[gPXE] [Etherboot-discuss] how big an image downloaded and booted by gPXE could be?

[Thomas Miletich]

On Wed, Dec 16, 2009 at 11:52 AM, Luca <lucarx76 at gmail.com> wrote:
> Thanks a lot.
>  Something is still not clear to me though. I'm using gPXE on a diskless
> workstation. I use gPXE to download a small ramdisk. Then I use this one to
> do a secondary download of the remaining contents.
> After the download I can do a pivot_root and switch to the new filesystem.
>
> Yet the space I have left is 1.5G (since I'm doing first download with gPXE
> and the image can't be bigger than 1.5GB).

The image you download with gPXE will be much smaller than 1.5GB.
Somewhere around max. 10MB. In this small initramfs you will have all
the modules and programs you need to access the network, and then you
simply have a script to mount a tmpfs or a ramdisk somewhere:

mount -t tmpfs -o size=4G none /rootfs

This command will create a 4GB tmpfs and mount it at /rootfs. Now you
can copy the root fs to /rootfs, in any way you like. You could mount
a NFS share and copy it, use sshfs, maybe even rsync.

It's a good start to take apart your distribution's initramfs( zcat
/boot/initrd.img |cpio -i ) and look at the scripts that usually mount
the HDD or NFS root. You should be able to modify them to copy your
files to your tmpfs and use that as the new rootfs.

Thomas

> I'm sure I'm missing something, but having everything in RAM, after I
> download a small ramdisk with gPXE, where do I save the content of the
> secondary download (which is going to be around 4G)?
>
> Thanks,
>  Luca
>
> On Fri, Dec 11, 2009 at 10:37 AM, H. Peter Anvin <hpa at zytor.com> wrote:
>>
>> On 12/11/2009 07:28 AM, Luca wrote:
>> > Thank you. So gPXE can address 4GB of RAM.
>> >
>> > Then, if I gPXE boot with the following script
>> >
>> > #!gpxe
>> >
>> >   kernel https://192.168.1.189/vmlinuz root=/dev/ram0 init=linuxrc
>> >   initrd https://192.168.1.189/ramdisk.img
>> >   boot
>> >
>> > the ramdisk's size could not exceed 2G (considering gPXE has to download
>> > it and then load it in memory)?
>> >
>>
>> Something like that.  In practice it is probably closer to 1-1.5 GB.
>>
>> You also don't want to actually use a ramdisk -- initramfs is a *lot*
>> more efficient.
>>
>> However, realistically you probably don't want to do that.  Instead you
>> want to bring up a reasonably small ramdisk that contains the contents
>> needed to do a secondary download of the remaining contents.  The
>> reasons are very simple: no need to double buffer, Linux has a *much*
>> more advanced IP stack that gPXE (for, I hope, obvious reasons -- not
>> picking on gPXE here), and no 4 GB limitation.
>>
>> As such, you should get much better preformace that way, and when you're
>> talking gigabytes, that matters.
>>
>>        -hpa
>>
>> --
>> H. Peter Anvin, Intel Open Source Technology Center
>> I work for Intel.  I don't speak on their behalf.
>>
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>
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