diff --git a/block_util/src/lib.rs b/block_util/src/lib.rs
index c4d9364fd..1ffbe5397 100644
--- a/block_util/src/lib.rs
+++ b/block_util/src/lib.rs
@@ -249,6 +249,7 @@ impl Request {
                 return Err(Error::DescriptorChainTooShort);
             }
         } else {
+            req.data_descriptors.reserve_exact(1);
             while desc.has_next() {
                 if desc.is_write_only() && req.request_type == RequestType::Out {
                     return Err(Error::UnexpectedWriteOnlyDescriptor);
@@ -353,7 +354,7 @@ impl Request {
         let request_type = self.request_type;
         let offset = (sector << SECTOR_SHIFT) as libc::off_t;
 
-        let mut iovecs = Vec::new();
+        let mut iovecs = Vec::with_capacity(self.data_descriptors.len());
         for (data_addr, data_len) in &self.data_descriptors {
             if *data_len == 0 {
                 continue;
diff --git a/block_util/src/raw_async.rs b/block_util/src/raw_async.rs
index f1ce670ad..9c2e32260 100644
--- a/block_util/src/raw_async.rs
+++ b/block_util/src/raw_async.rs
@@ -156,9 +156,8 @@ impl AsyncIo for RawFileAsync {
     }
 
     fn complete(&mut self) -> Vec<(u64, i32)> {
-        let mut completion_list = Vec::new();
-
         let cq = self.io_uring.completion();
+        let mut completion_list = Vec::with_capacity(cq.len());
         for cq_entry in cq {
             completion_list.push((cq_entry.user_data(), cq_entry.result()));
         }