Initial ramdisk

[1] To further complicate matters, the root file system may be on a software RAID volume, LVM, NFS (on diskless workstations), or on an encrypted partition.

[2] Another complication is kernel support for hibernation, which suspends the computer to disk by dumping an image of the entire contents of memory to a swap partition or a regular file, then powering off.

To avoid having to hardcode handling for so many special cases into the kernel, an initial boot stage with a temporary root file-system – now dubbed early user space – is used.

[5] For some applications, initramfs can use the casper utility to create a writable environment using unionfs to overlay a persistence layer over a read-only root filesystem image.

[7][8] Depending on which algorithms were compiled statically into it, the kernel can unpack initrd/initramfs images compressed with gzip, bzip2, LZMA, XZ, LZO, LZ4,[9] and zstd.

Some Linux distributions such as Debian will generate a customized initrd image which contains only whatever is necessary to boot some particular computer, such as ATA, SCSI and filesystem kernel modules.

In this case, the software must perform a complex cascade of tasks to get the root file system mounted: Some distributions use an event-driven hotplug agent such as udev, which invokes helper programs as hardware devices, disk partitions and storage volumes matching certain rules come online.

This allows discovery to run in parallel, and to progressively cascade into arbitrary nestings of LVM, RAID or encryption to get at the root file system.

However, this is rarely used since the Windows boot loader is capable of loading the .sys files for boot-time kernel modules itself, which is the task that requires initrd in Linux.