[syslinux] Memory usage with memdisk vs initrd a squashfs

Mattias Schlenker ms at mattiasschlenker.de
Thu Mar 13 13:27:41 PDT 2014

Am 13.03.2014 21:08, schrieb Gene Cumm:
>  Generally, the RAM that MEMDISK uses is permanently reserved however 
> that doesn't mean it's unusable. You can still mount the ISO assuming 
> you have the right tools like memdiskfind. As far as efficiency, a 
> small initrd to fetch another file will be more efficient (an ISO 
> might have extra unnecessary stuff like a copy of the kernel and 
> initrd) and would load faster. In my experience TFTP with PXE tops out 
> around 1 MB/s while an HTTP load can easily see 17MB/s or more. 
> lpxelinux.0 (the lwIP-enabled PXELINUX for BIOS) can achieive this as 
> can gpxelinux.0/ipxelinux.0 or PXELINUX ontop of gPXE/iPXE. You can 
> also see this when loading the file after the kernel is booted which 
> would often be a little bit faster than HTTP-capable PXELINUX. 

I maintain a distribution that loads a rather large initramfs containing 
all kernel modules and all firmware blobs. This allows for an easy 
network configuration at early stages and early initialization of 
framebuffer graphics/KMS. The distribution allows several boot modes 
over the network:

  * create an CPIO image containing the ISO and append it as second
    initramfs (syslinux allows concatenating multiple initramfs)
  * download the ISO immediately after configuring the network via TFTP,
    FTP or HTTP
  * mount a CIFS or NFS share containing the ISO and from there mount
    the ISO loopback

TFTPing large files is dead slow. wgetting FTP or HTTP usually is OK. 
CIFS and NFS boot is really fast, since just some 200MB of the whole ISO 
are actually used on systems booting to a thin client mode. And since 
enough gets cached it doesn't feel notably slower than systems that 
downloaded the whole ISO during startup.

So I would also suggest either downloading the system (stripped ISO 
without kernel, initramfs and stuff) via HTTP or accessing CIFS/NFS 
shares containing the ISO and mounting loopback  - if thats possible 
with your distribution.


Mattias Schlenker - Redaktion + EDV-Beratung + Linux-CD/DVD-Konzepte
August-Bebel-Str. 74 - 04275 LEIPZIG - GERMANY

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