From 32975ce2970fee62b6d5884889b64f8953e9c11d Mon Sep 17 00:00:00 2001
From: Simon Rettberg
Date: Fri, 2 Oct 2020 09:47:14 +0200
Subject: [slx-dmsetup] Add overprovisioning formula for CoW volume

---
 modules.d/slx-dmsetup/scripts/dmsetup-slx-device | 28 ++++++++++++++----------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/modules.d/slx-dmsetup/scripts/dmsetup-slx-device b/modules.d/slx-dmsetup/scripts/dmsetup-slx-device
index f890cd44..ff25e167 100755
--- a/modules.d/slx-dmsetup/scripts/dmsetup-slx-device
+++ b/modules.d/slx-dmsetup/scripts/dmsetup-slx-device
@@ -566,18 +566,22 @@ if [ -n "$thin_snapshot" ] || [ -n "$thin_volume" ]; then
 			# create thin-snapshot, use first one
 			read -r name crypt min max ignore <<< "$thin_snapshot"
 			# min/max was used for the pool data device, ignore it here!
-			# NOTE: the filesystem will most likely malfunction if the size of the
-			# thin-snapshot is smaller than what it was upon creation.
-			# As such, the size of the thin-snapshot can only be $scratch_device_sz
-			# if it is larger than $read_only_device_sz, otherwise we should only
-			# use $read_only_device_sz. While live-shrinking the filesystem might be
-			# an option, it is not supported throughout all fileystems (xfs can't).
-			if (( scratch_device_sz >= read_only_device_sz )); then
-				thin_snapshot_sz="$scratch_device_sz"
-			else
-				thin_snapshot_sz="$read_only_device_sz"
-			fi
-			# For later on-demand growing
+			# Calculate how much of the CoW space we reserve for changes in the base
+			# system. Usually all the files in the base system should be static, but
+			# if someone decided to run apt dist-upgrade, this would change a lot of
+			# existing blocks, which is bad.
+			# Use MIN( readonly_size / 2, scratch_size / 10 )
+			# until we come up with anything better.
+			# Given an RO image of 10GB, this gives us:
+			# 40GB scratch -> 46GB, so initially 36GB free space
+			# 5GB scratch -> 14.5GB, initially 4.5GB free space
+			declare -r max_reserved_sz="$(( scratch_device_sz / 10 ))"
+			reserved_sz="$(( read_only_device_sz / 2 ))"
+			(( reserved_sz > max_reserved_sz )) && reserved_sz="$max_reserved_sz"
+			thin_snapshot_sz="$(( scratch_device_sz + read_only_device_sz - reserved_sz ))"
+			# For later on-demand growing, overprovision by free space we found on
+			# clean NTFS volumes. This requires a user-space helper to listen for
+			# dm events in stage4, which should in turn add that free space to the pool-data
 			if (( root_ntfs_extra > 0 )); then
 				thin_snapshot_sz="$(( thin_snapshot_sz + root_ntfs_extra ))"
 			fi
-- 
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