where
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if !partitions.contains_key(&nodes) {
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partitions.insert(nodes.clone(), vec![]);
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}
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partitions.get_mut(&nodes).unwrap().push((k, vhash, v));
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partitions.get_mut(&nodes).unwrap().push(entry);
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}
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// For each set of nodes that contains some items,
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// ensure they are aware of the tombstone status, and once they
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// are, instruct them to delete the entries.
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let resps = join_all(
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partitions
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.into_iter()
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@ -146,6 +171,8 @@ where
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)
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.await;
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// Collect errors and return a single error value even if several
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// errors occurred.
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let mut errs = vec![];
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for resp in resps {
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if let Err(e) = resp {
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@ -162,23 +189,40 @@ where
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.collect::<Vec<_>>()
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.join(", "),
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))
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.err_context("in try_send_and_delete:")
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}
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}
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async fn try_send_and_delete(
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&self,
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nodes: Vec<Uuid>,
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items: Vec<(ByteBuf, Hash, ByteBuf)>,
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items: Vec<GcTodoEntry>,
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) -> Result<(), Error> {
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let n_items = items.len();
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// Strategy: we first send all of the values to the remote nodes,
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// to ensure that they are aware of the tombstone state.
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// (if they have a newer state that overrides the tombstone, that's fine).
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// Second, once everyone is at least at the tombstone state,
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// we instruct everyone to delete the tombstone IF that is still their current state.
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// If they are now at a different state, it means that that state overrides the
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// tombstone in the CRDT lattice, and it will be propagated back to us at some point
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// (either just a regular update that hasn't reached us yet, or later when the
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// table is synced).
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// Here, we store in updates all of the tombstones to send for step 1,
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// and in deletes the list of keys and hashes of value for step 2.
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let mut updates = vec![];
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let mut deletes = vec![];
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for (k, vhash, v) in items {
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updates.push(v);
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deletes.push((k, vhash));
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for item in items {
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updates.push(ByteBuf::from(item.value.unwrap()));
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deletes.push((ByteBuf::from(item.key), item.value_hash));
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}
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// Step 1: ensure everyone is at least at tombstone in CRDT lattice
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// Here the quorum is nodes.len(): we cannot tolerate even a single failure,
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// otherwise old values before the tombstone might come back in the data.
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// GC'ing is not a critical function of the system, so it's not a big
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// deal if we can't do it right now.
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self.system
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.rpc
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.try_call_many(
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@ -189,13 +233,18 @@ where
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.with_quorum(nodes.len())
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.with_timeout(TABLE_GC_RPC_TIMEOUT),
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)
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.await?;
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.await
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.err_context("GC: send tombstones")?;
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info!(
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"({}) GC: {} items successfully pushed, will try to delete.",
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self.data.name, n_items
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);
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// Step 2: delete tombstones everywhere.
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// Here we also fail if even a single node returns a failure:
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// it means that the garbage collection wasn't completed and has
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// to be retried later.
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self.system
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.rpc
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.try_call_many(
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@ -206,11 +255,17 @@ where
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.with_quorum(nodes.len())
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.with_timeout(TABLE_GC_RPC_TIMEOUT),
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)
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.await?;
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.await
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.err_context("GC: remote delete tombstones")?;
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// GC has been successfull for all of these entries.
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// We now remove them all from our local table and from the GC todo list.
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for (k, vhash) in deletes {
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self.data.delete_if_equal_hash(&k[..], vhash)?;
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self.todo_remove_if_equal(&k[..], vhash)?;
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self.data
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.delete_if_equal_hash(&k[..], vhash)
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.err_context("GC: local delete tombstones")?;
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self.todo_remove_if_equal(&k[..], vhash)
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.err_context("GC: remove from todo list after successfull GC")?;
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}
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Ok(())
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@ -248,3 +303,54 @@ where
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}
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}
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}
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/// An entry stored in the gc_todo Sled tree associated with the table
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/// Contains helper function for parsing, saving, and removing
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/// such entry in Sled
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pub(crate) struct GcTodoEntry {
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key: Vec<u8>,
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value_hash: Hash,
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value: Option<Vec<u8>>,
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}
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impl GcTodoEntry {
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/// Creates a new GcTodoEntry (not saved in Sled) from its components:
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/// the key of an entry in the table, and the hash of the associated
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/// serialized value
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pub(crate) fn new(key: Vec<u8>, value_hash: Hash) -> Self {
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Self {
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key,
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value_hash,
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value: None,
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}
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}
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/// Parses a GcTodoEntry from a (k, v) pair stored in the gc_todo tree
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pub(crate) fn parse(sled_k: &[u8], sled_v: &[u8]) -> Self {
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Self {
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key: sled_k.to_vec(),
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value_hash: Hash::try_from(sled_v).unwrap(),
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value: None,
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}
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}
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/// Saves the GcTodoEntry in the gc_todo tree
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pub(crate) fn save(&self, gc_todo_tree: &sled::Tree) -> Result<(), Error> {
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gc_todo_tree.insert(&self.key[..], self.value_hash.as_slice())?;
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Ok(())
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}
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/// Removes the GcTodoEntry from the gc_todo tree if the
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/// hash of the serialized value is the same here as in the tree.
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/// This is usefull to remove a todo entry only under the condition
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/// that it has not changed since the time it was read, i.e.
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/// what we have to do is still the same
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pub(crate) fn remove_if_equal(&self, gc_todo_tree: &sled::Tree) -> Result<(), Error> {
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let _ = gc_todo_tree.compare_and_swap::<_, _, Vec<u8>>(
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&self.key[..],
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Some(self.value_hash),
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None,
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)?;
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Ok(())
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}
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}
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