diff -r d96313048f17 src/lib-index/mail-cache-compress.c
--- a/src/lib-index/mail-cache-compress.c	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-cache-compress.c	Mon Oct 08 04:40:15 2012 +0300
@@ -353,6 +353,7 @@
 	uint32_t file_seq, old_offset;
 	ARRAY_TYPE(uint32_t) ext_offsets;
 	const uint32_t *offsets;
+	const void *data;
 	unsigned int i, count;
 	int fd, ret;
 
@@ -440,7 +441,7 @@
 	if (cache->file_cache != NULL)
 		file_cache_set_fd(cache->file_cache, cache->fd);
 
-	if (mail_cache_map(cache, 0, 0) < 0)
+	if (mail_cache_map(cache, 0, 0, &data) < 0)
 		return -1;
 	if (mail_cache_header_fields_read(cache) < 0)
 		return -1;
@@ -460,6 +461,9 @@
 	if (MAIL_INDEX_IS_IN_MEMORY(cache->index) || cache->index->readonly)
 		return 0;
 
+	/* compression isn't very efficient with small read()s */
+	cache->map_with_read = FALSE;
+
 	if (cache->index->lock_method == FILE_LOCK_METHOD_DOTLOCK) {
 		/* we're using dotlocking, cache file creation itself creates
 		   the dotlock file we need. */
@@ -494,5 +498,6 @@
 bool mail_cache_need_compress(struct mail_cache *cache)
 {
 	return cache->need_compress_file_seq != 0 &&
+		(cache->index->flags & MAIL_INDEX_OPEN_FLAG_SAVEONLY) == 0 &&
 		!cache->index->readonly;
 }
diff -r d96313048f17 src/lib-index/mail-cache-fields.c
--- a/src/lib-index/mail-cache-fields.c	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-cache-fields.c	Mon Oct 08 04:40:15 2012 +0300
@@ -198,11 +198,14 @@
 	return list;
 }
 
-static int mail_cache_header_fields_get_offset(struct mail_cache *cache,
-					       uint32_t *offset_r, bool map)
+static int
+mail_cache_header_fields_get_offset(struct mail_cache *cache,
+				    uint32_t *offset_r,
+				    const struct mail_cache_header_fields **field_hdr_r)
 {
 	const struct mail_cache_header_fields *field_hdr;
 	struct mail_cache_header_fields tmp_field_hdr;
+	const void *data;
 	uint32_t offset = 0, next_offset;
 	unsigned int next_count = 0;
 	bool invalidate = FALSE;
@@ -210,6 +213,8 @@
 
 	if (MAIL_CACHE_IS_UNUSABLE(cache)) {
 		*offset_r = 0;
+		if (field_hdr_r != NULL)
+			*field_hdr_r = NULL;
 		return 0;
 	}
 
@@ -228,16 +233,16 @@
 		invalidate = TRUE;
 
 		if (cache->mmap_base != NULL) {
-			if (mail_cache_map(cache, offset,
-					   sizeof(*field_hdr)) < 0)
-				return -1;
-			if (offset >= cache->mmap_length) {
+			ret = mail_cache_map(cache, offset, sizeof(*field_hdr),
+					     &data);
+			if (ret <= 0) {
+				if (ret < 0)
+					return -1;
 				mail_cache_set_corrupted(cache,
 					"header field next_offset points outside file");
 				return -1;
 			}
-
-			field_hdr = CONST_PTR_OFFSET(cache->data, offset);
+			field_hdr = data;
 		} else {
 			/* if we need to follow multiple offsets to get to
 			   the last one, it's faster to just pread() the file
@@ -270,7 +275,7 @@
 	if (next_count > MAIL_CACHE_HEADER_FIELD_CONTINUE_COUNT)
 		cache->need_compress_file_seq = cache->hdr->file_seq;
 
-	if (map) {
+	if (field_hdr_r != NULL) {
 		if (cache->file_cache != NULL && invalidate) {
 			/* if this isn't the first header in file and we hadn't
 			   read this before, we can't trust that the cached
@@ -278,17 +283,23 @@
 			file_cache_invalidate(cache->file_cache, offset,
 					      field_hdr->size);
 		}
-		if (mail_cache_map(cache, offset, field_hdr->size) < 0)
+		if ((ret = mail_cache_map(cache, offset, field_hdr->size,
+					  &data)) < 0)
 			return -1;
+		if (ret == 0) {
+			mail_cache_set_corrupted(cache,
+				"header field size outside file");
+			return -1;
+		}
+		*field_hdr_r = data;
 	}
-
 	*offset_r = offset;
 	return 0;
 }
 
 int mail_cache_header_fields_read(struct mail_cache *cache)
 {
-	const struct mail_cache_header_fields *field_hdr = NULL;
+	const struct mail_cache_header_fields *field_hdr;
 	struct mail_cache_field field;
 	const uint32_t *last_used, *sizes;
 	const uint8_t *types, *decisions;
@@ -299,7 +310,7 @@
 	time_t max_drop_time;
 	uint32_t offset, i;
 
-	if (mail_cache_header_fields_get_offset(cache, &offset, TRUE) < 0)
+	if (mail_cache_header_fields_get_offset(cache, &offset, &field_hdr) < 0)
 		return -1;
 
 	if (offset == 0) {
@@ -307,13 +318,6 @@
 		return 0;
 	}
 
-	field_hdr = CONST_PTR_OFFSET(cache->data, offset);
-	if (offset + field_hdr->size > cache->mmap_length) {
-		mail_cache_set_corrupted(cache,
-					 "field header points outside file");
-		return -1;
-	}
-
 	/* check the fixed size of the header. name[] has to be checked
 	   separately */
 	if (field_hdr->size < sizeof(*field_hdr) +
@@ -322,9 +326,7 @@
 		return -1;
 	}
 
-	field_hdr = CONST_PTR_OFFSET(cache->data, offset);
 	new_fields_count = field_hdr->fields_count;
-
 	if (new_fields_count != 0) {
 		cache->file_field_map =
 			i_realloc(cache->file_field_map,
diff -r d96313048f17 src/lib-index/mail-cache-lookup.c
--- a/src/lib-index/mail-cache-lookup.c	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-cache-lookup.c	Mon Oct 08 04:40:15 2012 +0300
@@ -14,6 +14,7 @@
 			  const struct mail_cache_record **rec_r)
 {
 	const struct mail_cache_record *rec;
+	const void *data;
 
 	i_assert(offset != 0);
 
@@ -24,14 +25,15 @@
 	}
 
 	/* we don't know yet how large the record is, so just guess */
-	if (mail_cache_map(cache, offset, sizeof(*rec) + CACHE_PREFETCH) < 0)
+	if (mail_cache_map(cache, offset, sizeof(*rec) + CACHE_PREFETCH,
+			   &data) < 0)
 		return -1;
 
 	if (offset + sizeof(*rec) > cache->mmap_length) {
 		mail_cache_set_corrupted(cache, "record points outside file");
 		return -1;
 	}
-	rec = CACHE_RECORD(cache, offset);
+	rec = data;
 
 	if (rec->size < sizeof(*rec)) {
 		mail_cache_set_corrupted(cache, "invalid record size");
@@ -39,9 +41,9 @@
 	}
 	if (rec->size > CACHE_PREFETCH) {
 		/* larger than we guessed. map the rest of the record. */
-		if (mail_cache_map(cache, offset, rec->size) < 0)
+		if (mail_cache_map(cache, offset, rec->size, &data) < 0)
 			return -1;
-		rec = CACHE_RECORD(cache, offset);
+		rec = data;
 	}
 
 	if (rec->size > cache->mmap_length ||
@@ -270,6 +272,7 @@
 	field_r->field_idx = field_idx;
 	field_r->data = CONST_PTR_OFFSET(ctx->rec, ctx->pos);
 	field_r->size = data_size;
+	field_r->offset = ctx->offset + ctx->pos;
 
 	/* each record begins from 32bit aligned position */
 	ctx->pos += (data_size + sizeof(uint32_t)-1) & ~(sizeof(uint32_t)-1);
@@ -444,8 +447,7 @@
 	lines_count = i;
 
 	hdr_data = t_new(struct header_lookup_data, 1);
-	hdr_data->offset = (const char *)&lines[lines_count+1] -
-		(const char *)ctx->view->cache->data;
+	hdr_data->offset = field->offset + (lines_count+1) * sizeof(uint32_t);
 	hdr_data->data_size = data_size;
 
 	for (i = 0; i < lines_count; i++) {
@@ -471,6 +473,7 @@
 	struct mail_cache_iterate_field field;
 	struct header_lookup_context ctx;
 	struct header_lookup_line *lines;
+	const void *data;
 	const unsigned char *p, *start, *end;
 	uint8_t *field_state;
 	unsigned int i, count, max_field = 0;
@@ -535,7 +538,10 @@
 
 	/* then start filling dest buffer from the headers */
 	for (i = 0; i < count; i++) {
-		start = CONST_PTR_OFFSET(cache->data, lines[i].data->offset);
+		if (mail_cache_map(cache, lines[i].data->offset,
+				   lines[i].data->data_size, &data) <= 0)
+			return -1;
+		start = data;
 		end = start + lines[i].data->data_size;
 
 		/* find the end of the (multiline) header */
diff -r d96313048f17 src/lib-index/mail-cache-private.h
--- a/src/lib-index/mail-cache-private.h	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-cache-private.h	Mon Oct 08 04:40:15 2012 +0300
@@ -134,10 +134,14 @@
 	ino_t st_ino;
 	dev_t st_dev;
 
+	size_t mmap_length;
+	/* a) mmaping the whole file */
 	void *mmap_base;
-	const void *data;
-	size_t mmap_length;
+	/* b) using file cache */
 	struct file_cache *file_cache;
+	/* c) using small read() calls with MAIL_INDEX_OPEN_FLAG_SAVEONLY */
+	uoff_t read_offset;
+	buffer_t *read_buf;
 	/* mail_cache_map() increases this always. */
 	unsigned int remap_counter;
 
@@ -174,6 +178,7 @@
 	unsigned int hdr_modified:1;
 	unsigned int field_header_write_pending:1;
 	unsigned int compressing:1;
+	unsigned int map_with_read:1;
 };
 
 struct mail_cache_loop_track {
@@ -205,6 +210,7 @@
 	unsigned int field_idx;
 	const void *data;
 	unsigned int size;
+	uoff_t offset;
 };
 
 struct mail_cache_lookup_iterate_ctx {
@@ -256,7 +262,8 @@
 int mail_cache_lookup_iter_next(struct mail_cache_lookup_iterate_ctx *ctx,
 				struct mail_cache_iterate_field *field_r);
 
-int mail_cache_map(struct mail_cache *cache, size_t offset, size_t size);
+int mail_cache_map(struct mail_cache *cache, size_t offset, size_t size,
+		   const void **data_r);
 void mail_cache_file_close(struct mail_cache *cache);
 int mail_cache_reopen(struct mail_cache *cache);
 
diff -r d96313048f17 src/lib-index/mail-cache-transaction.c
--- a/src/lib-index/mail-cache-transaction.c	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-cache-transaction.c	Mon Oct 08 04:40:15 2012 +0300
@@ -1091,6 +1091,8 @@
 		    uint32_t new_offset)
 {
 	const struct mail_cache_record *rec;
+	const void *data;
+	int ret;
 
 	i_assert(cache->locked);
 
@@ -1107,15 +1109,16 @@
 	   records at the same time. we'd rather not lose those additions, so
 	   force the linking order to be new_offset -> old_offset if it isn't
 	   already. */
-	if (mail_cache_map(cache, new_offset, sizeof(*rec)) < 0)
-		return -1;
-	if (new_offset + sizeof(*rec) > cache->mmap_length) {
+	if ((ret = mail_cache_map(cache, new_offset, sizeof(*rec),
+				  &data)) <= 0) {
+		if (ret < 0)
+			return -1;
 		mail_cache_set_corrupted(cache,
 			"Cache record offset %u points outside file",
 			new_offset);
 		return -1;
 	}
-	rec = CACHE_RECORD(cache, new_offset);
+	rec = data;
 	if (rec->prev_offset == old_offset) {
 		/* link is already correct */
 		return 0;
diff -r d96313048f17 src/lib-index/mail-cache.c
--- a/src/lib-index/mail-cache.c	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-cache.c	Mon Oct 08 04:40:15 2012 +0300
@@ -7,6 +7,7 @@
 #include "nfs-workarounds.h"
 #include "file-cache.h"
 #include "mmap-util.h"
+#include "read-full.h"
 #include "write-full.h"
 #include "mail-cache-private.h"
 
@@ -59,7 +60,6 @@
 		file_cache_set_fd(cache->file_cache, -1);
 
 	cache->mmap_base = NULL;
-	cache->data = NULL;
 	cache->hdr = NULL;
 	cache->mmap_length = 0;
 	cache->last_field_header_offset = 0;
@@ -145,6 +145,7 @@
 {
 	struct mail_index_view *view;
 	const struct mail_index_ext *ext;
+	const void *data;
 
 	i_assert(!cache->locked);
 
@@ -167,7 +168,7 @@
 
 	mail_cache_init_file_cache(cache);
 
-	if (mail_cache_map(cache, 0, 0) < 0)
+	if (mail_cache_map(cache, 0, 0, &data) < 0)
 		return -1;
 
 	if (mail_cache_header_fields_read(cache) < 0)
@@ -213,10 +214,9 @@
 		cache->need_compress_file_seq = hdr->file_seq;
 }
 
-static bool mail_cache_verify_header(struct mail_cache *cache)
+static bool mail_cache_verify_header(struct mail_cache *cache,
+				     const struct mail_cache_header *hdr)
 {
-	const struct mail_cache_header *hdr = cache->data;
-
 	/* check that the header is still ok */
 	if (cache->mmap_length < sizeof(struct mail_cache_header)) {
 		mail_cache_set_corrupted(cache, "File too small");
@@ -265,17 +265,88 @@
 	return TRUE;
 }
 
-int mail_cache_map(struct mail_cache *cache, size_t offset, size_t size)
+static int
+mail_cache_map_finish(struct mail_cache *cache, uoff_t offset, size_t size,
+		      const void *data, bool copy_hdr)
 {
+	if (offset == 0) {
+		const struct mail_cache_header *hdr = data;
+
+		if (!mail_cache_verify_header(cache, hdr)) {
+			cache->need_compress_file_seq =
+				!MAIL_CACHE_IS_UNUSABLE(cache) &&
+				cache->hdr->file_seq != 0 ?
+				cache->hdr->file_seq : 0;
+			return -1;
+		}
+		if (!copy_hdr)
+			cache->hdr = data;
+		else {
+			memcpy(&cache->hdr_ro_copy, data,
+			       sizeof(cache->hdr_ro_copy));
+			cache->hdr = &cache->hdr_ro_copy;
+		}
+		mail_cache_update_need_compress(cache);
+	}
+
+	if (offset + size > cache->mmap_length)
+		return 0;
+	return 1;
+}
+
+static int
+mail_cache_map_with_read(struct mail_cache *cache, size_t offset, size_t size,
+			 const void **data_r)
+{
+	void *data;
 	ssize_t ret;
 
-	cache->remap_counter++;
+	if (cache->read_buf == NULL) {
+		cache->read_buf =
+			buffer_create_dynamic(default_pool, size);
+	} else if (cache->read_offset <= offset &&
+		   cache->read_offset + cache->read_buf->used >= offset+size) {
+		/* already mapped */
+		*data_r = CONST_PTR_OFFSET(cache->mmap_base,
+					   offset - cache->read_offset);
+		return mail_cache_map_finish(cache, offset, size, *data_r, TRUE);
+	} else {
+		buffer_set_used_size(cache->read_buf, 0);
+	}
+	data = buffer_append_space_unsafe(cache->read_buf, size);
+	ret = pread(cache->fd, data, size, offset);
+	if (ret < 0) {
+		if (errno != ESTALE)
+			mail_cache_set_syscall_error(cache, "read()");
+
+		buffer_set_used_size(cache->read_buf, 0);
+		cache->hdr = NULL;
+		cache->mmap_length = 0;
+		return -1;
+	}
+	buffer_set_used_size(cache->read_buf, ret);
+
+	cache->read_offset = offset;
+	cache->mmap_length = offset + size;
+
+	*data_r = data;
+	return mail_cache_map_finish(cache, offset, size, data, TRUE);
+}
+
+int mail_cache_map(struct mail_cache *cache, size_t offset, size_t size,
+		   const void **data_r)
+{
+	const void *data;
+	ssize_t ret;
 
 	if (size == 0)
 		size = sizeof(struct mail_cache_header);
 
+	cache->remap_counter++;
+	if (cache->map_with_read)
+		return mail_cache_map_with_read(cache, offset, size, data_r);
+
 	if (cache->file_cache != NULL) {
-		cache->data = NULL;
 		cache->hdr = NULL;
 
 		ret = file_cache_read(cache->file_cache, offset, size);
@@ -292,30 +363,17 @@
 			return -1;
 		}
 
-		cache->data = file_cache_get_map(cache->file_cache,
-						 &cache->mmap_length);
-
-		if (offset == 0) {
-			if (!mail_cache_verify_header(cache)) {
-				cache->need_compress_file_seq =
-					!MAIL_CACHE_IS_UNUSABLE(cache) &&
-					cache->hdr->file_seq != 0 ?
-					cache->hdr->file_seq : 0;
-				return -1;
-			}
-			memcpy(&cache->hdr_ro_copy, cache->data,
-			       sizeof(cache->hdr_ro_copy));
-		}
-		cache->hdr = &cache->hdr_ro_copy;
-		if (offset == 0)
-			mail_cache_update_need_compress(cache);
-		return 0;
+		data = file_cache_get_map(cache->file_cache,
+					  &cache->mmap_length);
+		*data_r = CONST_PTR_OFFSET(data, offset);
+		return mail_cache_map_finish(cache, offset, size, *data_r, TRUE);
 	}
 
 	if (offset < cache->mmap_length &&
 	    size <= cache->mmap_length - offset) {
 		/* already mapped */
-		return 0;
+		*data_r = CONST_PTR_OFFSET(cache->mmap_base, offset);
+		return 1;
 	}
 
 	if (cache->mmap_base != NULL) {
@@ -338,28 +396,17 @@
 	cache->mmap_base = mmap_ro_file(cache->fd, &cache->mmap_length);
 	if (cache->mmap_base == MAP_FAILED) {
 		cache->mmap_base = NULL;
-		cache->data = NULL;
 		mail_cache_set_syscall_error(cache, "mmap()");
 		return -1;
 	}
-	cache->data = cache->mmap_base;
-
-	if (!mail_cache_verify_header(cache)) {
-		cache->need_compress_file_seq =
-			!MAIL_CACHE_IS_UNUSABLE(cache) &&
-			cache->hdr->file_seq != 0 ?
-			cache->hdr->file_seq : 0;
-		return -1;
-	}
-
-	cache->hdr = cache->data;
-	if (offset == 0)
-		mail_cache_update_need_compress(cache);
-	return 0;
+	*data_r = CONST_PTR_OFFSET(cache->mmap_base, offset);
+	return mail_cache_map_finish(cache, offset, size, *data_r, FALSE);
 }
 
 static int mail_cache_try_open(struct mail_cache *cache)
 {
+	const void *data;
+
 	cache->opened = TRUE;
 
 	if (MAIL_INDEX_IS_IN_MEMORY(cache->index))
@@ -379,9 +426,9 @@
 
 	mail_cache_init_file_cache(cache);
 
-	if (mail_cache_map(cache, 0, sizeof(struct mail_cache_header)) < 0)
+	if (mail_cache_map(cache, 0, sizeof(struct mail_cache_header),
+			   &data) < 0)
 		return -1;
-
 	return 1;
 }
 
@@ -425,6 +472,8 @@
 	if (!MAIL_INDEX_IS_IN_MEMORY(index) &&
 	    (index->flags & MAIL_INDEX_OPEN_FLAG_MMAP_DISABLE) != 0)
 		cache->file_cache = file_cache_new(-1);
+	cache->map_with_read =
+		(cache->index->flags & MAIL_INDEX_OPEN_FLAG_SAVEONLY) != 0;
 
 	cache->ext_id =
 		mail_index_ext_register(index, "cache", 0,
@@ -550,6 +599,7 @@
 		     bool nonblock)
 {
 	const struct mail_index_ext *ext;
+	const void *data;
 	struct mail_index_view *iview;
 	uint32_t reset_id;
 	int i, ret;
@@ -613,7 +663,7 @@
 			file_cache_invalidate(cache->file_cache, 0,
 					      sizeof(struct mail_cache_header));
 		}
-		if (mail_cache_map(cache, 0, 0) == 0)
+		if (mail_cache_map(cache, 0, 0, &data) == 0)
 			cache->hdr_copy = *cache->hdr;
 		else {
 			(void)mail_cache_unlock(cache);
@@ -678,13 +728,8 @@
 		return -1;
 	}
 
-	if (cache->file_cache != NULL) {
+	if (cache->file_cache != NULL)
 		file_cache_write(cache->file_cache, data, size, offset);
-
-		/* data pointer may change if file cache was grown */
-		cache->data = file_cache_get_map(cache->file_cache,
-						 &cache->mmap_length);
-	}
 	return 0;
 }
 
diff -r d96313048f17 src/lib-index/mail-index.h
--- a/src/lib-index/mail-index.h	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-index/mail-index.h	Mon Oct 08 04:40:15 2012 +0300
@@ -27,7 +27,10 @@
 	MAIL_INDEX_OPEN_FLAG_KEEP_BACKUPS	= 0x100,
 	/* If we run out of disk space, fail modifications instead of moving
 	   indexes to memory. */
-	MAIL_INDEX_OPEN_FLAG_NEVER_IN_MEMORY	= 0x200
+	MAIL_INDEX_OPEN_FLAG_NEVER_IN_MEMORY	= 0x200,
+	/* We're only going to save new messages to the index.
+	   Avoid unnecessary reads. */
+	MAIL_INDEX_OPEN_FLAG_SAVEONLY		= 0x400
 };
 
 enum mail_index_header_compat_flags {
diff -r d96313048f17 src/lib-storage/index/index-storage.c
--- a/src/lib-storage/index/index-storage.c	Mon Oct 08 03:14:12 2012 +0300
+++ b/src/lib-storage/index/index-storage.c	Mon Oct 08 04:40:15 2012 +0300
@@ -304,6 +304,8 @@
 	ibox->list_index_sync_ext_id = (uint32_t)-1;
 	ibox->index_flags = MAIL_INDEX_OPEN_FLAG_CREATE |
 		mail_storage_settings_to_index_flags(box->storage->set);
+	if ((box->flags & MAILBOX_FLAG_SAVEONLY) != 0)
+		ibox->index_flags |= MAIL_INDEX_OPEN_FLAG_SAVEONLY;
 	ibox->next_lock_notify = time(NULL) + LOCK_NOTIFY_INTERVAL;
 	MODULE_CONTEXT_SET(box, index_storage_module, ibox);
 
