Linux

Lifetime setting for file

The fcntl function in Linux is used to manipulate file descriptors. It can perform various operations determined by the cmd argument¹.

If you’re looking to use read/write hints with fcntl, you might be referring to the F_SET_RW_HINT command. This command is used to set a hint for the expected read/write life-time of the file.

Here’s an example of how you might use it:

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdint.h>

static char const filename [] = "./file.txt";
static char const message [] = "/mnt/pool/my_file\\n";

int main (void) {
    int fp;
    int cnt = 0;
    errno = 0;
    uint64_t type = RWH_WRITE_LIFE_MEDIUM; // This is the write hint

    fp = open (filename, O_WRONLY | O_CREAT);
    if (fp == -1)
        return 1;

    fcntl (fp, F_SET_RW_HINT, &type); // Setting the write hint here

    cnt = write (fp, message, sizeof (message));
    if (cnt == -1)
        return 1;

    close (fp);
    return 0;
}

https://www.man7.org/linux/man-pages/man2/fcntl.2.html

In this example, RWH_WRITE_LIFE_MEDIUM is the write hint, which is set using the F_SET_RW_HINT command in the fcntl function².

Create filesystem across multiple disks

To create a filesystem across multiple disks in Linux, you can use Logical Volume Management (LVM). Here are the steps:

1. Identify the Disks: Use the fdisk -l command to find the attached disks³.
2. Create Partitions: Use the fdisk command to create partitions on the disks³.
3. Create Physical Volumes: After partitioning the disks, use pvcreate to prepare your new partitions for LVM².
4. Create a Volume Group: Assign both physical volumes to a new volume group⁴.
5. Create a Logical Volume: Create a new logical volume in that volume group⁴.
6. Create a Filesystem: Decide the type of filesystem you want to create (such as ext4, XFS, etc.) and use the appropriate mkfs command variation. For example, for ext4, you can use mkfs.ext4 /dev/sda1¹.
7. Mount the Filesystem: After creating your filesystem, you can mount it in your operating system¹.
8. Verify: Finally, verify using df -Th³.

Please replace /dev/sda1 with your actual device name. Be aware that these operations are potentially destructive and should be performed with care. Always ensure that your data is backed up before making such changes.

Here is an example of these commands:

# Create a physical volume on disk1 and disk2
pvcreate /dev/sda1 /dev/sdb1

# Create a volume group named myvg using the physical volumes
vgcreate myvg /dev/sda1 /dev/sdb1

# Create a logical volume named mylv in the volume group
lvcreate -l 100%FREE -n mylv myvg

# Create an ext4 filesystem in the logical volume
mkfs.ext4 /dev/myvg/mylv

# Mount the filesystem to a directory
mount /dev/myvg/mylv /mnt/mydir

• Locks