Enable zero-elimination if dedup is activated

Creating high-refcount dedup entries for checksums of each record size
is inefficient when the record could be recorded as a hole instead.

Normally, zero-elimination is disabled if compression is not enabled,
but in the dedup case, it is expected that the write will be skipped
anyway, we are just optimizing the way the write is skipped.

Signed-off-by: Allan Jude <[email protected]>

Author: allanjude

module/zfs/zio.c

@@ -1932,6 +1932,7 @@ zio_write_compress(zio_t *zio)
 	spa_t *spa = zio->io_spa;
 	zio_prop_t *zp = &zio->io_prop;
 	enum zio_compress compress = zp->zp_compress;
+	boolean_t dedup = zp->zp_dedup;
 	blkptr_t *bp = zio->io_bp;
 	uint64_t lsize = zio->io_lsize;
 	uint64_t psize = zio->io_size;
@@ -2087,6 +2088,21 @@ zio_write_compress(zio_t *zio)
 			zio_push_transform(zio, cdata,
 			    psize, rounded, NULL);
 		}
+	} else if (dedup == B_TRUE) {
+		/*
+		 * Storing many references to an all zeros block in the dedup
+		 * table would be expensive, and each different size of block
+		 * would have its own checksum.
+		 * Instead, if dedup is enabled, store all zero blocks as
+		 * holes even if compression is not enabled.
+		 */
+		if (abd_cmp_zero(zio->io_abd, lsize) == 0) {
+			psize = 0;
+			compress = ZIO_COMPRESS_OFF;
+			dedup = B_FALSE;
+		} else {
+			psize = lsize;
+		}
 	} else {
 		ASSERT3U(psize, !=, 0);
 	}
@@ -2129,7 +2145,7 @@ zio_write_compress(zio_t *zio)
 		BP_SET_PSIZE(bp, psize);
 		BP_SET_COMPRESS(bp, compress);
 		BP_SET_CHECKSUM(bp, zp->zp_checksum);
-		BP_SET_DEDUP(bp, zp->zp_dedup);
+		BP_SET_DEDUP(bp, dedup);
 		BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);
 		if (zp->zp_dedup) {
 			ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);