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Rino Bino
Asked: 2022-04-05 20:01:09 +0800 CST

Using a compressed initrd when pxe booting

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I'm using netboot/pxeboot with grub.

menuentry "Install Ubuntu 20.04" {
  set gfxpayload=keep
  echo 'Loading vmlinuz ...'
  linux  /tftp/vmlinuz ip=dhcp netboot=nfs nfsroot=10.0.0.20:/data/netboot/nfs/ubuntu2004/ boot=casper toram noquiet splash=off console=tty0 console=ttyS1,57600n8 ---
  echo 'Loading initrd, this takes a long time ...'
  initrd /tftp/initrd
}

It works fine, however, the loading of initrd over tftp is taking a long time (30+ minutes). I would like to compress (gz/bz2) this file to save on some file transfer time.

I had seen some examples out there referring to a initrd.gz (One example: https://unix.stackexchange.com/questions/217002/which-iso-file-vmlinuz-and-initrd-gz-to-use-for-installing-centos-from-multiboo) but when I tried to just compress the file with gzip and use it, I get an error such as:

[   12.543547] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[   12.558487] Please append a correct "root=" boot option; here are the available partitions:
[   12.575161] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Is there a way to compress this file so it can transfer a smaller file size, and have it uncompress before it tries to mount the root fs?

Alternatively, is there a way to transfer it over a different protocol (HTTP/FTP/SFTP/SCP/etc) ?

boot pxe-boot linux-kernel tftp

2 Answers

  1. Best Answer

    An initrd is a compressed CPIO structure created from a file tree structure with a command like:

    find . | cpio -o -c -R root:root | gzip -9 > /boot/new.img

    The ubuntu-20.04-desktop-amd64 initrd is already compressed and weights about 87MB. TFTP transfer of that file should be around 36 seconds.

    [04/05 08:17:46.445] TFTP Inf: <\NWA_PXE\ubuntu-20.04-desktop-amd64\casper\initrd>: sent blks=63901 blkSz=1408, Total 89971296 bytes in 36s, err recovery=0

    If your TFTP transfer takes half an hour you have a different problem in your network-TFTP setup. A Wireshark traffic capture can help you to pin point your issue.

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  2. @Pat's answer is accepted and more detailed, however here are the direct answers to the original questions:

    Is there a way to compress this file so it can transfer a smaller file size, and have it uncompress before it tries to mount the root fs?

    No, it's already compressed. The accepted answer gives an example of this.

    Alternatively, is there a way to transfer it over a different protocol (HTTP/FTP/SFTP/SCP/etc) ?

    Not with regular PXE. Use another network boot system, like iPXE, which allows http/nfs

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