[pve-devel] [RFC cluster] status: clear stale kv stores upon sync

[pve-devel] [RFC cluster] status: clear stale kv stores upon sync

[Fiona Ebner]

This avoids that stale kv entries stay around when a node leaves the
CPG. Now, each kv entry will be something a node sent after (or upon
joining) the CPG.

This avoids scenarios where a user of pmxcfs (like pvestatd) on node A
might not yet have had the time to broadcast up-to-date kv entries,
but a user of pmxcfs on node B sees node A as online and uses the
outdated value (which couldn't be detected as outdated).

In particular, this should be helpful for more static information
broadcast by pvestatd which (mostly) doesn't change while a node is
running.

Could also be done as part of the dfsm_confchg() callback, but then
(at least) the additional guarantee of "confchg always happens before
the kvstore messages during sync arrive" is needed (pointed out by
Fabian). It should hold in practice, but dfsm_process_state_update()
doesn't need such guarantees and is also a fitting place to do it.

The cfs_status_clear_other_kvstores() could take a hash table with the
IDs to avoid the quadratic loop, but since the number of nodes in
setups in practice is never too big, it's likely faster than the
additional work of constructing that hash at the call side.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Many thanks to Fabian for helpful discussions!

Another alternative to avoid the quadratic loop would be to copy the
hash table, iterating over skip_node_ids once to remove them from the
hash table (but need to make sure the original value isn't freed!) and
then iterate over the hash table once. Not sure how much nicer that
would be though.

 data/src/status.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/data/src/status.c b/data/src/status.c
index 9bceaeb..3da57cf 100644
--- a/data/src/status.c
+++ b/data/src/status.c
@@ -539,6 +539,46 @@ void cfs_status_set_clinfo(
 	g_mutex_unlock (&mutex);
 }
 
+void
+cfs_status_clear_other_kvstores(
+	uint32_t *skip_node_ids,
+	int count)
+{
+	g_mutex_lock (&mutex);
+
+	if (!cfs_status.clinfo || !cfs_status.clinfo->nodes_byid) {
+		goto unlock; /* ignore */
+	}
+
+	GHashTable *ht = cfs_status.clinfo->nodes_byid;
+	GHashTableIter iter;
+	gpointer key, value;
+
+	g_hash_table_iter_init (&iter, ht);
+
+	// Quadratic in the number of nodes, but it's safe to assume that the number is small enough
+	while (g_hash_table_iter_next (&iter, &key, &value)) {
+		uint32_t nodeid = *(uint32_t *)key;
+		cfs_clnode_t *clnode = (cfs_clnode_t *)value;
+		gboolean skip = FALSE;
+
+		for (int i = 0; i < count; i++) {
+			if (nodeid == skip_node_ids[i]) {
+				skip = TRUE;
+				break;
+			}
+		}
+
+		if (!skip && clnode->kvhash) {
+			cfs_debug("clearing kv store of node %d", nodeid);
+			g_hash_table_remove_all(clnode->kvhash);
+		}
+	}
+
+unlock:
+	g_mutex_unlock (&mutex);
+}
+
 static void
 dump_kvstore_versions(
 	GString *str,
@@ -1769,12 +1809,15 @@ dfsm_process_state_update(
 	g_return_val_if_fail(syncinfo != NULL, -1);
 
 	clog_base_t *clog[syncinfo->node_count];
+	uint32_t sync_node_ids[syncinfo->node_count];
 
 	int local_index = -1;
 	for (int i = 0; i < syncinfo->node_count; i++) {
 		dfsm_node_info_t *ni = &syncinfo->nodes[i];
 		ni->synced = 1;
 
+		sync_node_ids[i] = ni->nodeid;
+
 		if (syncinfo->local == ni)
 			local_index = i;
 
@@ -1791,6 +1834,10 @@ dfsm_process_state_update(
 		cfs_critical("unable to merge log files");
 	}
 
+	// Clear our copy of the kvstore of every node that is not part of the current sync. When
+	// such a node joins again, it will sync its current kvstore with cfs_kvstore_sync().
+	cfs_status_clear_other_kvstores(sync_node_ids, syncinfo->node_count);
+
 	cfs_kvstore_sync();
 
 	return 1;
-- 
2.30.2

Re: [pve-devel] [RFC cluster] status: clear stale kv stores upon sync

[Aaron Lauterer]

In the context of another patch to improve the broadcasting of 
version-info across a cluster, Fiona mentioned [0] that this RFC would 
also be of interested to invalidate KV data of nodes that left the 
cluster, to avoid stale infos.

Had to adapt the paths in the patch file, but then it still applied to 
current master.
/data/src/status.c
/src/pmxcfs/status.c

Did a quick test and so far it seems to do what it promises. Once I stop 
the corosync service of a node, the version-info of it disappears right 
away.

[0] 
https://lore.proxmox.com/pve-devel/17363828-136f-4f85-ad41-3a34b1a56689@proxmox.com/

Consider this

Tested-By: Aaron Lauterer <a.lauterer@proxmox.com>


On  2022-10-06  14:54, Fiona Ebner wrote:
> This avoids that stale kv entries stay around when a node leaves the
> CPG. Now, each kv entry will be something a node sent after (or upon
> joining) the CPG.
> 
> This avoids scenarios where a user of pmxcfs (like pvestatd) on node A
> might not yet have had the time to broadcast up-to-date kv entries,
> but a user of pmxcfs on node B sees node A as online and uses the
> outdated value (which couldn't be detected as outdated).
> 
> In particular, this should be helpful for more static information
> broadcast by pvestatd which (mostly) doesn't change while a node is
> running.
> 
> Could also be done as part of the dfsm_confchg() callback, but then
> (at least) the additional guarantee of "confchg always happens before
> the kvstore messages during sync arrive" is needed (pointed out by
> Fabian). It should hold in practice, but dfsm_process_state_update()
> doesn't need such guarantees and is also a fitting place to do it.
> 
> The cfs_status_clear_other_kvstores() could take a hash table with the
> IDs to avoid the quadratic loop, but since the number of nodes in
> setups in practice is never too big, it's likely faster than the
> additional work of constructing that hash at the call side.
> 
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
> 
> Many thanks to Fabian for helpful discussions!
> 
> Another alternative to avoid the quadratic loop would be to copy the
> hash table, iterating over skip_node_ids once to remove them from the
> hash table (but need to make sure the original value isn't freed!) and
> then iterate over the hash table once. Not sure how much nicer that
> would be though.
> 
>   data/src/status.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 47 insertions(+)
> 
> diff --git a/data/src/status.c b/data/src/status.c
> index 9bceaeb..3da57cf 100644
> --- a/data/src/status.c
> +++ b/data/src/status.c
> @@ -539,6 +539,46 @@ void cfs_status_set_clinfo(
>   	g_mutex_unlock (&mutex);
>   }
>   
> +void
> +cfs_status_clear_other_kvstores(
> +	uint32_t *skip_node_ids,
> +	int count)
> +{
> +	g_mutex_lock (&mutex);
> +
> +	if (!cfs_status.clinfo || !cfs_status.clinfo->nodes_byid) {
> +		goto unlock; /* ignore */
> +	}
> +
> +	GHashTable *ht = cfs_status.clinfo->nodes_byid;
> +	GHashTableIter iter;
> +	gpointer key, value;
> +
> +	g_hash_table_iter_init (&iter, ht);
> +
> +	// Quadratic in the number of nodes, but it's safe to assume that the number is small enough
> +	while (g_hash_table_iter_next (&iter, &key, &value)) {
> +		uint32_t nodeid = *(uint32_t *)key;
> +		cfs_clnode_t *clnode = (cfs_clnode_t *)value;
> +		gboolean skip = FALSE;
> +
> +		for (int i = 0; i < count; i++) {
> +			if (nodeid == skip_node_ids[i]) {
> +				skip = TRUE;
> +				break;
> +			}
> +		}
> +
> +		if (!skip && clnode->kvhash) {
> +			cfs_debug("clearing kv store of node %d", nodeid);
> +			g_hash_table_remove_all(clnode->kvhash);
> +		}
> +	}
> +
> +unlock:
> +	g_mutex_unlock (&mutex);
> +}
> +
>   static void
>   dump_kvstore_versions(
>   	GString *str,
> @@ -1769,12 +1809,15 @@ dfsm_process_state_update(
>   	g_return_val_if_fail(syncinfo != NULL, -1);
>   
>   	clog_base_t *clog[syncinfo->node_count];
> +	uint32_t sync_node_ids[syncinfo->node_count];
>   
>   	int local_index = -1;
>   	for (int i = 0; i < syncinfo->node_count; i++) {
>   		dfsm_node_info_t *ni = &syncinfo->nodes[i];
>   		ni->synced = 1;
>   
> +		sync_node_ids[i] = ni->nodeid;
> +
>   		if (syncinfo->local == ni)
>   			local_index = i;
>   
> @@ -1791,6 +1834,10 @@ dfsm_process_state_update(
>   		cfs_critical("unable to merge log files");
>   	}
>   
> +	// Clear our copy of the kvstore of every node that is not part of the current sync. When
> +	// such a node joins again, it will sync its current kvstore with cfs_kvstore_sync().
> +	cfs_status_clear_other_kvstores(sync_node_ids, syncinfo->node_count);
> +
>   	cfs_kvstore_sync();
>   
>   	return 1;



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