--- a/src/lib-storage/index/index-mailbox-size.c	Tue Feb 07 13:53:52 2017 +0200
+++ b/src/lib-storage/index/index-mailbox-size.c	Tue Feb 07 16:18:55 2017 +0200
@@ -1,6 +1,10 @@
 /* Copyright (c) 2002-2017 Dovecot authors, see the included COPYING file */
 
 #include "lib.h"
+#include "str.h"
+#include "strescape.h"
+#include "net.h"
+#include "write-full.h"
 #include "file-create-locked.h"
 #include "mail-search-build.h"
 #include "index-storage.h"
@@ -33,6 +37,9 @@
 #define VSIZE_LOCK_SUFFIX ".vsize.lock"
 #define VSIZE_UPDATE_MAX_LOCK_SECS 10
 
+#define INDEXER_SOCKET_NAME "indexer"
+#define INDEXER_HANDSHAKE "VERSION\tindexer\t1\t0\n"
+
 struct mailbox_vsize_update {
 	struct mailbox *box;
 	struct mail_index_view *view;
@@ -43,6 +50,7 @@
 	struct file_lock *lock;
 	bool rebuild;
 	bool written;
+	bool finish_in_background;
 };
 
 static void vsize_header_refresh(struct mailbox_vsize_update *update)
@@ -192,6 +200,36 @@
 	(void)mail_index_transaction_commit(&trans);
 }
 
+static void index_mailbox_vsize_notify_indexer(struct mailbox *box)
+{
+	string_t *str = t_str_new(256);
+	const char *path;
+	int fd;
+
+	path = t_strconcat(box->storage->user->set->base_dir,
+			   "/"INDEXER_SOCKET_NAME, NULL);
+	fd = net_connect_unix(path);
+	if (fd == -1) {
+		mail_storage_set_critical(box->storage,
+			"Can't start vsize building on background: "
+			"net_connect_unix(%s) failed: %m", path);
+		return;
+	}
+	str_append(str, INDEXER_HANDSHAKE);
+	str_append(str, "APPEND\t0\t");
+	str_append_tabescaped(str, box->storage->user->username);
+	str_append_c(str, '\t');
+	str_append_tabescaped(str, box->vname);
+	str_append_c(str, '\n');
+
+	if (write_full(fd, str_data(str), str_len(str)) < 0) {
+		mail_storage_set_critical(box->storage,
+			"Can't start vsize building on background: "
+			"write(%s) failed: %m", path);
+	}
+	i_close_fd(&fd);
+}
+
 void index_mailbox_vsize_update_deinit(struct mailbox_vsize_update **_update)
 {
 	struct mailbox_vsize_update *update = *_update;
@@ -206,6 +244,9 @@
 		file_lock_free(&update->lock);
 		i_close_fd(&update->lock_fd);
 	}
+	if (update->finish_in_background)
+		index_mailbox_vsize_notify_indexer(update->box);
+
 	mail_index_view_close(&update->view);
 	i_free(update->lock_path);
 	i_free(update);
@@ -243,6 +284,7 @@
 	struct mail_search_args *search_args;
 	struct mailbox_status status;
 	struct mail *mail;
+	unsigned int mails_left;
 	uint32_t seq1, seq2;
 	uoff_t vsize;
 	int ret = 0;
@@ -272,8 +314,33 @@
 	trans = mailbox_transaction_begin(update->box, 0);
 	search_ctx = mailbox_search_init(trans, search_args, NULL,
 					 MAIL_FETCH_VIRTUAL_SIZE, NULL);
+	mails_left = update->box->storage->set->mail_vsize_bg_after_count == 0 ?
+		UINT_MAX : update->box->storage->set->mail_vsize_bg_after_count;
 	while (mailbox_search_next(search_ctx, &mail)) {
-		if (mail_get_virtual_size(mail, &vsize) < 0) {
+		if (mails_left == UINT_MAX) {
+			/* we want to build the full vsize here */
+			ret = mail_get_virtual_size(mail, &vsize);
+		} else {
+			/* if vsize building wants to open too many mails from
+			   storage, return temporary failure and finish up the
+			   calculation in background. */
+			mail->lookup_abort = MAIL_LOOKUP_ABORT_NOT_IN_CACHE;
+			ret = mail_get_virtual_size(mail, &vsize);
+			mail->lookup_abort = MAIL_LOOKUP_ABORT_NEVER;
+			if (ret < 0 &&
+			    mailbox_get_last_mail_error(update->box) == MAIL_ERROR_NOTPOSSIBLE) {
+				/* size isn't in cache. */
+				if (mails_left == 0) {
+					mail_storage_set_error(update->box->storage, MAIL_ERROR_INUSE,
+						"Finishing vsize calculation on background");
+					update->finish_in_background = TRUE;
+					break;
+				}
+				mails_left--;
+				ret = mail_get_virtual_size(mail, &vsize);
+			}
+		}
+		if (ret < 0) {
 			if (mail->expunged)
 				continue;
 			ret = -1;

--- a/src/lib-storage/mail-storage-settings.c	Tue Feb 07 13:53:52 2017 +0200
+++ b/src/lib-storage/mail-storage-settings.c	Tue Feb 07 16:18:55 2017 +0200
@@ -42,6 +42,7 @@
 	DEF(SET_UINT, mail_max_keyword_length),
 	DEF(SET_TIME, mail_max_lock_timeout),
 	DEF(SET_TIME, mail_temp_scan_interval),
+	DEF(SET_UINT, mail_vsize_bg_after_count),
 	DEF(SET_BOOL, mail_save_crlf),
 	DEF(SET_ENUM, mail_fsync),
 	DEF(SET_BOOL, mmap_disable),
@@ -82,6 +83,7 @@
 	.mail_max_keyword_length = 50,
 	.mail_max_lock_timeout = 0,
 	.mail_temp_scan_interval = 7*24*60*60,
+	.mail_vsize_bg_after_count = 0,
 	.mail_save_crlf = FALSE,
 	.mail_fsync = "optimized:never:always",
 	.mmap_disable = FALSE,

--- a/src/lib-storage/mail-storage-settings.h	Tue Feb 07 13:53:52 2017 +0200
+++ b/src/lib-storage/mail-storage-settings.h	Tue Feb 07 16:18:55 2017 +0200
@@ -28,6 +28,7 @@
 	unsigned int mail_max_keyword_length;
 	unsigned int mail_max_lock_timeout;
 	unsigned int mail_temp_scan_interval;
+	unsigned int mail_vsize_bg_after_count;
 	bool mail_save_crlf;
 	const char *mail_fsync;
 	bool mmap_disable;

--- a/src/plugins/quota/quota-count.c	Tue Feb 07 13:53:52 2017 +0200
+++ b/src/plugins/quota/quota-count.c	Tue Feb 07 16:18:55 2017 +0200
@@ -55,6 +55,9 @@
 			i_error("quota: Couldn't get size of mailbox %s: %s",
 				vname, errstr);
 			ret = -1;
+		} else if (error == MAIL_ERROR_INUSE) {
+			/* started on background. don't log an error. */
+			ret = -1;
 		} else {
 			/* non-temporary error, e.g. ACLs denied access. */
 			ret = 0;