pageserver: add no_sync for use in regression tests (1/2) (#9677)

## Problem

In test environments, the `syncfs` that the pageserver does on startup
can take a long time, as other tests running concurrently might have
many gigabytes of dirty pages.

## Summary of changes

- Add a `no_sync` option to the pageserver's config.
- Skip syncfs on startup if this is set
- A subsequent PR (https://github.com/neondatabase/neon/pull/9678) will
enable this by default in tests. We need to wait until after the next
release to avoid breaking compat tests, which would fail if we set
no_sync & use an old pageserver binary.

Q: Why is this a different mechanism than safekeeper, which as a
--no-sync CLI?
A: Because the way we manage pageservers in neon_local depends on the
pageserver.toml containing the full configuration, whereas safekeepers
have a config file which is neon-local-specific and can drive a CLI
flag.

Q: Why is the option no_sync rather than sync?
A: For boolean configs with a dangerous value, it's preferable to make
"false" the safe option, so that any downstream future config tooling
that might have a "booleans are false by default" behavior (e.g. golang
structs) is safe by default.

Q: Why only skip the syncfs, and not all fsyncs?
A: Skipping all fsyncs would require more code changes, and the most
acute problem isn't fsyncs themselves (these just slow down a running
test), it's the syncfs (which makes a pageserver startup slow as a
result of _other_ tests)

control_plane/src/bin/neon_local.rs

@@ -944,6 +944,9 @@ fn handle_init(args: &InitCmdArgs) -> anyhow::Result<LocalEnv> {
                         pg_auth_type: AuthType::Trust,
                         http_auth_type: AuthType::Trust,
                         other: Default::default(),
+                        // Typical developer machines use disks with slow fsync, and we don't care
+                        // about data integrity: disable disk syncs.
+                        no_sync: true,
                     }
                 })
                 .collect(),

control_plane/src/local_env.rs

@@ -225,6 +225,7 @@ pub struct PageServerConf {
     pub listen_http_addr: String,
     pub pg_auth_type: AuthType,
     pub http_auth_type: AuthType,
+    pub no_sync: bool,
 }
 
 impl Default for PageServerConf {
@@ -235,6 +236,7 @@ impl Default for PageServerConf {
             listen_http_addr: String::new(),
             pg_auth_type: AuthType::Trust,
             http_auth_type: AuthType::Trust,
+            no_sync: false,
         }
     }
 }
@@ -249,6 +251,8 @@ pub struct NeonLocalInitPageserverConf {
     pub listen_http_addr: String,
     pub pg_auth_type: AuthType,
     pub http_auth_type: AuthType,
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub no_sync: bool,
     #[serde(flatten)]
     pub other: HashMap<String, toml::Value>,
 }
@@ -261,6 +265,7 @@ impl From<&NeonLocalInitPageserverConf> for PageServerConf {
             listen_http_addr,
             pg_auth_type,
             http_auth_type,
+            no_sync,
             other: _,
         } = conf;
         Self {
@@ -269,6 +274,7 @@ impl From<&NeonLocalInitPageserverConf> for PageServerConf {
             listen_http_addr: listen_http_addr.clone(),
             pg_auth_type: *pg_auth_type,
             http_auth_type: *http_auth_type,
+            no_sync: *no_sync,
         }
     }
 }
@@ -569,6 +575,8 @@ impl LocalEnv {
                     listen_http_addr: String,
                     pg_auth_type: AuthType,
                     http_auth_type: AuthType,
+                    #[serde(default)]
+                    no_sync: bool,
                 }
                 let config_toml_path = dentry.path().join("pageserver.toml");
                 let config_toml: PageserverConfigTomlSubset = toml_edit::de::from_str(
@@ -591,6 +599,7 @@ impl LocalEnv {
                     listen_http_addr,
                     pg_auth_type,
                     http_auth_type,
+                    no_sync,
                 } = config_toml;
                 let IdentityTomlSubset {
                     id: identity_toml_id,
@@ -607,6 +616,7 @@ impl LocalEnv {
                     listen_http_addr,
                     pg_auth_type,
                     http_auth_type,
+                    no_sync,
                 };
                 pageservers.push(conf);
             }

control_plane/src/pageserver.rs

@@ -273,6 +273,7 @@ impl PageServerNode {
             )
         })?;
         let args = vec!["-D", datadir_path_str];
+
         background_process::start_process(
             "pageserver",
             &datadir,

libs/pageserver_api/src/config.rs

@@ -106,6 +106,8 @@ pub struct ConfigToml {
     pub ephemeral_bytes_per_memory_kb: usize,
     pub l0_flush: Option<crate::models::L0FlushConfig>,
     pub virtual_file_io_mode: Option<crate::models::virtual_file::IoMode>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub no_sync: Option<bool>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
@@ -389,6 +391,7 @@ impl Default for ConfigToml {
             l0_flush: None,
             virtual_file_io_mode: None,
             tenant_config: TenantConfigToml::default(),
+            no_sync: None,
         }
     }
 }

pageserver/src/bin/pageserver.rs

@@ -154,13 +154,17 @@ fn main() -> anyhow::Result<()> {
             },
         };
 
-        let started = Instant::now();
-        syncfs(dirfd)?;
-        let elapsed = started.elapsed();
-        info!(
-            elapsed_ms = elapsed.as_millis(),
-            "made tenant directory contents durable"
-        );
+        if conf.no_sync {
+            info!("Skipping syncfs on startup");
+        } else {
+            let started = Instant::now();
+            syncfs(dirfd)?;
+            let elapsed = started.elapsed();
+            info!(
+                elapsed_ms = elapsed.as_millis(),
+                "made tenant directory contents durable"
+            );
+        }
     }
 
     // Initialize up failpoints support

pageserver/src/config.rs

@@ -178,6 +178,9 @@ pub struct PageServerConf {
 
     /// Direct IO settings
     pub virtual_file_io_mode: virtual_file::IoMode,
+
+    /// Optionally disable disk syncs (unsafe!)
+    pub no_sync: bool,
 }
 
 /// Token for authentication to safekeepers
@@ -332,6 +335,7 @@ impl PageServerConf {
             concurrent_tenant_size_logical_size_queries,
             virtual_file_io_engine,
             tenant_config,
+            no_sync,
         } = config_toml;
 
         let mut conf = PageServerConf {
@@ -409,6 +413,7 @@ impl PageServerConf {
                 .map(crate::l0_flush::L0FlushConfig::from)
                 .unwrap_or_default(),
             virtual_file_io_mode: virtual_file_io_mode.unwrap_or(virtual_file::IoMode::preferred()),
+            no_sync: no_sync.unwrap_or(false),
         };
 
         // ------------------------------------------------------------