vgoyal added a new comment to an issue you are following: ``
Flipping from one to the other will take free space somewhere for the 'atomic storage export/import' operation to temporarily store docker images and containers to. A way around the xfs lack of shrink issue is to put the filesystem containing /var onto a thinly provisioned LV (be it a dir on rootfs or its own volume). After 'atomic storage reset' wipes the docker storage, issue fstrim, and all the previously used extents will be returned to the thin pool, which can then be returned to the VG, which can then be reassigned to a new docker thin pool. Convoluted in my opinion, but doable.
IIUC, you are saying that use a thin LV for rootfs to work around xfs shrink issue? People have tried that in the past and there have been talks about that many a times. There are still issues with xfs on top of thin lv and how no space situation is handled etc. Bottom line, we are not there yet.
So if we can't use rootfs on thin LV and if xfs can't be shrinked, then only way to flip back to devicemapper is don't allow rootfs to use all free space. And have free space which can either be used by overlay2 or devicemapper for container storage. ``
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