--- a/src/lib-storage/mail-storage.c	Thu Jun 22 02:44:12 2017 +0300
+++ b/src/lib-storage/mail-storage.c	Thu Jun 22 01:28:57 2017 +0300
@@ -6,7 +6,9 @@
 #include "llist.h"
 #include "str.h"
 #include "str-sanitize.h"
+#include "sha1.h"
 #include "unichar.h"
+#include "hex-binary.h"
 #include "file-create-locked.h"
 #include "istream.h"
 #include "eacces-error.h"
@@ -2782,7 +2784,26 @@
 	set.gid = perm->file_create_gid;
 	set.gid_origin = perm->file_create_gid_origin;
 
-	lock_path = t_strdup_printf("%s/%s", box->index->dir, lock_fname);
+	if (box->list->set.volatile_dir == NULL)
+		lock_path = t_strdup_printf("%s/%s", box->index->dir, lock_fname);
+	else {
+		unsigned char box_name_sha1[SHA1_RESULTLEN];
+		string_t *str = t_str_new(128);
+
+		/* Keep this simple: Use the lock_fname with a SHA1 of the
+		   mailbox name as the suffix. The mailbox name itself could
+		   be too large as a filename and creating the full directory
+		   structure would be pretty troublesome. It would also make
+		   it more difficult to perform the automated deletion of empty
+		   lock directories. */
+		str_printfa(str, "%s/%s.", box->list->set.volatile_dir,
+			    lock_fname);
+		sha1_get_digest(box->name, strlen(box->name), box_name_sha1);
+		binary_to_hex_append(str, box_name_sha1, sizeof(box_name_sha1));
+		lock_path = str_c(str);
+		set.mkdir_mode = 0700;
+	}
+
 	if (file_create_locked(lock_path, &set, lock_r, &created, error_r) == -1) {
 		*error_r = t_strdup_printf("file_create_locked(%s) failed: %s",
 					   lock_path, *error_r);

--- a/src/lib-storage/mail-user.c	Thu Jun 22 02:44:12 2017 +0300
+++ b/src/lib-storage/mail-user.c	Thu Jun 22 01:28:57 2017 +0300
@@ -23,6 +23,7 @@
 #include "mail-storage-service.h"
 #include "mail-namespace.h"
 #include "mail-storage.h"
+#include "mailbox-list-private.h"
 #include "mail-autoexpunge.h"
 #include "mail-user.h"
 
@@ -523,7 +524,16 @@
 		.lock_timeout_secs = lock_secs,
 		.lock_method = mail_set->parsed_lock_method,
 	};
-	path = t_strdup_printf("%s/%s", home, lock_fname);
+	struct mailbox_list *inbox_list =
+		mail_namespace_find_inbox(user->namespaces)->list;
+	if (inbox_list->set.volatile_dir == NULL)
+		path = t_strdup_printf("%s/%s", home, lock_fname);
+	else {
+		path = t_strdup_printf("%s/%s", inbox_list->set.volatile_dir,
+				       lock_fname);
+		lock_set.mkdir_mode = 0700;
+	}
+
 	if (file_create_locked(path, &lock_set, lock_r, &created, &error) == -1) {
 		*error_r = t_strdup_printf("file_create_locked(%s) failed: %s", path, error);
 		return errno == EAGAIN ? 0 : -1;

--- a/src/lib-storage/mailbox-list.c	Thu Jun 22 02:44:12 2017 +0300
+++ b/src/lib-storage/mailbox-list.c	Thu Jun 22 01:28:57 2017 +0300
@@ -4,6 +4,7 @@
 #include "array.h"
 #include "abspath.h"
 #include "ioloop.h"
+#include "file-create-locked.h"
 #include "mkdir-parents.h"
 #include "str.h"
 #include "sha1.h"
@@ -171,6 +172,7 @@
 		p_strdup(list->pool, set->mailbox_dir_name);
 	list->set.alt_dir = p_strdup(list->pool, set->alt_dir);
 	list->set.alt_dir_nocheck = set->alt_dir_nocheck;
+	list->set.volatile_dir = p_strdup(list->pool, set->volatile_dir);
 	list->set.index_control_use_maildir_name =
 		set->index_control_use_maildir_name;
 
@@ -336,6 +338,8 @@
 			dest = &set_r->maildir_name;
 		else if (strcmp(key, "MAILBOXDIR") == 0)
 			dest = &set_r->mailbox_dir_name;
+		else if (strcmp(key, "VOLATILEDIR") == 0)
+			dest = &set_r->volatile_dir;
 		else if (strcmp(key, "LISTINDEX") == 0)
 			dest = &set_r->list_index_fname;
 		else if (strcmp(key, "FULLDIRNAME") == 0) {

--- a/src/lib-storage/mailbox-list.h	Thu Jun 22 02:44:12 2017 +0300
+++ b/src/lib-storage/mailbox-list.h	Thu Jun 22 01:28:57 2017 +0300
@@ -98,6 +98,11 @@
 	const char *index_pvt_dir;
 	const char *control_dir;
 	const char *alt_dir; /* FIXME: dbox-specific.. */
+	/* Backend-local directory where volatile data, such as lock files,
+	   can be temporarily created. This setting allows specifying a
+	   separate directory for them to reduce disk I/O on the real storage.
+	   The volatile_dir can point to an in-memory filesystem. */
+	const char *volatile_dir;
 
 	const char *inbox_path;
 	const char *subscription_fname;