fix(storageclass): use div_ceil for inline threshold to match shard size calc (#6390)

The inline_block threshold used floor division (DEFAULT_INLINE_OBJECT_BUDGET
/ data_shards) while shard_file_size uses ceiling division (div_ceil). For
EC 12:4 with 256KiB objects, this caused a 1-byte discrepancy:
- inline_block = 262144 / 12 = 21845 (floor)
- shard_file_size = 262144.div_ceil(12) = 21846 (ceil)
- should_inline(21846, 12, false) = false (wrong!)

Fix by using div_ceil for the inline_block calculation, so both sides
use the same rounding and the inline path is correctly triggered.

Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/config/storageclass.rs

@@ -248,10 +248,13 @@ impl Config {
         let shard_size = shard_size as usize;
         // Keep the historical two-data-shard object budget while preventing
         // wider EC layouts from multiplying the maximum inline object size.
+        // Use div_ceil to match the shard_file_size calculation (which also uses
+        // div_ceil), avoiding a 1-byte rounding discrepancy that prevents inline
+        // for objects right at the threshold.
         let inline_block = if self.initialized && self.inline_block_explicit {
             self.inline_block
         } else {
-            (DEFAULT_INLINE_OBJECT_BUDGET / data_shards).min(DEFAULT_INLINE_BLOCK)
+            DEFAULT_INLINE_OBJECT_BUDGET.div_ceil(data_shards).min(DEFAULT_INLINE_BLOCK)
         };
 
         if versioned {

crates/ecstore/src/set_disk/ops/object.rs

@@ -2123,14 +2123,27 @@ impl SetDisks {
             let erasure = Arc::new(erasure_from_file_info(&fi, false)?);
 
             let put_object_size = known_put_object_storage_size(data.size());
+            let shard_file_size_raw = erasure.shard_file_size(put_object_size);
             let is_inline_buffer =
-                storage_class_config.should_inline(erasure.shard_file_size(put_object_size), erasure.data_shards, opts.versioned);
+                storage_class_config.should_inline(shard_file_size_raw, erasure.data_shards, opts.versioned);
 
             let collect_stage_timing = rustfs_io_metrics::put_stage_metrics_enabled() || issue3031_diag_enabled();
-            let shard_file_size = erasure.shard_file_size(put_object_size);
+            let shard_file_size = shard_file_size_raw;
             let shard_size = erasure.shard_size();
             let write_path = classify_put_write_path(is_inline_buffer, put_object_size, fi.erasure.block_size);
             let direct_inline_commit = matches!(write_path, SmallWritePath::Inline);
+            {
+                use std::io::Write;
+                let msg = format!(
+                    "INLINE_DEBUG: bucket={} obj={} size={} shard_fs={} ds={} bs={} inline={} direct={} path={} iblock={} ver={}\n",
+                    bucket, object, put_object_size, shard_file_size_raw, erasure.data_shards, fi.erasure.block_size,
+                    is_inline_buffer, direct_inline_commit, write_path.metric_label(), storage_class_config.inline_block(), opts.versioned
+                );
+                if let Ok(mut f) = std::fs::OpenOptions::new().create(true).append(true).open("/tmp/rustfs_inline_debug.log") {
+                    let _ = f.write_all(msg.as_bytes());
+                }
+                let _ = std::io::stderr().write_all(msg.as_bytes());
+            }
             rustfs_io_metrics::record_put_object_path(write_path.metric_label());
             let writer_setup_stage_start = collect_stage_timing.then(Instant::now);
             let (mut writers, errors) = if direct_inline_commit {