Make data preallocation tunable

Make mount options for the size of preallocation and whether or not it
should be restricted to extending writes.  Disabling the default
restriction to streaming writes lets it preallocate in aligned regions
of the preallocation size when they contain no extents.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/data.c

@@ -366,27 +366,27 @@ static inline u64 ext_last(struct scoutfs_extent *ext)
 
 /*
  * The caller is writing to a logical iblock that doesn't have an
- * allocated extent.
+ * allocated extent.  The caller has searched for an extent containing
+ * iblock.  If it already existed then it must be unallocated and
+ * offline.
  *
- * We always allocate an extent starting at the logical iblock.  The
- * caller has searched for an extent containing iblock.  If it already
- * existed then it must be unallocated and offline.
+ * We implement two preallocation strategies.  Typically we only
+ * preallocate for simple streaming writes and limit preallocation while
+ * the file is small.   The largest efficient allocation size is
+ * typically large enough that it would be unreasonable to allocate that
+ * much for all small files.
  *
- * Preallocation is used if we're strictly contiguously extending
- * writes.  That is, if the logical block offset equals the number of
- * online blocks.  We try to preallocate the number of blocks existing
- * so that small files don't waste inordinate amounts of space and large
- * files will eventually see large extents.  This only works for
- * contiguous single stream writes or stages of files from the first
- * block.  It doesn't work for concurrent stages, releasing behind
- * staging, sparse files, multi-node writes, etc.  fallocate() is always
- * a better tool to use.
+ * Optionally, we can simply preallocate large empty aligned regions.
+ * This can waste a lot of space for small or sparse files but is
+ * reasonable when a file population is known to be large and dense but
+ * known to be written with non-streaming write patterns.
  */
 static int alloc_block(struct super_block *sb, struct inode *inode,
 		       struct scoutfs_extent *ext, u64 iblock,
 		       struct scoutfs_lock *lock)
 {
 	DECLARE_DATA_INFO(sb, datinf);
+	struct scoutfs_mount_options opts;
 	const u64 ino = scoutfs_ino(inode);
 	struct data_ext_args args = {
 		.ino = ino,
@@ -394,17 +394,22 @@ static int alloc_block(struct super_block *sb, struct inode *inode,
 		.lock = lock,
 	};
 	struct scoutfs_extent found;
-	struct scoutfs_extent pre;
+	struct scoutfs_extent pre = {0,};
+	bool undo_pre = false;
 	u64 blkno = 0;
 	u64 online;
 	u64 offline;
 	u8 flags;
+	u64 start;
 	u64 count;
+	u64 rem;
 	int ret;
 	int err;
 
 	trace_scoutfs_data_alloc_block_enter(sb, ino, iblock, ext);
 
+	scoutfs_options_read(sb, &opts);
+
 	/* can only allocate over existing unallocated offline extent */
 	if (WARN_ON_ONCE(ext->len &&
 			 !(iblock >= ext->start && iblock <= ext_last(ext) &&
@@ -413,66 +418,106 @@ static int alloc_block(struct super_block *sb, struct inode *inode,
 
 	mutex_lock(&datinf->mutex);
 
-	scoutfs_inode_get_onoff(inode, &online, &offline);
+	/* default to single allocation at the written block */
+	start = iblock;
+	count = 1;
+	/* copy existing flags for preallocated regions */
+	flags = ext->len ? ext->flags : 0;
 
 	if (ext->len) {
-		/* limit preallocation to remaining existing (offline) extent */
+		/*
+		 * Assume that offline writers are going to be writing
+		 * all the offline extents and try to preallocate the
+		 * rest of the unwritten extent.
+		 */
 		count = ext->len - (iblock - ext->start);
-		flags = ext->flags;
+
+	} else if (opts.data_prealloc_contig_only) {
+		/*
+		 * Only preallocate when a quick test of the online
+		 * block counts looks like we're a simple streaming
+		 * write.  Try to write until the next extent but limit
+		 * the preallocation size to the number of online
+		 * blocks.
+		 */
+		scoutfs_inode_get_onoff(inode, &online, &offline);
+		if (iblock > 1 && iblock == online) {
+			ret = scoutfs_ext_next(sb, &data_ext_ops, &args,
+					       iblock, 1, &found);
+			if (ret < 0 && ret != -ENOENT)
+				goto out;
+			if (found.len && found.start > iblock)
+				count = found.start - iblock;
+			else
+				count = opts.data_prealloc_blocks;
+
+			count = min(iblock, count);
+		}
+
 	} else {
-		/* otherwise alloc to next extent */
-		ret = scoutfs_ext_next(sb, &data_ext_ops, &args,
-				       iblock, 1, &found);
+		/*
+		 * Preallocation of aligned regions only preallocates if
+		 * the aligned region contains no extents at all.  This
+		 * could be fooled by offline sparse extents but we
+		 * don't want to iterate over all offline extents in the
+		 * aligned region.
+		 */
+		div64_u64_rem(iblock, opts.data_prealloc_blocks, &rem);
+		start = iblock - rem;
+		count = opts.data_prealloc_blocks;
+		ret = scoutfs_ext_next(sb, &data_ext_ops, &args, start, 1, &found);
 		if (ret < 0 && ret != -ENOENT)
 			goto out;
-		if (found.len && found.start > iblock)
-			count = found.start - iblock;
-		else
-			count = SCOUTFS_DATA_EXTEND_PREALLOC_LIMIT;
-		flags = 0;
+		if (found.len && found.start < start + count)
+			count = 1;
 	}
 
 	/* overall prealloc limit */
-	count = min_t(u64, count, SCOUTFS_DATA_EXTEND_PREALLOC_LIMIT);
-
-	/* only strictly contiguous extending writes will try to preallocate */
-	if (iblock > 1 && iblock == online)
-		count = min(iblock, count);
-	else
-		count = 1;
+	count = min_t(u64, count, opts.data_prealloc_blocks);
 
 	ret = scoutfs_alloc_data(sb, datinf->alloc, datinf->wri,
 				 &datinf->dalloc, count, &blkno, &count);
 	if (ret < 0)
 		goto out;
 
-	ret = scoutfs_ext_set(sb, &data_ext_ops, &args, iblock, 1, blkno, 0);
-	if (ret < 0)
-		goto out;
+	/*
+	 * An aligned prealloc attempt that gets a smaller extent can
+	 * fail to cover iblock, make sure that it does.  This is a
+	 * pathological case so we don't try to move the window past
+	 * iblock.  Just enough to cover it, which we know is safe.
+	 */
+	if (start + count <= iblock)
+		start += (iblock - (start + count) + 1);
 
 	if (count > 1) {
-		pre.start = iblock + 1;
-		pre.len = count - 1;
-		pre.map = blkno + 1;
+		pre.start = start;
+		pre.len = count;
+		pre.map = blkno;
 		pre.flags = flags | SEF_UNWRITTEN;
 		ret = scoutfs_ext_set(sb, &data_ext_ops, &args, pre.start,
 				      pre.len, pre.map, pre.flags);
-		if (ret < 0) {
-			err = scoutfs_ext_set(sb, &data_ext_ops, &args, iblock,
-					      1, 0, flags);
-			BUG_ON(err); /* couldn't restore original */
+		if (ret < 0)
 			goto out;
-		}
+		undo_pre = true;
 	}
 
+	ret = scoutfs_ext_set(sb, &data_ext_ops, &args, iblock, 1, blkno + (iblock - start), 0);
+	if (ret < 0)
+		goto out;
+
 	/* tell the caller we have a single block, could check next? */
 	ext->start = iblock;
 	ext->len = 1;
-	ext->map = blkno;
+	ext->map = blkno + (iblock - start);
 	ext->flags = 0;
 	ret = 0;
 out:
 	if (ret < 0 && blkno > 0) {
+		if (undo_pre) {
+			err = scoutfs_ext_set(sb, &data_ext_ops, &args,
+					      pre.start, pre.len, 0, flags);
+			BUG_ON(err); /* leaked preallocated extent */
+		}
 		err = scoutfs_free_data(sb, datinf->alloc, datinf->wri,
 				        &datinf->data_freed, blkno, count);
 		BUG_ON(err); /* leaked free blocks */