[LEDE-DEV] [OpenWrt-Devel] [RFC] Pre-seeded files/directories for UBIFS

[Richard Weinberger]

Ralph,

Am 20.05.2017 um 21:36 schrieb Ralph Sennhauser:
>>> These days I had an interesting discussion with Christoph about
>>> overlayfs and its burden. The main use-case of overlayfs in
>>> combination with UBIFS is having a squashfs as lower and UBIFS as
>>> upper directory. Such that all changes to the read-only squashfs go
>>> into UBIFS. Upon a factory reset all files within the UBIFS will be
>>> removed  and the merged directory is clean again. Christoph argued
>>> that such a functionality  could be achieved without overlayfs if
>>> the filesystem supported something like pre-seeded  files or
>>> directories. This would lower memory pressure and complexity.  
>>
>> As you may know, OpenWrt/LEDE have been using this scheme for many
>> years now (before it was named overlayfs, this was called mini fanout
>> overlay ~10 yrs ago) with squashfs + jffs2 before on P-NOR flashes.
>> There are still devices like those that benefit from
>> squashfs(ro)+jffs2(rw), so while bringing a similar functionality
>> using UBIFS exclusively would be interesting, it would still make
>> Linux distribution want to support a more generic scheme which is
>> using overlayfs as well.
>>
> 
> There is also the size consideration. Unless a seeded ubifs can get
> close to squashfs in terms of compression there would still be a
> use-case for squashfs with an ubifs overlay. My current root as ubifs
> instead of squashfs is 76.8% bigger.

You seem to misunderstand this feature, the goal is not to void all
uses of squashfs.
I'm pretty sure for the LEDE usecase squashfs is the better choice.

Thanks,
//richard