diff --git a/Makefile b/Makefile
index 94c63fc..efb279f 100644
--- a/Makefile
+++ b/Makefile
@@ -22,7 +22,7 @@ C = $(shell find . '(' -name '*.c' -o -name '*.cc' ')')
 INCLUDES = -I/usr/local/include
 LIBS = -L/usr/local/lib
 
-tippecanoe: geojson.o jsonpull.o vector_tile.pb.o tile.o clip.o pool.o mbtiles.o geometry.o projection.o
+tippecanoe: geojson.o jsonpull.o vector_tile.pb.o tile.o clip.o pool.o mbtiles.o geometry.o projection.o memfile.o
 	g++ $(PG) $(LIBS) -O3 -g -Wall -o $@ $^ -lm -lz -lprotobuf-lite -lsqlite3
 
 enumerate: enumerate.o
diff --git a/geojson.c b/geojson.c
index 46b8184..1146360 100644
--- a/geojson.c
+++ b/geojson.c
@@ -20,6 +20,7 @@
 #include "mbtiles.h"
 #include "projection.h"
 #include "version.h"
+#include "memfile.h"
 
 int low_detail = 10;
 int full_detail = -1;
@@ -396,43 +397,71 @@ static void merge(struct merge *merges, int nmerges, unsigned char *map, FILE *f
 }
 
 struct stringpool {
-	char *s;  // first byte is type
-	struct stringpool *left;
-	struct stringpool *right;
+	long long left;
+	long long right;
 	long long off;
-} *pooltree = NULL;
+};
+long long pooltree = 0;
 
-long long addpool(FILE *poolfile, long long *poolpos, char *s, char type) {
-	struct stringpool **sp = &pooltree;
+long long addpool(struct memfile *poolfile, char *s, char type) {
+	long long *sp = &pooltree;
 
-	while (*sp != NULL) {
-		int cmp = strcmp(s, (*sp)->s + 1);
+	while (*sp != 0) {
+		int cmp = strcmp(s, poolfile->map + ((struct stringpool *) (poolfile->map + *sp))->off + 1);
 
 		if (cmp == 0) {
-			cmp = type - (*sp)->s[0];
+			cmp = type - (poolfile->map + ((struct stringpool *) (poolfile->map + *sp))->off)[0];
 		}
 
 		if (cmp < 0) {
-			sp = &((*sp)->left);
+			sp = &(((struct stringpool *) (poolfile->map + *sp))->left);
 		} else if (cmp > 0) {
-			sp = &((*sp)->right);
+			sp = &(((struct stringpool *) (poolfile->map + *sp))->right);
 		} else {
-			return (*sp)->off;
+			return ((struct stringpool *) (poolfile->map + *sp))->off;
 		}
 	}
 
-	*sp = malloc(sizeof(struct stringpool));
-	(*sp)->s = malloc(strlen(s) + 2);
-	(*sp)->s[0] = type;
-	strcpy((*sp)->s + 1, s);  // XXX really should be mapped from the pool itself
-	(*sp)->left = NULL;
-	(*sp)->right = NULL;
-	(*sp)->off = *poolpos;
+	// *sp is probably in the memory-mapped file, and will move if the file grows.
+	long long ssp;
+	if (sp == &pooltree) {
+		ssp = -1;
+	} else {
+		ssp = ((char *) sp) - poolfile->map;
+	}
 
-	fwrite_check((*sp)->s, strlen(s) + 2, sizeof(char), poolfile, "string pool");
-	*poolpos += strlen(s) + 2;
+	long long off = poolfile->off;
+	if (memfile_write(poolfile, &type, 1) < 0) {
+		perror("memfile write");
+		exit(EXIT_FAILURE);
+	}
+	if (memfile_write(poolfile, s, strlen(s) + 1) < 0) {
+		perror("memfile write");
+		exit(EXIT_FAILURE);
+	}
 
-	return (*sp)->off;
+	struct stringpool tsp;
+	tsp.left = 0;
+	tsp.right = 0;
+	tsp.off = off;
+
+	// alignment
+	while (poolfile->off % sizeof(long long) != 0) {
+		memfile_write(poolfile, "", 1);
+	}
+
+	long long p = poolfile->off;
+	if (memfile_write(poolfile, &tsp, sizeof(struct stringpool)) < 0) {
+		perror("memfile write");
+		exit(EXIT_FAILURE);
+	}
+
+	if (ssp == -1) {
+		pooltree = p;
+	} else {
+		*((long long *) (poolfile->map + ssp)) = p;
+	}
+	return off;
 }
 
 int read_json(int argc, char **argv, char *fname, const char *layername, int maxzoom, int minzoom, sqlite3 *outdb, struct pool *exclude, struct pool *include, int exclude_all, double droprate, int buffer, const char *tmpdir, double gamma, char *prevent) {
@@ -474,7 +503,7 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		perror(metaname);
 		exit(EXIT_FAILURE);
 	}
-	FILE *poolfile = fopen(poolname, "wb");
+	struct memfile *poolfile = memfile_open(poolfd);
 	if (poolfile == NULL) {
 		perror(poolname);
 		exit(EXIT_FAILURE);
@@ -490,18 +519,16 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		exit(EXIT_FAILURE);
 	}
 	long long metapos = 0;
-	long long poolpos = 0;
 	long long geompos = 0;
 	long long indexpos = 0;
 
 	unlink(metaname);
-	//unlink(poolname);
+	unlink(poolname);
 	unlink(geomname);
 	unlink(indexname);
 
-	// So we still have a legitimate map even if no metadata
-	fprintf(poolfile, "\n");
-	poolpos++;
+	// To distinguish a null value
+	memfile_write(poolfile, "", 1);
 
 	unsigned file_bbox[] = {UINT_MAX, UINT_MAX, 0, 0};
 	unsigned midx = 0, midy = 0;
@@ -663,8 +690,8 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 
 				serialize_int(metafile, m, &metapos, fname);
 				for (i = 0; i < m; i++) {
-					serialize_long_long(metafile, addpool(poolfile, &poolpos, metakey[i], VT_STRING), &metapos, fname);
-					serialize_long_long(metafile, addpool(poolfile, &poolpos, metaval[i], metatype[i]), &metapos, fname);
+					serialize_long_long(metafile, addpool(poolfile, metakey[i], VT_STRING), &metapos, fname);
+					serialize_long_long(metafile, addpool(poolfile, metaval[i], metatype[i]), &metapos, fname);
 				}
 
 				long long geomstart = geompos;
@@ -739,13 +766,11 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 	}
 
 	fclose(metafile);
-	fclose(poolfile);
 	fclose(geomfile);
 	fclose(indexfile);
 
 	struct stat geomst;
 	struct stat metast;
-	struct stat poolst;
 
 	if (fstat(geomfd, &geomst) != 0) {
 		perror("stat geom\n");
@@ -755,10 +780,6 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		perror("stat meta\n");
 		exit(EXIT_FAILURE);
 	}
-	if (fstat(poolfd, &poolst) != 0) {
-		perror("stat pool\n");
-		exit(EXIT_FAILURE);
-	}
 
 	if (geomst.st_size == 0 || metast.st_size == 0) {
 		fprintf(stderr, "did not read any valid geometries\n");
@@ -771,11 +792,7 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		exit(EXIT_FAILURE);
 	}
 
-	char *stringpool = (char *) mmap(NULL, poolst.st_size, PROT_READ, MAP_PRIVATE, poolfd, 0);
-	if (stringpool == MAP_FAILED) {
-		perror("mmap stringpool");
-		exit(EXIT_FAILURE);
-	}
+	char *stringpool = poolfile->map;
 
 	struct pool file_keys1[nlayers];
 	struct pool *file_keys[nlayers];
@@ -856,6 +873,7 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 			merges[start / unit].end = end;
 			merges[start / unit].next = NULL;
 
+			// MAP_PRIVATE to avoid disk writes if it fits in memory
 			void *map = mmap(NULL, end - start, PROT_READ | PROT_WRITE, MAP_PRIVATE, indexfd, start);
 			if (map == MAP_FAILED) {
 				perror("mmap");
@@ -999,7 +1017,7 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		size[j] = 0;
 	}
 
-	fprintf(stderr, "%lld features, %lld bytes of geometry, %lld bytes of metadata, %lld bytes of string pool\n", seq, (long long) geomst.st_size, (long long) metast.st_size, (long long) poolst.st_size);
+	fprintf(stderr, "%lld features, %lld bytes of geometry, %lld bytes of metadata, %lld bytes of string pool\n", seq, (long long) geomst.st_size, (long long) metast.st_size, poolfile->off);
 
 	int written = traverse_zooms(fd, size, meta, stringpool, file_bbox, file_keys, &midx, &midy, layernames, maxzoom, minzoom, outdb, droprate, buffer, fname, tmpdir, gamma, nlayers, prevent);
 
@@ -1016,10 +1034,7 @@ int read_json(int argc, char **argv, char *fname, const char *layername, int max
 		perror("close meta");
 	}
 
-	if (munmap(stringpool, poolst.st_size) != 0) {
-		perror("munmap pool");
-	}
-	if (close(poolfd) < 0) {
+	if (memfile_close(poolfile) != 0) {
 		perror("close pool");
 	}
 
@@ -1184,7 +1199,6 @@ int main(int argc, char **argv) {
 	}
 
 	geometry_scale = 32 - (full_detail + maxzoom);
-	printf("geometry scale is %d\n", geometry_scale);
 
 	if (outdir == NULL) {
 		fprintf(stderr, "%s: must specify -o out.mbtiles\n", argv[0]);
diff --git a/memfile.c b/memfile.c
new file mode 100644
index 0000000..454628c
--- /dev/null
+++ b/memfile.c
@@ -0,0 +1,69 @@
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include "memfile.h"
+
+#define INCREMENT 131072
+
+struct memfile *memfile_open(int fd) {
+	if (ftruncate(fd, INCREMENT) != 0) {
+		return NULL;
+	}
+
+	char *map = mmap(NULL, INCREMENT, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (map == MAP_FAILED) {
+		return NULL;
+	}
+
+	struct memfile *mf = malloc(sizeof(struct memfile));
+	if (mf == NULL) {
+		munmap(map, INCREMENT);
+		return NULL;
+	}
+
+	mf->fd = fd;
+	mf->map = map;
+	mf->len = INCREMENT;
+	mf->off = 0;
+
+	return mf;
+}
+
+int memfile_close(struct memfile *file) {
+	if (munmap(file->map, file->len) != 0) {
+		return -1;
+	}
+
+	if (file->fd >= 0) {
+		if (close(file->fd) != 0) {
+			return -1;
+		}
+	}
+
+	free(file);
+	return 0;
+}
+
+int memfile_write(struct memfile *file, void *s, long long len) {
+	if (file->off + len > file->len) {
+		if (munmap(file->map, file->len) != 0) {
+			return -1;
+		}
+
+		file->len += INCREMENT;
+
+		if (ftruncate(file->fd, file->len) != 0) {
+			return -1;
+		}
+
+		file->map = mmap(NULL, file->len, PROT_READ | PROT_WRITE, MAP_SHARED, file->fd, 0);
+		if (file->map == MAP_FAILED) {
+			return -1;
+		}
+	}
+
+	memcpy(file->map + file->off, s, len);
+	file->off += len;
+	return len;
+}
diff --git a/memfile.h b/memfile.h
new file mode 100644
index 0000000..b536a6c
--- /dev/null
+++ b/memfile.h
@@ -0,0 +1,10 @@
+struct memfile {
+	int fd;
+	char *map;
+	long long len;
+	long long off;
+};
+
+struct memfile *memfile_open(int fd);
+int memfile_close(struct memfile *file);
+int memfile_write(struct memfile *file, void *s, long long len);