zio_compress: introduce max size threshold

Now default compression is lz4, which can stop
compression process by itself on incompressible data.
If there are additional size checks -
we will only make our compressratio worse.

New usable compression thresholds are:
- less than BPE_PAYLOAD_SIZE (embedded_data feature);
- at least one saved sector.

Old 12.5% threshold is left to minimize affect
on existing user expectations of CPU utilization.

If data wasn't compressed - it will be saved as
ZIO_COMPRESS_OFF, so if we really need to recompress
data without ashift info and check anything -
we can just compress it with zero threshold.
So, we don't need a new feature flag here!

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #9416

module/zfs/zio.c

@@ -26,6 +26,7 @@
  * Copyright (c) 2019, 2023, 2024, Klara Inc.
  * Copyright (c) 2019, Allan Jude
  * Copyright (c) 2021, Datto, Inc.
+ * Copyright (c) 2021, 2024 by George Melikov. All rights reserved.
  */
 
 #include <sys/sysmacros.h>
@@ -1704,6 +1705,21 @@ zio_roundup_alloc_size(spa_t *spa, uint64_t size)
 	return (spa->spa_min_alloc);
 }
 
+size_t
+zio_get_compression_max_size(uint64_t gcd_alloc, uint64_t min_alloc,
+    size_t s_len)
+{
+	size_t d_len;
+
+	/* minimum 12.5% must be saved (legacy value, may be changed later) */
+	d_len = s_len - (s_len >> 3);
+
+	d_len = d_len - d_len % gcd_alloc;
+	if (d_len < min_alloc)
+		return (BPE_PAYLOAD_SIZE);
+	return (d_len);
+}
+
 /*
  * ==========================================================================
  * Prepare to read and write logical blocks
@@ -1885,7 +1901,10 @@ zio_write_compress(zio_t *zio)
 			psize = lsize;
 		else
 			psize = zio_compress_data(compress, zio->io_abd, &cabd,
-			    lsize, zp->zp_complevel);
+			    lsize,
+			    zio_get_compression_max_size(spa->spa_gcd_alloc,
+			    spa->spa_min_alloc, lsize),
+			    zp->zp_complevel);
 		if (psize == 0) {
 			compress = ZIO_COMPRESS_OFF;
 		} else if (psize >= lsize) {