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diff --git a/t/perf/p5551-fetch-rescan.sh b/t/perf/p5551-fetch-rescan.sh
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+#!/bin/sh
+
+test_description='fetch performance with many packs
+
+It is common for fetch to consider objects that we might not have, and it is an
+easy mistake for the code to use a function like `parse_object` that might
+give the correct _answer_ on such an object, but do so slowly (due to
+re-scanning the pack directory for lookup failures).
+
+The resulting performance drop can be hard to notice in a real repository, but
+becomes quite large in a repository with a large number of packs. So this
+test creates a more pathological case, since any mistakes would produce a more
+noticeable slowdown.
+'
+. ./perf-lib.sh
+. "$TEST_DIRECTORY"/perf/lib-pack.sh
+
+test_expect_success 'create parent and child' '
+ git init parent &&
+ git clone parent child
+'
+
+
+test_expect_success 'create refs in the parent' '
+ (
+ cd parent &&
+ git commit --allow-empty -m foo &&
+ head=$(git rev-parse HEAD) &&
+ test_seq 1000 |
+ sed "s,.*,update refs/heads/& $head," |
+ $MODERN_GIT update-ref --stdin
+ )
+'
+
+test_expect_success 'create many packs in the child' '
+ (
+ cd child &&
+ setup_many_packs
+ )
+'
+
+test_perf 'fetch' '
+ # start at the same state for each iteration
+ obj=$($MODERN_GIT -C parent rev-parse HEAD) &&
+ (
+ cd child &&
+ $MODERN_GIT for-each-ref --format="delete %(refname)" refs/remotes |
+ $MODERN_GIT update-ref --stdin &&
+ rm -vf .git/objects/$(echo $obj | sed "s|^..|&/|") &&
+
+ git fetch
+ )
+'
+
+test_done