diff --git a/kolibri.h b/kolibri.h
index 82e1bcb..849b068 100644
--- a/kolibri.h
+++ b/kolibri.h
@@ -111,7 +111,7 @@ typedef struct {
     int (*read)(void *userdata, void *buffer, off_t startsector, size_t *numsectors) __attribute__((__stdcall__));
     int (*write)(void *userdata, void *buffer, off_t startsector, size_t *numsectors) __attribute__((__stdcall__));
     int (*flush)(void *userdata) __attribute__((__stdcall__));
-    unsigned int (*adjust_cache_size)(void *userdata, uint32_t suggested_size) __attribute__((__stdcall__));
+    unsigned int (*adjust_cache_size)(void *userdata, size_t suggested_size) __attribute__((__stdcall__));
 } diskfunc_t;
 
 struct disk_t {
diff --git a/umka_fuse.c b/umka_fuse.c
index e035286..7c96866 100644
--- a/umka_fuse.c
+++ b/umka_fuse.c
@@ -123,24 +123,13 @@ static struct fuse_operations umka_oper = {
         .read           = umka_read,
 };
 
-diskfunc_t vdisk_functions = {
-                                 .strucsize = sizeof(diskfunc_t),
-                                 .close = vdisk_close,
-                                 .closemedia = NULL,
-                                 .querymedia = vdisk_querymedia,
-                                 .read = vdisk_read,
-                                 .write = vdisk_write,
-                                 .flush = NULL,
-                                 .adjust_cache_size = NULL,
-                                };
-
 int main(int argc, char *argv[]) {
         if (argc != 3) {
                 printf("usage: umka_fuse dir img\n");
                 exit(1);
         }
         kos_init();
-        void *userdata = vdisk_init(argv[2], 0u);
+        void *userdata = vdisk_init(argv[2], 1, 0u);
         void *vdisk = disk_add(&vdisk_functions, "hd0", userdata, 0);
         disk_media_changed(vdisk, 1);
         return fuse_main(argc-1, argv, &umka_oper, NULL);
diff --git a/umka_shell.c b/umka_shell.c
index 66ebdfc..8072306 100644
--- a/umka_shell.c
+++ b/umka_shell.c
@@ -50,17 +50,6 @@
 #define DEFAULT_READDIR_ENCODING UTF8
 #define DEFAULT_PATH_ENCODING UTF8
 
-diskfunc_t vdisk_functions = {
-                                 .strucsize = sizeof(diskfunc_t),
-                                 .close = vdisk_close,
-                                 .closemedia = NULL,
-                                 .querymedia = vdisk_querymedia,
-                                 .read = vdisk_read,
-                                 .write = vdisk_write,
-                                 .flush = NULL,
-                                 .adjust_cache_size = NULL,
-                                };
-
 net_device_t vnet = {
                             .device_type = NET_TYPE_ETH,
                             .mtu = 1514,
@@ -275,7 +264,8 @@ void shell_disk_add(int argc, char **argv) {
         puts(usage);
         return;
     }
-    unsigned cache_size = 0u;
+    size_t cache_size = 0;
+    int adjust_cache_size = 0;
     int opt;
     optind = 1;
     const char *file_name = argv[optind++];
@@ -284,7 +274,7 @@ void shell_disk_add(int argc, char **argv) {
         switch (opt) {
         case 'c':
             cache_size = strtoul(optarg, NULL, 0);
-            vdisk_functions.adjust_cache_size = vdisk_adjust_cache_size;
+            adjust_cache_size = 1;
             break;
         default:
             puts(usage);
@@ -292,7 +282,7 @@ void shell_disk_add(int argc, char **argv) {
         }
     }
 
-    void *userdata = vdisk_init(file_name, cache_size);
+    void *userdata = vdisk_init(file_name, adjust_cache_size, cache_size);
     if (userdata) {
         COVERAGE_ON();
         void *vdisk = disk_add(&vdisk_functions, disk_name, userdata, 0);
diff --git a/vdisk.c b/vdisk.c
index 2d1eb81..c3f53c4 100644
--- a/vdisk.c
+++ b/vdisk.c
@@ -5,15 +5,17 @@
 #include <errno.h>
 #include "kolibri.h"
 #include "trace.h"
+#include "vdisk.h"
 
 typedef struct {
     FILE *file;
     uint32_t sect_size;
     uint64_t sect_cnt;
     unsigned cache_size;
+    int      adjust_cache_size;
 } vdisk_t;
 
-void *vdisk_init(const char *fname, unsigned cache_size) {
+void *vdisk_init(const char *fname, int adjust_cache_size, size_t cache_size) {
     FILE *f = fopen(fname, "r+");
     if (!f) {
         printf("vdisk: can't open file '%s': %s\n", fname, strerror(errno));
@@ -30,7 +32,8 @@ void *vdisk_init(const char *fname, unsigned cache_size) {
     *vdisk = (vdisk_t){.file = f,
                        .sect_size = sect_size,
                        .sect_cnt = (uint64_t)fsize / sect_size,
-                       .cache_size = cache_size};
+                       .cache_size = cache_size,
+                       .adjust_cache_size = adjust_cache_size};
     return vdisk;
 }
 
@@ -77,7 +80,22 @@ int vdisk_querymedia(void *userdata, diskmediainfo_t *minfo) {
 }
 
 __attribute__((__stdcall__))
-unsigned vdisk_adjust_cache_size(vdisk_t *vdisk, unsigned suggested_size) {
-    (void)suggested_size;
-    return vdisk->cache_size;
+size_t vdisk_adjust_cache_size(void *userdata, size_t suggested_size) {
+    vdisk_t *vdisk = userdata;
+    if (vdisk->adjust_cache_size) {
+        return vdisk->cache_size;
+    } else {
+        return suggested_size;
+    }
 }
+
+diskfunc_t vdisk_functions = {
+                                 .strucsize = sizeof(diskfunc_t),
+                                 .close = vdisk_close,
+                                 .closemedia = NULL,
+                                 .querymedia = vdisk_querymedia,
+                                 .read = vdisk_read,
+                                 .write = vdisk_write,
+                                 .flush = NULL,
+                                 .adjust_cache_size = vdisk_adjust_cache_size,
+                                };
diff --git a/vdisk.h b/vdisk.h
index 1f54ed6..b80e159 100644
--- a/vdisk.h
+++ b/vdisk.h
@@ -5,7 +5,7 @@
 #include <inttypes.h>
 #include "kolibri.h"
 
-void *vdisk_init(const char *fname, unsigned cache_size);
+void *vdisk_init(const char *fname, int adjust_cache_size, size_t cache_size);
 
 __attribute__((__stdcall__))
 void vdisk_close(void *userdata);
@@ -24,4 +24,6 @@ int vdisk_querymedia(void *userdata, diskmediainfo_t *minfo);
 __attribute__((__stdcall__))
 unsigned int vdisk_adjust_cache_size(void *userdata, unsigned suggested_size);
 
+extern diskfunc_t vdisk_functions;
+
 #endif  // VDISK_H_INCLUDED