For discussion: "slack" compression option

include/sys/zio_compress.h

@@ -54,6 +54,7 @@ enum zio_compress {
 	ZIO_COMPRESS_ZLE,
 	ZIO_COMPRESS_LZ4,
 	ZIO_COMPRESS_ZSTD,
+	ZIO_COMPRESS_SLACK,
 	ZIO_COMPRESS_FUNCTIONS
 };
 
@@ -169,6 +170,10 @@ extern size_t zfs_lz4_compress(abd_t *src, abd_t *dst, size_t s_len,
     size_t d_len, int level);
 extern int zfs_lz4_decompress(abd_t *src, abd_t *dst, size_t s_len,
     size_t d_len, int level);
+extern size_t zfs_slack_compress(abd_t *src, abd_t *dst, size_t s_len,
+    size_t d_len, int level);
+extern int zfs_slack_decompress(abd_t *src, abd_t *dst, size_t s_len,
+    size_t d_len, int level);
 
 /*
  * Compress and decompress data if necessary.

include/zfeature_common.h

@@ -83,6 +83,7 @@ typedef enum spa_feature {
 	SPA_FEATURE_REDACTION_LIST_SPILL,
 	SPA_FEATURE_RAIDZ_EXPANSION,
 	SPA_FEATURE_FAST_DEDUP,
+	SPA_FEATURE_SLACK_COMPRESS,
 	SPA_FEATURES
 } spa_feature_t;
 

lib/libzpool/Makefile.am

@@ -126,6 +126,7 @@ nodist_libzpool_la_SOURCES = \
 	module/zfs/rrwlock.c \
 	module/zfs/sa.c \
 	module/zfs/sha2_zfs.c \
+	module/zfs/slack.c \
 	module/zfs/skein_zfs.c \
 	module/zfs/spa.c \
 	module/zfs/spa_checkpoint.c \

module/Kbuild.in

@@ -368,6 +368,7 @@ ZFS_OBJS := \
 	sa.o \
 	sha2_zfs.o \
 	skein_zfs.o \
+	slack.o \
 	spa.o \
 	spa_checkpoint.o \
 	spa_config.o \

module/Makefile.bsd

@@ -297,6 +297,7 @@ SRCS+=	abd.c \
 	sa.c \
 	sha2_zfs.c \
 	skein_zfs.c \
+	slack.c \
 	spa.c \
 	space_map.c \
 	space_reftree.c \

module/zcommon/zfeature_common.c

@@ -760,6 +760,20 @@ zpool_feature_init(void)
 	    ZFEATURE_FLAG_READONLY_COMPAT, ZFEATURE_TYPE_BOOLEAN, NULL,
 	    sfeatures);
 
+	{
+		{
+		static const spa_feature_t slack_deps[] = {
+			SPA_FEATURE_EXTENSIBLE_DATASET,
+			SPA_FEATURE_NONE
+		};
+		zfeature_register(SPA_FEATURE_SLACK_COMPRESS,
+		    "com.klarasystems:slack_compress", "slack_compress",
+		    "slack compression support",
+		    ZFEATURE_FLAG_PER_DATASET, ZFEATURE_TYPE_BOOLEAN,
+		    slack_deps, sfeatures);
+		}
+	}
+
 	zfs_mod_list_supported_free(sfeatures);
 }
 

module/zcommon/zfs_prop.c

@@ -130,6 +130,7 @@ zfs_prop_init(void)
 		{ "gzip-9",	ZIO_COMPRESS_GZIP_9 },
 		{ "zle",	ZIO_COMPRESS_ZLE },
 		{ "lz4",	ZIO_COMPRESS_LZ4 },
+		{ "slack",	ZIO_COMPRESS_SLACK },
 		{ "zstd",	ZIO_COMPRESS_ZSTD },
 		{ "zstd-fast",
 		    ZIO_COMPLEVEL_ZSTD(ZIO_ZSTD_LEVEL_FAST_DEFAULT) },

module/zfs/slack.c

@@ -0,0 +1,72 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * Copyright (c) 2023, Klara Inc.
+ */
+
+#include <sys/zio_compress.h>
+#include <sys/types.h>
+
+/*
+ * Slack compression simply searches for the last non-zero byte in the buffer,
+ * and sets the position as the size of the "compressed" data.
+ */
+
+static size_t
+zfs_slack_compress_buf(void *src, void *dst, size_t s_len, size_t d_len,
+    int level)
+{
+	(void) level;
+
+	ASSERT3U(s_len, >, 0);
+	ASSERT0(P2PHASE(s_len, sizeof (uint64_t)));
+
+	uint64_t *buf = (uint64_t *)src;
+
+	int p = (s_len / sizeof (uint64_t)) - 1;
+	for (; p >= 0; p--)
+		if (buf[p] != 0)
+			break;
+
+	if (p < 0)
+		return (s_len);
+
+	size_t c_len = (p + 1) * sizeof (uint64_t);
+	if (c_len > d_len)
+		return (s_len);
+
+	memcpy(dst, src, c_len);
+	return (c_len);
+}
+
+static int
+zfs_slack_decompress_buf(void *src, void *dst, size_t s_len, size_t d_len,
+    int level)
+{
+	(void) level;
+	ASSERT3U(d_len, >=, s_len);
+	memcpy(dst, src, s_len);
+	return (0);
+}
+
+ZFS_COMPRESS_WRAP_DECL(zfs_slack_compress)
+ZFS_DECOMPRESS_WRAP_DECL(zfs_slack_decompress)

module/zfs/zfs_ioctl.c

@@ -4806,6 +4806,20 @@ zfs_check_settable(const char *dsname, nvpair_t *pair, cred_t *cr)
 				}
 				spa_close(spa, FTAG);
 			}
+
+			if (compval == ZIO_COMPRESS_SLACK) {
+				spa_t *spa;
+
+				if ((err = spa_open(dsname, &spa, FTAG)) != 0)
+					return (err);
+
+				if (!spa_feature_is_enabled(spa,
+				    SPA_FEATURE_SLACK_COMPRESS)) {
+					spa_close(spa, FTAG);
+					return (SET_ERROR(ENOTSUP));
+				}
+				spa_close(spa, FTAG);
+			}
 		}
 		break;
 

module/zfs/zio_compress.c

@@ -84,6 +84,8 @@ zio_compress_info_t zio_compress_table[ZIO_COMPRESS_FUNCTIONS] = {
 	    zfs_lz4_compress,	zfs_lz4_decompress, NULL},
 	{"zstd",	ZIO_ZSTD_LEVEL_DEFAULT,
 	    zfs_zstd_compress,	zfs_zstd_decompress, zfs_zstd_decompress_level},
+	{"slack",	0,
+	    zfs_slack_compress,	zfs_slack_decompress, NULL },
 };
 
 uint8_t
@@ -200,6 +202,8 @@ zio_compress_to_feature(enum zio_compress comp)
 	switch (comp) {
 	case ZIO_COMPRESS_ZSTD:
 		return (SPA_FEATURE_ZSTD_COMPRESS);
+	case ZIO_COMPRESS_SLACK:
+		return (SPA_FEATURE_SLACK_COMPRESS);
 	default:
 		break;
 	}