filesetup: add fallocate=truncate option.

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 64c6a033982b41c01b94cfe8b5088a64b4f36e93..88dbb03fc23859ad39ba351b3fa06de81c96512b 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -1173,6 +1173,10 @@ I/O type
                        Pre-allocate via :manpage:`fallocate(2)` with
                        FALLOC_FL_KEEP_SIZE set.
 
+               **truncate**
+                       Extend file to final size via :manpage:`ftruncate(2)`
+                       instead of allocating.
+
                **0**
                        Backward-compatible alias for **none**.
 
@@ -1182,7 +1186,15 @@ I/O type
        May not be available on all supported platforms. **keep** is only available
        on Linux. If using ZFS on Solaris this cannot be set to **posix**
        because ZFS doesn't support pre-allocation. Default: **native** if any
-       pre-allocation methods are available, **none** if not.
+       pre-allocation methods except **truncate** are available, **none** if not.
+
+       Note that using **truncate** on Windows will interact surprisingly
+       with non-sequential write patterns. When writing to a file that has
+       been extended by setting the end-of-file information, Windows will
+       backfill the unwritten portion of the file up to that offset with
+       zeroes before issuing the new write. This means that a single small
+       write to the end of an extended file will stall until the entire
+       file has been filled with zeroes.
 
 .. option:: fadvise_hint=str