Help me understand installation procedure , please .


Nov 9, 2018
I have found the description of installation in :

1. ROMs for devices with locked bootloader:
Wipe /data partition (NEVER wipe System or Persist!)

2. Installation via TWRP:
Install - Select zip

I know 2 types of installation clean and dirty.
The clean has to be initiated wipping/formating all partitions exept external card, the dirty we wipping cache, dalvik, data .

My first question.
If I install (2) without wipping anything , will the rom work correct with not compatible system data and cache ? If I will delete (1) only data , will the cache and dalvik not crash new installed rom ?

The second question is about not wiped system.
a. Can I install on every existing rom in the internal flash ? Each rom has own files in system and if I will not delete it ... what does it mean ? will not install own data to system and will use system from previous installed rom ?
b. If do not have own system, what version of rom I have to install before installling ? Stock ? If stock , what android should I have 8.1 or 9 for each or specific version of ?
c. I read , is debloated . How is it possible if we do not wipe system before installing , and system is place where bloateware is installed ?

If someone will find a moment for answer my doubts, thank you.


Oct 30, 2016
Clean and dirty does not depend on locked bootloader of TWRP.
if you switch ROM, (e.g. from MIUI global to, or MIUI global stable to developer ROM) do a clean install, which means wipe cache, dalvik, data
if you upgrade (e.g. updating from an older version to a new version), a dirty install should be fine. which means no need to wipe.

a. If you install, you are deleting whatever rom you currently have on your phone. so system will be replaced with new ROM files.
b. it does not matter what ROM you have before switching to because you are deleting it in order to put on
c. read point a.

Also, as a side point:
1. if your phone has a locked bootloader, you will need to unlock it first. nothing can be done UNTIL your bootloader is unlocked. the purpose of bootloader lock is to prevent you from flashing ROM. keeping the phone secure.
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Nov 9, 2018
Thank you. As I understand , the process of first installation of does not need wipping system partition for security reason... if the instalation will fail , we have previous system not deleted , if it will finish with success , we have system partition wiped and new installed by installator.
Thank you for clarification.


Apr 29, 2016
System partition always will be automatically wiped by installing eu-ROM. Wiping cache/dalvik cache is normallly done by ROM-installation itself, so not necessary to be done manually when your phone is running smooth.
With first installation of eu-ROM coming from official China or Global ROM FORMAT DATA is necessary to remove encryption. This can't be done with wiping data partition.