From f89ba54198b27d1dfc233f126ee8bcfd09c65d34 Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Mon, 10 Nov 2025 13:31:24 +0100
Subject: [PATCH 1/4] concurrency limit 64 -> 10

---
 src/zarr/core/config.py | 2 +-
 tests/test_config.py    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/zarr/core/config.py b/src/zarr/core/config.py
index 5acf242ef7..908922a5f2 100644
--- a/src/zarr/core/config.py
+++ b/src/zarr/core/config.py
@@ -97,7 +97,7 @@ def enable_gpu(self) -> ConfigSet:
                 "order": "C",
                 "write_empty_chunks": False,
             },
-            "async": {"concurrency": 64, "timeout": None},
+            "async": {"concurrency": 10, "timeout": None},
             "threading": {"max_workers": None},
             "json_indent": 2,
             "codec_pipeline": {
diff --git a/tests/test_config.py b/tests/test_config.py
index 150aca7c96..55d2349e49 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -54,7 +54,7 @@ def test_config_defaults_set() -> None:
                     "order": "C",
                     "write_empty_chunks": False,
                 },
-                "async": {"concurrency": 64, "timeout": None},
+                "async": {"concurrency": 10, "timeout": None},
                 "threading": {"max_workers": None},
                 "json_indent": 2,
                 "codec_pipeline": {

From 9ac0e6bae960e99b80633c367c2cbb5afc3b8b4c Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Mon, 10 Nov 2025 13:43:05 +0100
Subject: [PATCH 2/4] update reference to concurrency limit in docs

---
 docs/user-guide/performance.md | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/docs/user-guide/performance.md b/docs/user-guide/performance.md
index 88d8e69936..8b338547fb 100644
--- a/docs/user-guide/performance.md
+++ b/docs/user-guide/performance.md
@@ -175,13 +175,18 @@ Coming soon.
 
 ## Parallel computing and synchronization
 
-Zarr is designed to support parallel computing and enables concurrent reads and writes to arrays. This section covers how to optimize Zarr's concurrency settings for different parallel computing scenarios.
+Zarr is designed to support parallel computing and enables concurrent reads and writes to arrays.
+This section covers how to optimize Zarr's concurrency settings for different parallel computing
+scenarios.
 
 ### Concurrent I/O operations
 
-Zarr uses asynchronous I/O internally to enable concurrent reads and writes across multiple chunks. The level of concurrency is controlled by the `async.concurrency` configuration setting, which determines the maximum number of concurrent I/O operations.
+Zarr uses asynchronous I/O internally to enable concurrent reads and writes across multiple chunks.
+The level of concurrency is controlled by the `async.concurrency` configuration setting, which
+determines the maximum number of concurrent I/O operations.
 
-The default value is 64, which provides good performance for most workloads. You can adjust this value based on your specific needs:
+The default value is 10, which is a conservative value. You may get improved performance by tuning
+the concurrency limit. You can adjust this value based on your specific needs:
 
 ```python
 import zarr

From a502daeb3b959b5073af8b3667075aea824af880 Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Thu, 13 Nov 2025 16:10:12 +0100
Subject: [PATCH 3/4] update tests

---
 tests/test_config.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/test_config.py b/tests/test_config.py
index 55d2349e49..ec72967b98 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -100,7 +100,7 @@ def test_config_defaults_set() -> None:
         ]
     )
     assert config.get("array.order") == "C"
-    assert config.get("async.concurrency") == 64
+    assert config.get("async.concurrency") == 10
     assert config.get("async.timeout") is None
     assert config.get("codec_pipeline.batch_size") == 1
     assert config.get("json_indent") == 2
@@ -108,7 +108,7 @@ def test_config_defaults_set() -> None:
 
 @pytest.mark.parametrize(
     ("key", "old_val", "new_val"),
-    [("array.order", "C", "F"), ("async.concurrency", 64, 128), ("json_indent", 2, 0)],
+    [("array.order", "C", "F"), ("async.concurrency", 10, 128), ("json_indent", 2, 0)],
 )
 def test_config_defaults_can_be_overridden(key: str, old_val: Any, new_val: Any) -> None:
     assert config.get(key) == old_val

From 51a8ead8f78226d8149666b5b85107f337cdd24d Mon Sep 17 00:00:00 2001
From: Davis Vann Bennett <davis.v.bennett@gmail.com>
Date: Thu, 13 Nov 2025 16:15:52 +0100
Subject: [PATCH 4/4] remove release note

---
 changes/3526.feature.md | 1 -
 1 file changed, 1 deletion(-)
 delete mode 100644 changes/3526.feature.md

diff --git a/changes/3526.feature.md b/changes/3526.feature.md
deleted file mode 100644
index 7746611855..0000000000
--- a/changes/3526.feature.md
+++ /dev/null
@@ -1 +0,0 @@
-Increased the default value of `async.concurrency` from 10 to 64 to improve parallelism and throughput for concurrent I/O operations. This change enables better performance out-of-the-box for most workloads. Users with specific resource constraints or when using many Dask threads may want to lower this value via the `ZARR_ASYNC_CONCURRENCY` environment variable or by setting `zarr.config.set({'async.concurrency': N})`.