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Subject[PATCH 52/52] ext4: Documention update for new ordered mode and delayed allocation
From: Mingming Cao <>

Adding some documentations for delayed allocation and new ordered mode.

Signed-off-by: Mingming Cao <>
Signed-off-by: "Theodore Ts'o" <>
Documentation/filesystems/ext4.txt | 21 ++++++++++++++-------
1 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 77ed8fe..e2afb6e 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -62,7 +62,7 @@ Mailing list:
* extent format reduces metadata overhead (RAM, IO for access, transactions)
* extent format more robust in face of on-disk corruption due to magics,
* internal redunancy in tree
-* improved file allocation (multi-block alloc, delayed alloc)
+* improved file allocation (multi-block alloc)
* fix 32000 subdirectory limit
* nsec timestamps for mtime, atime, ctime, create time
* inode version field on disk (NFSv4, Lustre)
@@ -73,6 +73,10 @@ Mailing list:
flex_bg feature
* large file support
* Inode allocation using large virtual block groups via flex_bg
+* delayed allocation
+* large block (up to pagesize) support
+* efficent new ordered mode in JBD2 and ext4(avoid using buffer head to force
+ the ordering)

2.2 Candidate features for future inclusion

@@ -235,7 +239,9 @@ stripe=n Number of filesystem blocks that mballoc will try
to use for allocation size and alignment. For RAID5/6
systems this should be the number of data
disks * RAID chunk size in file system blocks.
+delalloc (*) Deferring block allocation until write-out time.
+nodelalloc Disable delayed allocation. Blocks are allocation
+ when data is copied from user to page cache.
Data Mode
There are 3 different data modes:
@@ -249,10 +255,10 @@ typically provide the best ext4 performance.

* ordered mode
In data=ordered mode, ext4 only officially journals metadata, but it logically
-groups metadata and data blocks into a single unit called a transaction. When
-it's time to write the new metadata out to disk, the associated data blocks
-are written first. In general, this mode performs slightly slower than
-writeback but significantly faster than journal mode.
+groups metadata information related to data changes with the data blocks into a
+single unit called a transaction. When it's time to write the new metadata
+out to disk, the associated data blocks are written first. In general,
+this mode performs slightly slower than writeback but significantly faster than journal mode.

* journal mode
data=journal mode provides full data and metadata journaling. All new data is
@@ -260,7 +266,8 @@ written to the journal first, and then to its final location.
In the event of a crash, the journal can be replayed, bringing both data and
metadata into a consistent state. This mode is the slowest except when data
needs to be read from and written to disk at the same time where it
-outperforms all others modes.
+outperforms all others modes. Curently ext4 does not have delayed
+allocation support if this data journalling mode is selected.


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