[pbs-devel] [PATCH proxmox-backup v5 1/4] partial fix #6049: config: enable config version cache for datastore

[Samuel Rufinatscha]

Repeated /status requests caused lookup_datastore() to re-read and
parse datastore.cfg on every call. The issue was mentioned in report
#6049 [1]. cargo-flamegraph [2] confirmed that the hot path is
dominated by pbs_config::datastore::config() (config parsing).

To solve the issue, this patch prepares the config version cache,
so that datastore config caching can be built on top of it.
This patch specifically:
(1) implements increment function in order to invalidate generations
(2) removes obsolete comments

Links

[1] Bugzilla: https://bugzilla.proxmox.com/show_bug.cgi?id=6049
[2] cargo-flamegraph: https://github.com/flamegraph-rs/flamegraph

Fixes: #6049
Signed-off-by: Samuel Rufinatscha <s.rufinatscha at proxmox.com>
---
Changes:

>From v1 → v2 (original introduction), thanks @Fabian
- Split the ConfigVersionCache changes out of the large datastore patch
into their own config-only patch
    * removed the obsolete // FIXME comment on datastore_generation
    * added ConfigVersionCache::datastore_generation() as getter

>From v2 → v3
No changes

>From v3 → v4
No changes

>From v4 → v5
- Rebased only, no changes

 pbs-config/src/config_version_cache.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/pbs-config/src/config_version_cache.rs b/pbs-config/src/config_version_cache.rs
index e8fb994f..b875f7e0 100644
--- a/pbs-config/src/config_version_cache.rs
+++ b/pbs-config/src/config_version_cache.rs
@@ -26,7 +26,6 @@ struct ConfigVersionCacheDataInner {
     // Traffic control (traffic-control.cfg) generation/version.
     traffic_control_generation: AtomicUsize,
     // datastore (datastore.cfg) generation/version
-    // FIXME: remove with PBS 3.0
     datastore_generation: AtomicUsize,
     // Add further atomics here
 }
@@ -145,8 +144,15 @@ impl ConfigVersionCache {
             .fetch_add(1, Ordering::AcqRel);
     }
 
+    /// Returns the datastore generation number.
+    pub fn datastore_generation(&self) -> usize {
+        self.shmem
+            .data()
+            .datastore_generation
+            .load(Ordering::Acquire)
+    }
+
     /// Increase the datastore generation number.
-    // FIXME: remove with PBS 3.0 or make actually useful again in datastore lookup
     pub fn increase_datastore_generation(&self) -> usize {
         self.shmem
             .data()
-- 
2.47.3