[Bug 63232] New: CacheDirLength is not a good default

[bu...@apache.org]

https://bz.apache.org/bugzilla/show_bug.cgi?id=63232

            Bug ID: 63232
           Summary: CacheDirLength is not a good default
           Product: Apache httpd-2
           Version: 2.4-HEAD
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: mod_cache_disk / mod_disk_cache
          Assignee: bugs@httpd.apache.org
          Reporter: hchuang95@gmail.com
  Target Milestone: ---

The default CacheDirLength is 2.

However, the document(https://httpd.apache.org/docs/2.4/caching.html#examples)
says: "Unless you have a good reason not to, using a setting of "1" for
CacheDirLength is recommended."

So maybe the default CacheDirLength should be changed to 1.

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[Bug 63232] CacheDirLength is not a good default

[bu...@apache.org]

https://bz.apache.org/bugzilla/show_bug.cgi?id=63232

--- Comment #1 from Rich Bowen <rb...@apache.org> ---
The documentation for CacheDirLength correctly states the default
is 2, which matches the code (DEFAULT_DIRLENGTH in
mod_cache_disk.h, line 66).

However, the caching guide (caching.html) recommends a value of 1,
stating: "Unless you have a good reason not to, using a setting of
1 for CacheDirLength is recommended."

When our own documentation recommends a value different from the
default, that's a signal that the default should change. Defaults
should reflect best practice so that users get good behavior out of
the box without needing to discover and follow a recommendation
buried in a guide page.

Proposing we change DEFAULT_DIRLENGTH from 2 to 1 in
modules/cache/mod_cache_disk.h to match the documented
recommendation.

Patch:

--- a/modules/cache/mod_cache_disk.h
+++ b/modules/cache/mod_cache_disk.h
@@ -65,2 +65,2 @@
 #define DEFAULT_DIRLEVELS 2
-#define DEFAULT_DIRLENGTH 2
+#define DEFAULT_DIRLENGTH 1

Update directive docs default in
docs/manual/mod/mod_cache_disk.xml:

-    <default>CacheDirLength 2</default>
+    <default>CacheDirLength 1</default>

And update the recommendation in docs/manual/caching.xml
(around line 497) since the advice now matches the default:

-      <p>The overall aim of this technique is to reduce the number of
-      subdirectories or files that may be in a particular directory,
-      as most file-systems slow down as this number increases. With
-      setting of "1" for
-      <directive module="mod_cache_disk">CacheDirLength</directive>
-      there can at most be 64 subdirectories at any particular level.
-      With a setting of 2 there can be 64 * 64 subdirectories, and so on.
-      Unless you have a good reason not to, using a setting of "1"
-      for <directive module="mod_cache_disk">CacheDirLength</directive>
-      is recommended.</p>
+      <p>The overall aim of this technique is to reduce the number of
+      subdirectories or files that may be in a particular directory,
+      as most file-systems slow down as this number increases. With
+      the default setting of "1" for
+      <directive module="mod_cache_disk">CacheDirLength</directive>
+      there can at most be 64 subdirectories at any particular level.
+      With a setting of 2 there can be 64 * 64 subdirectories, and so on.
+      The default of "1" is appropriate for most configurations.</p>

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