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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FSADM(8) FSADM(8)
NAME
fsadm - utility to resize or check filesystem on a device
SYNOPSIS
fsadm [options] check device
fsadm [options] resize device [new_size[BKMGTEP]]
DESCRIPTION
fsadm utility checks or resizes the filesystem on a device. It tries to use the same API for ext2, ext3, ext4, ReiserFS and XFS filesys-
tem.
OPTIONS
-e, --ext-offline
Unmount ext2/ext3/ext4 filesystem before doing resize.
-f, --force
Bypass some sanity checks.
-h, --help
Display the help text.
-n, --dry-run
Print commands without running them.
-v, --verbose
Be more verbose.
-y, --yes
Answer "yes" at any prompts.
new_size
Absolute number of filesystem blocks to be in the filesystem, or an absolute size using a suffix (in powers of 1024). If new_size
is not supplied, the whole device is used.
DIAGNOSTICS
On successful completion, the status code is 0. A status code of 2 indicates the operation was interrupted by the user. A status code of
3 indicates the requested check operation could not be performed because the filesystem is mounted and does not support an online fsck(8).
A status code of 1 is used for other failures.
EXAMPLES
Resize the filesystem on logical volume /dev/vg/test to 1000 megabytes. If /dev/vg/test contains ext2/ext3/ext4 filesystem it will be
unmounted prior the resize. All [y|n] questions will be answered 'y'.
fsadm -e -y resize /dev/vg/test 1000M
ENVIRONMENT VARIABLES
TMPDIR The temporary directory name for mount points. Defaults to "/tmp".
DM_DEV_DIR
The device directory name. Defaults to "/dev" and must be an absolute path.
SEE ALSO
lvm(8), lvresize(8), lvm.conf(5), fsck(8), tune2fs(8), resize2fs(8), reiserfstune(8), resize_reiserfs(8), xfs_info(8), xfs_growfs(8),
xfs_check(8)
Red Hat, Inc LVM TOOLS 2.02.105(2)-RHEL7 (2014-03-26) FSADM(8)