add command line option for the inline metadata size

Author: iwanbk

README.md

@@ -18,12 +18,26 @@ cd s3-cas
 cargo build --release --features binary
 ```
 
+## Inline metadata
+
+Objects smaller than or equal to a configurable threshold can be stored directly in their metadata records,
+improving performance for small objects.
+
+Configure this feature using the command-line option:
+```console
+--inline-metadata-size <size>    # omit to disable inlining
+```
+
+When the size is set:
+- If the size of object data + metadata smaller than or equal to the threshold, the object data is stored in the metadata,
+  otherwise use the standard block storage
+- Setting size to 0 or omitting the option disables inlining completely
+
+Currently, objects uploaded using the multipart method will never be inlined
+because they are assumed to be large objects.
+
 ## Known issues
 
-- The metadata database (sled) has unbounded memory growth related to the objects stored. This means
-  the server will eventually consume all memory on the host and crash. To fix this the metadata database
-  should either be replaced with a new version of sled (still in development) or a different one entirely
 - Only the basic API is implemented, and even then it is not entirely implemented (for instance copy
   between servers is not implemented).
-- The codebase is very much POC and not following a good abstraction structure
 - Single key only, no support to add multiple keys with different permissions.

src/main.rs

@@ -31,6 +31,9 @@ struct Args {
     #[structopt(long, default_value = "9100")]
     metric_port: u16,
 
+    #[structopt(long, help = "leave empty to disable it")]
+    inline_metadata_size: Option<usize>,
+
     #[structopt(long, requires("secret-key"), display_order = 1000)]
     access_key: Option<String>,
 
@@ -71,7 +74,7 @@ async fn run(args: Args) -> anyhow::Result<()> {
         args.meta_root.clone(),
         metrics.clone(),
         storage_engine,
-        None,
+        args.inline_metadata_size,
     );
     let s3fs = s3_cas::s3fs::S3FS::new(args.fs_root, args.meta_root, casfs, metrics.clone());
     let s3fs = s3_cas::metrics::MetricFs::new(s3fs, metrics.clone());

src/metastore/stores/fjall.rs

@@ -26,7 +26,7 @@ impl std::fmt::Debug for FjallStore {
     }
 }
 
-const DEFAULT_INLINED_METADATA_SIZE: usize = 1024;
+const DEFAULT_INLINED_METADATA_SIZE: usize = 1; // setting very low will practically disable it by default
 
 impl FjallStore {
     pub fn new(path: PathBuf, inlined_metadata_size: Option<usize>) -> Self {