Merge pull request #101 from versity/zab/data_prealloc_options

Zab/data prealloc options

kmod/src/alloc.h

@@ -19,14 +19,11 @@
 	(128ULL * 1024 * 1024 >> SCOUTFS_BLOCK_SM_SHIFT)
 
 /*
- * The largest aligned region that we'll try to allocate at the end of
- * the file as it's extended.  This is also limited to the current file
- * size so we can only waste at most twice the total file size when
- * files are less than this.  We try to keep this around the point of
- * diminishing returns in streaming performance of common data devices
- * to limit waste.
+ * The default size that we'll try to preallocate.  This is trying to
+ * hit the limit of large efficient device writes while minimizing
+ * wasted preallocation that is never used.
  */
-#define SCOUTFS_DATA_EXTEND_PREALLOC_LIMIT \
+#define SCOUTFS_DATA_PREALLOC_DEFAULT_BLOCKS \
 	(8ULL * 1024 * 1024 >> SCOUTFS_BLOCK_SM_SHIFT)
 
 /*

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 */

kmod/src/options.c

@@ -27,9 +27,12 @@
 #include "options.h"
 #include "super.h"
 #include "inode.h"
+#include "alloc.h"
 
 enum {
 	Opt_acl,
+	Opt_data_prealloc_blocks,
+	Opt_data_prealloc_contig_only,
 	Opt_metadev_path,
 	Opt_noacl,
 	Opt_orphan_scan_delay_ms,
@@ -39,6 +42,8 @@ enum {
 
 static const match_table_t tokens = {
 	{Opt_acl, "acl"},
+	{Opt_data_prealloc_blocks, "data_prealloc_blocks=%s"},
+	{Opt_data_prealloc_contig_only, "data_prealloc_contig_only=%s"},
 	{Opt_metadev_path, "metadev_path=%s"},
 	{Opt_noacl, "noacl"},
 	{Opt_orphan_scan_delay_ms, "orphan_scan_delay_ms=%s"},
@@ -110,9 +115,15 @@ static void free_options(struct scoutfs_mount_options *opts)
 #define DEFAULT_ORPHAN_SCAN_DELAY_MS	(10 * MSEC_PER_SEC)
 #define MAX_ORPHAN_SCAN_DELAY_MS	(60 * MSEC_PER_SEC)
 
+#define MIN_DATA_PREALLOC_BLOCKS	1ULL
+#define MAX_DATA_PREALLOC_BLOCKS	((unsigned long long)SCOUTFS_BLOCK_SM_MAX)
+
 static void init_default_options(struct scoutfs_mount_options *opts)
 {
 	memset(opts, 0, sizeof(*opts));
+
+	opts->data_prealloc_blocks = SCOUTFS_DATA_PREALLOC_DEFAULT_BLOCKS;
+	opts->data_prealloc_contig_only = 1;
 	opts->quorum_slot_nr = -1;
 	opts->orphan_scan_delay_ms = -1;
 }
@@ -126,6 +137,7 @@ static void init_default_options(struct scoutfs_mount_options *opts)
 static int parse_options(struct super_block *sb, char *options, struct scoutfs_mount_options *opts)
 {
 	substring_t args[MAX_OPT_ARGS];
+	u64 nr64;
 	int nr;
 	int token;
 	char *p;
@@ -142,6 +154,30 @@ static int parse_options(struct super_block *sb, char *options, struct scoutfs_m
 			sb->s_flags |= MS_POSIXACL;
 			break;
 
+		case Opt_data_prealloc_blocks:
+			ret = match_u64(args, &nr64);
+			if (ret < 0 ||
+			    nr64 < MIN_DATA_PREALLOC_BLOCKS || nr64 > MAX_DATA_PREALLOC_BLOCKS) {
+				scoutfs_err(sb, "invalid data_prealloc_blocks option, must be between %llu and %llu",
+					    MIN_DATA_PREALLOC_BLOCKS, MAX_DATA_PREALLOC_BLOCKS);
+				if (ret == 0)
+					ret = -EINVAL;
+				return ret;
+			}
+			opts->data_prealloc_blocks = nr64;
+			break;
+
+		case Opt_data_prealloc_contig_only:
+			ret = match_int(args, &nr);
+			if (ret < 0 || nr < 0 || nr > 1) {
+				scoutfs_err(sb, "invalid data_prealloc_contig_only option, bool must only be 0 or 1");
+				if (ret == 0)
+					ret = -EINVAL;
+				return ret;
+			}
+			opts->data_prealloc_contig_only = nr;
+			break;
+
 		case Opt_metadev_path:
 			ret = parse_bdev_path(sb, &args[0], &opts->metadev_path);
 			if (ret < 0)
@@ -271,6 +307,8 @@ int scoutfs_options_show(struct seq_file *seq, struct dentry *root)
 
 	if (is_acl)
 		seq_puts(seq, ",acl");
+	seq_printf(seq, ",data_prealloc_blocks=%llu", opts.data_prealloc_blocks);
+	seq_printf(seq, ",data_prealloc_contig_only=%u", opts.data_prealloc_contig_only);
 	seq_printf(seq, ",metadev_path=%s", opts.metadev_path);
 	if (!is_acl)
 		seq_puts(seq, ",noacl");
@@ -281,6 +319,83 @@ int scoutfs_options_show(struct seq_file *seq, struct dentry *root)
 	return 0;
 }
 
+static ssize_t data_prealloc_blocks_show(struct kobject *kobj, struct kobj_attribute *attr,
+					 char *buf)
+{
+	struct super_block *sb = SCOUTFS_SYSFS_ATTRS_SB(kobj);
+	struct scoutfs_mount_options opts;
+
+	scoutfs_options_read(sb, &opts);
+
+	return snprintf(buf, PAGE_SIZE, "%llu", opts.data_prealloc_blocks);
+}
+static ssize_t data_prealloc_blocks_store(struct kobject *kobj, struct kobj_attribute *attr,
+					  const char *buf, size_t count)
+{
+	struct super_block *sb = SCOUTFS_SYSFS_ATTRS_SB(kobj);
+	DECLARE_OPTIONS_INFO(sb, optinf);
+	char nullterm[30]; /* more than enough for octal -U64_MAX */
+	u64 val;
+	int len;
+	int ret;
+
+	len = min(count, sizeof(nullterm) - 1);
+	memcpy(nullterm, buf, len);
+	nullterm[len] = '\0';
+
+	ret = kstrtoll(nullterm, 0, &val);
+	if (ret < 0 || val < MIN_DATA_PREALLOC_BLOCKS || val > MAX_DATA_PREALLOC_BLOCKS) {
+		scoutfs_err(sb, "invalid data_prealloc_blocks option, must be between %llu and %llu",
+			    MIN_DATA_PREALLOC_BLOCKS, MAX_DATA_PREALLOC_BLOCKS);
+		return -EINVAL;
+	}
+
+	write_seqlock(&optinf->seqlock);
+	optinf->opts.data_prealloc_blocks = val;
+	write_sequnlock(&optinf->seqlock);
+
+	return count;
+}
+SCOUTFS_ATTR_RW(data_prealloc_blocks);
+
+static ssize_t data_prealloc_contig_only_show(struct kobject *kobj, struct kobj_attribute *attr,
+					 char *buf)
+{
+	struct super_block *sb = SCOUTFS_SYSFS_ATTRS_SB(kobj);
+	struct scoutfs_mount_options opts;
+
+	scoutfs_options_read(sb, &opts);
+
+	return snprintf(buf, PAGE_SIZE, "%u", opts.data_prealloc_contig_only);
+}
+static ssize_t data_prealloc_contig_only_store(struct kobject *kobj, struct kobj_attribute *attr,
+					  const char *buf, size_t count)
+{
+	struct super_block *sb = SCOUTFS_SYSFS_ATTRS_SB(kobj);
+	DECLARE_OPTIONS_INFO(sb, optinf);
+	char nullterm[20]; /* more than enough for octal -U32_MAX */
+	long val;
+	int len;
+	int ret;
+
+	len = min(count, sizeof(nullterm) - 1);
+	memcpy(nullterm, buf, len);
+	nullterm[len] = '\0';
+
+	ret = kstrtol(nullterm, 0, &val);
+	if (ret < 0 || val < 0 || val > 1) {
+		scoutfs_err(sb, "invalid data_prealloc_contig_only option, bool must be 0 or 1");
+		return -EINVAL;
+	}
+
+	write_seqlock(&optinf->seqlock);
+	optinf->opts.data_prealloc_contig_only = val;
+	write_sequnlock(&optinf->seqlock);
+
+	return count;
+}
+SCOUTFS_ATTR_RW(data_prealloc_contig_only);
+
 static ssize_t metadev_path_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
 {
 	struct super_block *sb = SCOUTFS_SYSFS_ATTRS_SB(kobj);
@@ -345,6 +460,8 @@ static ssize_t quorum_slot_nr_show(struct kobject *kobj, struct kobj_attribute *
 SCOUTFS_ATTR_RO(quorum_slot_nr);
 
 static struct attribute *options_attrs[] = {
+	SCOUTFS_ATTR_PTR(data_prealloc_blocks),
+	SCOUTFS_ATTR_PTR(data_prealloc_contig_only),
 	SCOUTFS_ATTR_PTR(metadev_path),
 	SCOUTFS_ATTR_PTR(orphan_scan_delay_ms),
 	SCOUTFS_ATTR_PTR(quorum_slot_nr),

kmod/src/options.h

@@ -6,6 +6,8 @@
 #include "format.h"
 
 struct scoutfs_mount_options {
+	u64 data_prealloc_blocks;
+	bool data_prealloc_contig_only;
 	char *metadev_path;
 	unsigned int orphan_scan_delay_ms;
 	int quorum_slot_nr;

tests/funcs/fs.sh

@@ -405,7 +405,7 @@ t_save_all_sysfs_mount_options() {
 
 	for i in $(t_fs_nrs); do
 		opt="$(t_sysfs_path $i)/mount_options/$name"
-		ind="$name_$i"
+		ind="${name}_${i}"
 
 		_saved_opts[$ind]="$(cat $opt)"
 	done
@@ -417,7 +417,7 @@ t_restore_all_sysfs_mount_options() {
 	local i
 
 	for i in $(t_fs_nrs); do
-		ind="$name_$i"
+		ind="${name}_${i}"
 
 		t_set_sysfs_mount_option $i $name "${_saved_opts[$ind]}"
 	done

tests/golden/data-prealloc

@@ -0,0 +1,26 @@
+== initial writes smaller than prealloc grow to prealloc size
+/mnt/test/test/data-prealloc/file-1: 7 extents found
+/mnt/test/test/data-prealloc/file-2: 7 extents found
+== larger files get full prealloc extents
+/mnt/test/test/data-prealloc/file-1: 9 extents found
+/mnt/test/test/data-prealloc/file-2: 9 extents found
+== non-streaming writes with contig have per-block extents
+/mnt/test/test/data-prealloc/file-1: 32 extents found
+/mnt/test/test/data-prealloc/file-2: 32 extents found
+== any writes to region prealloc get full extents
+/mnt/test/test/data-prealloc/file-1: 4 extents found
+/mnt/test/test/data-prealloc/file-2: 4 extents found
+/mnt/test/test/data-prealloc/file-1: 4 extents found
+/mnt/test/test/data-prealloc/file-2: 4 extents found
+== streaming offline writes get full extents either way
+/mnt/test/test/data-prealloc/file-1: 4 extents found
+/mnt/test/test/data-prealloc/file-2: 4 extents found
+/mnt/test/test/data-prealloc/file-1: 4 extents found
+/mnt/test/test/data-prealloc/file-2: 4 extents found
+== goofy preallocation amounts work
+/mnt/test/test/data-prealloc/file-1: 5 extents found
+/mnt/test/test/data-prealloc/file-2: 5 extents found
+/mnt/test/test/data-prealloc/file-1: 5 extents found
+/mnt/test/test/data-prealloc/file-2: 5 extents found
+/mnt/test/test/data-prealloc/file-1: 3 extents found
+/mnt/test/test/data-prealloc/file-2: 3 extents found

tests/run-tests.sh

@@ -58,6 +58,7 @@ $(basename $0) options:
     -m        | Run mkfs on the device before mounting and running
               | tests.  Implies unmounting existing mounts first.
     -n <nr>   | The number of devices and mounts to test.
+    -o <opts> | Add option string to all mounts during all tests.
     -P        | Enable trace_printk.
     -p        | Exit script after preparing mounts only, don't run tests.
     -q <nr>   | The first <nr> mounts will be quorum members.  Must be
@@ -68,6 +69,7 @@ $(basename $0) options:
     -s        | Skip git repo checkouts.
     -t        | Enabled trace events that match the given glob argument.
               | Multiple options enable multiple globbed events.
+    -T <nr>   | Multiply the original trace buffer size by nr during the run.
     -X        | xfstests git repo. Used by tests/xfstests.sh.
     -x        | xfstests git branch to checkout and track.
     -y        | xfstests ./check additional args
@@ -136,6 +138,12 @@ while true; do
 		T_NR_MOUNTS="$2"
 		shift
 		;;
+	-o)
+		test -n "$2" || die "-o must have option string argument"
+		# always appending to existing options
+		T_MNT_OPTIONS+=",$2"
+		shift
+		;;
 	-P)
 		T_TRACE_PRINTK="1"
 		;;
@@ -160,6 +168,11 @@ while true; do
 		T_TRACE_GLOB+=("$2")
 		shift
 		;;
+	-T)
+		test -n "$2" || die "-T must have trace buffer size multiplier argument"
+		T_TRACE_MULT="$2"
+		shift
+		;;
 	-X)
 		test -n "$2" || die "-X requires xfstests git repo dir argument"
 		T_XFSTESTS_REPO="$2"
@@ -345,6 +358,13 @@ if [ -n "$T_INSMOD" ]; then
 	cmd insmod "$T_KMOD/src/scoutfs.ko"
 fi
 
+if [ -n "$T_TRACE_MULT" ]; then
+	orig_trace_size=$(cat /sys/kernel/debug/tracing/buffer_size_kb)
+	mult_trace_size=$((orig_trace_size * T_TRACE_MULT))
+	msg "increasing trace buffer size from $orig_trace_size KiB to $mult_trace_size KiB"
+	echo $mult_trace_size > /sys/kernel/debug/tracing/buffer_size_kb
+fi
+
 nr_globs=${#T_TRACE_GLOB[@]}
 if [ $nr_globs -gt 0 ]; then
 	echo 0 > /sys/kernel/debug/tracing/events/scoutfs/enable
@@ -374,6 +394,7 @@ fi
 # always describe tracing in the logs
 cmd cat /sys/kernel/debug/tracing/set_event
 cmd grep .  /sys/kernel/debug/tracing/options/trace_printk \
+	    /sys/kernel/debug/tracing/buffer_size_kb \
 	    /proc/sys/kernel/ftrace_dump_on_oops
 
 #
@@ -430,6 +451,7 @@ for i in $(seq 0 $((T_NR_MOUNTS - 1))); do
 	if [ "$i" -lt "$T_QUORUM" ]; then
 		opts="$opts,quorum_slot_nr=$i"
 	fi
+	opts="${opts}${T_MNT_OPTIONS}"
 
 	msg "mounting $meta_dev|$data_dev on $dir"
 	cmd mount -t scoutfs $opts "$data_dev" "$dir" &
@@ -604,6 +626,9 @@ if [ -n "$T_TRACE_GLOB" -o -n "$T_TRACE_PRINTK" ]; then
 	echo 0 > /sys/kernel/debug/tracing/events/scoutfs/enable
 	echo 0 > /sys/kernel/debug/tracing/options/trace_printk
 	cat /sys/kernel/debug/tracing/trace > "$T_RESULTS/traces"
+	if [ -n "$orig_trace_size" ]; then
+		echo $orig_trace_size > /sys/kernel/debug/tracing/buffer_size_kb
+	fi
 fi
 
 if [ "$skipped" == 0 -a "$failed" == 0 ]; then

tests/sequence

@@ -6,6 +6,7 @@ simple-inode-index.sh
 simple-staging.sh
 simple-release-extents.sh
 fallocate.sh
+data-prealloc.sh
 setattr_more.sh
 offline-extent-waiting.sh
 move-blocks.sh

tests/tests/data-prealloc.sh

@@ -0,0 +1,136 @@
+#
+# test that the data prealloc options behave as expected.  We write to
+# two files a block at a time so that a single file doesn't naturally
+# merge adjacent consecutive allocations.  (we don't have multiple
+# allocation cursors)
+#
+t_require_commands scoutfs stat filefrag dd touch truncate
+
+write_forwards()
+{
+	local prefix="$1"
+	local nr="$2"
+	local blk
+
+	touch "$prefix"-{1,2}
+	truncate -s 0 "$prefix"-{1,2}
+
+	for blk in $(seq 0 1 $((nr - 1))); do
+		dd if=/dev/zero of="$prefix"-1 bs=4096 seek=$blk count=1 conv=notrunc status=none
+		dd if=/dev/zero of="$prefix"-2 bs=4096 seek=$blk count=1 conv=notrunc status=none
+	done
+}
+
+write_backwards()
+{
+	local prefix="$1"
+	local nr="$2"
+	local blk
+
+	touch "$prefix"-{1,2}
+	truncate -s 0 "$prefix"-{1,2}
+
+	for blk in $(seq $((nr - 1)) -1 0); do
+		dd if=/dev/zero of="$prefix"-1 bs=4096 seek=$blk count=1 conv=notrunc status=none
+		dd if=/dev/zero of="$prefix"-2 bs=4096 seek=$blk count=1 conv=notrunc status=none
+	done
+}
+
+release_files() {
+	local prefix="$1"
+	local size=$(($2 * 4096))
+	local vers
+	local f
+
+	for f in "$prefix"*; do
+		size=$(stat -c "%s" "$f")
+		vers=$(scoutfs stat -s data_version "$f")
+		scoutfs release "$f" -V "$vers" -o 0 -l $size
+	done
+}
+
+stage_files() {
+	local prefix="$1"
+	local nr="$2"
+	local vers
+	local f
+
+	for blk in $(seq 0 1 $((nr - 1))); do
+		for f in "$prefix"*; do
+			vers=$(scoutfs stat -s data_version "$f")
+			scoutfs stage /dev/zero "$f" -V "$vers" -o $((blk * 4096)) -l 4096
+		done
+	done
+}
+
+print_extents_found()
+{
+	local prefix="$1"
+
+	filefrag "$prefix"* 2>&1 | grep "extent.*found" | t_filter_fs
+}
+
+t_save_all_sysfs_mount_options data_prealloc_blocks
+t_save_all_sysfs_mount_options data_prealloc_contig_only
+restore_options()
+{
+	t_restore_all_sysfs_mount_options data_prealloc_blocks
+	t_restore_all_sysfs_mount_options data_prealloc_contig_only
+}
+trap restore_options EXIT
+
+prefix="$T_D0/file"
+
+echo "== initial writes smaller than prealloc grow to prealloc size"
+t_set_sysfs_mount_option 0 data_prealloc_blocks 32
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 1
+write_forwards $prefix 64
+print_extents_found $prefix
+
+echo "== larger files get full prealloc extents"
+t_set_sysfs_mount_option 0 data_prealloc_blocks 32
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 1
+write_forwards $prefix 128
+print_extents_found $prefix
+
+echo "== non-streaming writes with contig have per-block extents"
+t_set_sysfs_mount_option 0 data_prealloc_blocks 32
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 1
+write_backwards $prefix 32
+print_extents_found $prefix
+
+echo "== any writes to region prealloc get full extents"
+t_set_sysfs_mount_option 0 data_prealloc_blocks 16
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 0
+write_forwards $prefix 64
+print_extents_found $prefix
+write_backwards $prefix 64
+print_extents_found $prefix
+
+echo "== streaming offline writes get full extents either way"
+t_set_sysfs_mount_option 0 data_prealloc_blocks 16
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 1
+write_forwards $prefix 64
+release_files $prefix 64
+stage_files $prefix 64
+print_extents_found $prefix
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 0
+release_files $prefix 64
+stage_files $prefix 64
+print_extents_found $prefix
+
+echo "== goofy preallocation amounts work"
+t_set_sysfs_mount_option 0 data_prealloc_blocks 7
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 1
+write_forwards $prefix 14
+print_extents_found $prefix
+t_set_sysfs_mount_option 0 data_prealloc_blocks 13
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 0
+write_forwards $prefix 53
+print_extents_found $prefix
+t_set_sysfs_mount_option 0 data_prealloc_blocks 1
+t_set_sysfs_mount_option 0 data_prealloc_contig_only 0
+write_forwards $prefix 3
+print_extents_found $prefix
+
+t_pass

utils/man/scoutfs.5

@@ -21,6 +21,40 @@ as detailed in
 .BR acl (5) .
 Support for POSIX ACLs is the default.
 .TP
+.B data_prealloc_blocks=<blocks>
+Set the size of preallocation regions of data files, in 4KiB blocks.
+Writes to these regions that contain no extents will attempt to
+preallocate the size of the full region.  This can waste a lot of space
+with small files, files with sparse regions, and files whose final
+length isn't a multiple of the preallocation size.  The following
+data_prealloc_contig_only option, which is the default, restricts this
+behaviour to waste less space.
+.sp
+All the preallocation options can be changed in an active mount by
+writing to their respective files in the options directory in the
+mount's sysfs directory.
+.sp
+It is worth noting that it is always more efficient in every way to use 
+.BR fallocate (2)
+to precisely allocate large extents for the resulting size of the file.
+Always attempt to enable it in software that supports it.
+.TP
+.B data_prealloc_contig_only=<0|1>
+This option, currently the default, limits file data preallocation in
+two ways.  First, it will only preallocate when extending a fully
+allocated file.  Second, it will limit the size of preallocation to the
+existing length of the file.  These limits reduce the amount of
+preallocation wasted per file at the cost of multiple initial extents in
+all files.  It only supports simple streaming writes, any other write
+pattern will not be recognized and could result in many fragmented
+extent allocations.
+.sp
+This option can be disabled to encourage large allocated extents
+regardless of write patterns.  This can be helpful if files are written
+with initial sparse regions (perhaps by multiple threads writing to
+different regions) and wasted space isn't an issue (perhaps because the
+file population contains few small files).
+.TP
 .B metadev_path=<device>
 The metadev_path option specifies the path to the block device that
 contains the filesystem's metadata.