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-rw-r--r--Documentation/technical/index-format.txt19
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diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index ade0b0c44..db3572626 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -295,3 +295,22 @@ The remaining data of each directory block is grouped by type:
in the previous ewah bitmap.
- One NUL.
+
+== File System Monitor cache
+
+ The file system monitor cache tracks files for which the core.fsmonitor
+ hook has told us about changes. The signature for this extension is
+ { 'F', 'S', 'M', 'N' }.
+
+ The extension starts with
+
+ - 32-bit version number: the current supported version is 1.
+
+ - 64-bit time: the extension data reflects all changes through the given
+ time which is stored as the nanoseconds elapsed since midnight,
+ January 1, 1970.
+
+ - 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.
+
+ - An ewah bitmap, the n-th bit indicates whether the n-th index entry
+ is not CE_FSMONITOR_VALID.