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This patch adds a reproducer for issue #7586 - that Alternator queries (Query) operating in reverse order (ScanIndexForward = false) are artificially limited to 100 MB partitions because of their memory use. This test generates a partition over 100 MB in size and then tries various reverse queries on it - with or without Limit, starting at the end or the middle of the partition. The test currently fails when a reverse query refuses to operate on such a large partition - the log reports this: ERROR ... Memory usage of reversed read exceeds hard limit of 104857600 (configured via max_memory_for_unlimited_query_hard_limit), while reading partition K1H6ON3A1C With yet-uncommitted reverse-scan improvements, the test proceeds further, but still fails where we test that a reverse query with Limit not explicitly specified should still be limited to a certain size (e.g. 1MB) and cannot return the entire 100 MB partition in one response. Please note that this is not a comprehensive test for Scylla's reverse scan implementation: In particular we do not have separate tests for reverse scan's implementation on different sources - memtables, sstables, or the cache. Nor do we check all sorts of edge cases. We assume that Scylla's reverse scan implementation will have its own unit tests elsewhere that will check these things - and this test can focus on the Alternator use case. This test is marked "xfail" because it still fails on Alternator. It is marked "veryslow" because it's a (relatively) slow test, taking multiple seconds to set up the 100 MB partition. So run the test with the pytest options "--runxfail --runveryslow" to see how it fails. Refs #7586 Signed-off-by: Nadav Har'El <nyh@scylladb.com> Message-Id: <20210930063700.407511-1-nyh@scylladb.com>