09-26-2016
What is the difference between o_direct and DAX with ext4 filesystem?
I'm trying to understand the difference between o_direct flag of open system call and dax (direct access) with ext4 filesystem.
According to my understanding both bypass page cache.
But I'm still unclear about the crucial difference between these 2 techniques. If there is a huge difference then what does it mean to use o_direct flag with DAX? Can anyone please explain about this in detail?
NOTE: Here I have partitioned a pmem device on top of DRAM using memmap parameter.
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DUMPE2FS(8) System Manager's Manual DUMPE2FS(8)
NAME
dumpe2fs - dump ext2/ext3/ext4 filesystem information
SYNOPSIS
dumpe2fs [ -bfghixV ] [ -o superblock=superblock ] [ -o blocksize=blocksize ] device
DESCRIPTION
dumpe2fs prints the super block and blocks group information for the filesystem present on device.
Note: When used with a mounted filesystem, the printed information may be old or inconsistent.
OPTIONS
-b print the blocks which are reserved as bad in the filesystem.
-o superblock=superblock
use the block superblock when examining the filesystem. This option is not usually needed except by a filesystem wizard who is
examining the remains of a very badly corrupted filesystem.
-o blocksize=blocksize
use blocks of blocksize bytes when examining the filesystem. This option is not usually needed except by a filesystem wizard who is
examining the remains of a very badly corrupted filesystem.
-f force dumpe2fs to display a filesystem even though it may have some filesystem feature flags which dumpe2fs may not understand (and
which can cause some of dumpe2fs's display to be suspect).
-g display the group descriptor information in a machine readable colon-separated value format. The fields displayed are the group
number; the number of the first block in the group; the superblock location (or -1 if not present); the range of blocks used by the
group descriptors (or -1 if not present); the block bitmap location; the inode bitmap location; and the range of blocks used by the
inode table.
-h only display the superblock information and not any of the block group descriptor detail information.
-i display the filesystem data from an image file created by e2image, using device as the pathname to the image file.
-x print the detailed group information block numbers in hexadecimal format
-V print the version number of dumpe2fs and exit.
BUGS
You need to know the physical filesystem structure to understand the output.
AUTHOR
dumpe2fs was written by Remy Card <Remy.Card@linux.org>. It is currently being maintained by Theodore Ts'o <tytso@alum.mit.edu>.
AVAILABILITY
dumpe2fs is part of the e2fsprogs package and is available from http://e2fsprogs.sourceforge.net.
SEE ALSO
e2fsck(8), mke2fs(8), tune2fs(8). ext4(5)
E2fsprogs version 1.44.1 March 2018 DUMPE2FS(8)