Win10XPE - Build Your Own Rescue Media [2]

tralam · 04 Oct 2021

Hi All,

So I've been playing with Win10XPE and have some (basic) questions:

I have a Custom folder where I add some (portable) apps to X:\Program Files\ which works great. However, I have 1024mb set for X:\ in WinBuilder and because of me adding all those apps the subsequent boot.wim is almost 1GB in size (compressed).

How does this exactly relate to the 1024mb size limit? I have not gotten any errors or messages when loading the PE in a VM (VM RAM allocation: 4096mb) but it would seem I'm dangerously close to the 1024 limit. The way I understand it, the X:\ (Boot) drive loads the contents of the boot.wim into memory. If so, what would happen if my boot.wim exceeds this limit? How would I know? Inside the PE the drives read almost empty, so I'm having a hard time determining where the limit lies and how close I am to it.

Another question regards the B:\ or Scratch drive. Mine is set (default, I think) to 1.28GB. Is the B:\ drive exclusively used for loading applications in the PE, or is this space also used for the actual memory load of Windows (PE)? Or how do I see that? I'm just trying to understand exactly how it works and where I'm bound to run into limitations. Thank you.

EDIT: Is the default 1024mb allocation for X:\ *in addition* to the contents loaded from boot.wim? So if boot.wim has 1.5gb of files, that means 1.5gb + 1gb (1024mb) + B: drive/scratch disk?

MaloK · 04 Oct 2021

Hi, @tralam,

The size of windows XPE cache remains Whatever size is boot.wim... It set what free space your XPE will have "writable" in the RAM boot disk . Since change done to drive X: is never commited to the disk. they go in a disk overlay that fills up with use. This tell XPE the writable space on boot disk.

No matter how many programs you add to your boot.wim If the computer has the necesssairy memory availlable it will always set the overlay close to what you have configured. So even if you set it to 4 gig and boot on a computer with only 4gig ram The cache will be automatically reduced to a Size that Fits in availlable Ram. I cannot really talk about The Ram drive I never used it. Some say that it is needed if you want to perform Dism operations on large images.

tralam · 04 Oct 2021

Thank you for the reply, @MaloK

I think I understand part of what you said. But if you don't mind, to clarify:

So what if I have the XPE cache set to 1024mb as I do, but instead of having a 500 or 600mb boot.wim, I add apps in it and have a 1gb boot.wim, do I not end up with 500/400mb less available space on drive X:\ in the loaded PE?

Also, what do you mean by "never committed to the disk, they go in a disk overlay"?

Sorry about all the questions, but I can't seem to get it clear in my mind.

Also, the B:\ ram drive, it is solely meant to run programs (temp files, etc), correct?

MaloK · 04 Oct 2021

1- Yes it will never increase the size of the boot.wim. It configure the size of the Writable Overlay.(take it as availlable writable space on your boot RAM Disk... It's size is always 0 when booting... with uses and time the overlay will slowly fill up with written data increasing the size of the overlay until it reaches the limit amount you selected or run of of ram before.)

2-changes are not written to disk but instead in the previousely configured Overlay (By default 1GB)

3-Yes Ram Drive is there for temporary files program may use etc...

tralam · 04 Oct 2021

Thank you! So how is it able to get the data from the boot.wim, then, if it doesn't load the entire boot.wim contents into memory?

EDIT: To clarify, when you wrote "The size of windows XPE cache remains Whatever size is boot.wim..." did you mean: "The size of Windows XPE cache remains the same, regardless of the size of the boot.wim." ? Because that is how I read that.

MaloK · 04 Oct 2021

tralam said:

Thank you! So how is it able to get the data from the boot.wim, then, if it doesn't load the entire boot.wim contents into memory?

It's a trick that comes from the Linux World

The boot.wim is all there and decompressed. Look into how wimboot manages memory, you will understand how it works.

tralam · 04 Oct 2021

MaloK said:

It's a trick that comes from the Linux World

The boot.wim is all there and decompressed. Look into how wimboot manages memory, you will understand how it works.

So, to be sure, the boot.wim does decompress into RAM, but not inside the allocated 1024mb set as PE cache? Sorry, I know I ask too many questions

EDIT: I found this, is this what you're referring to? "...wimboot is a boot loader for Windows Imaging Format (.wim) files. It enables you to boot a Windows PE (WinPE) environment from a RAM disk, without wasting the memory used for the RAM disk image."

MaloK · 04 Oct 2021

loll... Noooooooooooooooooooo....

You cannot control that aspect

like that.

Once your boot.wim is decompressed in ram. XPE will attempt to boot the Boot Ram disk. then during the boot process XPE will add the Cache to you existing RAM Disk. But its size will remain 0 until it start to get written to. So it a good balance of compatibility and power. Booting Windows PE alone does not require lots of ram but the boot.wim + ram disk (b:) are the main memory hog so even if PE needs 256 mo ram to run You still need 2 gig to fit the boot.wim and ram disk + the cache...

XPE is fine for Good operation on 4GB++ and will work on 2GB But... Another thing is that PE has no memory cap management. If you go over what you have, it will BSOD.

Edit: Yes that is how it's done.

To give a little more information on how this works. Once the ram drive is installed and running. Since all program are already in memory... Instead of copying them to memory to run them they are runned directly on the disk... First time I read that... I was like Wow !!!

tralam · 04 Oct 2021

Thanks for all this. However, some things are just not "gelling" for me :)

Okay, the PE cache size is set to 1024mb/1gb (the X:\ drive). In addition, there is a RAM disk initiated as the PE loads (1.28gb, currently) (the B:\ drive).

How does the (uncompressed) boot.wim contents relate to the X: boot drive? You say it "loads into RAM" but how does that relate to either X:\ or B:\ drives? I understand it all loads into RAM, so saying it loads into RAM is not really making it clear for me.

When loading into the PE, the X: drive shows up as empty (or 2mb used out of 1gb). The RAM drive B:\ also shows up as empty or 2mb or whatever used. However, my boot.wim is 1gb (compressed). So how can the boot.wim contents load into RAM, yet not show up in the PE drives X:\ and B:\ ? Does it mean the boot.wim contents get loaded into RAM *besides* the X and B drives, so that "true" RAM usage is actually higher -- or, depending on what apps your run as you indicated? So, additionally, in other words? I'm having real trouble visualizing this.

I guess mostly I'm trying to just figure out this: if I keep the PE cache size to 1024 (X:\), and the RAM drive to 1.28 or whatever it is set by default, will me adding 500mb of apps into the boot.wim cause the PE to run out of memory? From what you say, it would appear the extra apps do get loaded into memory, even if not used. I just can't visualize it :)

EDIT: Someone else here had a similar question.

This is what resources usages and the X and B drives look like in loaded PE (with the 1gb boot.wim):

Win10XPE - Build Your Own Rescue Media [2]-image-1.png

MaloK · 04 Oct 2021

ok.

Ill try to put it differently...

If your booted Wim takes 1.6 gig, starting Windows PE will take very little more space (Since the files are running directly from the disk instead of being copied to another place in memory before execution.)

Then, there is your cache settings... The size you select there, means the limit of writable space you want to have on drive X:... Before theorical cache space depletes. If there is not enough memory PE wil cut back on this. But the space used is always 0 at boot time and grows with uses. Setting Maximum does not change any other ability to work... It just means that you want to try to reach this point before stopping. Windows may think otherwise.

In blunt words the bigger the cache, the longer it works before crash.

Then, there is you RAM disk... It will occupate 50% of the memory remaining... This is configurable in XPE. And is not needed at all for most of the thing I tried under PE.

Dont worry it's just Basic Ram booting with a lemon twist.

Edit: Yes. Your boot wim is part of the memory used.