os/bluestore/write-v2: Tune down usage of 'unused'

We should not use 'unused' when allocation unit = disk block.
Blobs with 'unused' cannot be split which makes shard resharding
more problematic and spanning blobs more pronounced.

Signed-off-by: Adam Kupczyk <[email protected]>

Author: aclamk

src/os/bluestore/Writer.cc

@@ -499,7 +499,9 @@ BlueStore::BlobRef BlueStore::Writer::_blob_create_with_data(
   _get_disk_space(blob_length - alloc_offset, blob_allocs);
   bblob.allocated(alloc_offset, blob_length - alloc_offset, blob_allocs);
   //^sets also logical_length = blob_length
-  bblob.add_unused_all();
+  if (min_alloc_size != block_size) {
+    bblob.add_unused_all();
+  }
   dout(25) << __func__ << " @0x" << std::hex << in_blob_offset
     << "~" << disk_data.length()
     << " alloc_offset=" << alloc_offset