On Mon, 3 Apr 2023 at 21:00, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > > > > > > > > I think lazyfollow could be enabled by default after we hashed out > > > > all the bugs and corner cases and most importantly remove the > > > > POC limitation of lower-only overlay. > > > > > [...] > > > > > > > > > > Lazy follow seems to make sense. Why does it need to be optional? > > > > It doesn't. > > > > > Does it have any advantage to *not* do lazy follow? > > > > > > > Not that I can think of. > > Miklos, > > I completed writing the lazy lookup patches [1]. > > It wasn't trivial and the first versions had many traps that took time to > trip on, so I've made some design choices to make it safer and easier to > land an initial improvement that will cater the composefs use case. > > The main design choice has to do with making lazy lowerdata lookup > completely opt-in by defining a new type of data-only layers, such as > the content addressable lower layer of composefs. > The request for the data-only layers came from Alexander. > > The current patches only do lazy lookup in data-only layers and the lookup > in data-only layers is always lazy. > > Data-only layers have some other advantages, for example, multiple > data-only uuid-less layers are allowed. > Please see the text below taken from the patches. > > What do you think about this direction? > > Alexander has started to test these patches. > If he finds no issues and if you have no objections to the concept, > then I will post the patches for wider review. > > > Thanks, > Amir. > > [1] https://github.com/amir73il/linux/commits/ovl-lazy-lowerdata-rc2 > > Data-only lower layers > ---------------------- > > With "metacopy" feature enabled, an overlayfs regular file may be a > composition of information from up to three different layers: > > 1) metadata from a file in the upper layer > > 2) st_ino and st_dev object identifier from a file in a lower layer > > 3) data from a file in another lower layer (further below) > > The "lower data" file can be on any lower layer, except from the top most > lower layer. > > Below the top most lower layer, any number of lower most layers may be > defined as "data-only" lower layers, using the double collon ("::") separator. > > For example: > > mount -t overlay overlay -olowerdir=/lower1::/lower2:/lower3 /merged What are the rules? Is "do1::do2::lower" allowed? Is "do1::lower1:do2::lower2 allowed? > > The paths of files in the "data-only" lower layers are not visible in the > merged overlayfs directories and the metadata and st_ino/st_dev of files > in the "data-only" lower layers are not visible in overlayfs inodes. > > Only the data of the files in the "data-only" lower layers may be visible > when a "metacopy" file in one of the lower layers above it, has a "redirect" > to the absolute path of the "lower data" file in the "data-only" lower layer. Okay. Thanks, Miklos
