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Commit 49f1487b authored by Mingming Cao's avatar Mingming Cao Committed by Theodore Ts'o
Browse files

ext4: Documention update for new ordered mode and delayed allocation 



Adding some documentations for delayed allocation and new ordered mode.

Signed-off-by: default avatarMingming Cao <cmm@us.ibm.com>
Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
parent e4079a11

Documentation/filesystems/ext4.txt

+14 −7
Original line number Diff line number Diff line
@@ -66,7 +66,7 @@ Mailing list: linux-ext4@vger.kernel.org 
* 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)
@@ -77,6 +77,10 @@ Mailing list: linux-ext4@vger.kernel.org 
  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
@@ -239,7 +243,9 @@ 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:
@@ -253,10 +259,10 @@ 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
@@ -264,7 +270,8 @@ 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.



References

==========