From: Robert Pengelly <robertapengelly@hotmail.com>
Date: Sat, 22 Mar 2025 11:00:08 +0000 (+0000)
Subject: Explain why a disk image needs to be way bigger than it should be
X-Git-Url: https://git.candlhat.org/?a=commitdiff_plain;h=d3c50eea6da9e3419ed4746337da4f3a1daff8e8;p=parted.git

Explain why a disk image needs to be way bigger than it should be
---

diff --git a/parted.c b/parted.c
index 2c3bf27..a369b38 100644
--- a/parted.c
+++ b/parted.c
@@ -24,6 +24,7 @@
 static long cylinders = 0;
 static long heads_per_cylinder = 0;
 static long sectors_per_track = 0;
+static long sectors_per_cylinder = 0;
 
 struct parted_state *state = 0;
 const char *program_name = 0;
@@ -146,12 +147,32 @@ static void calculate_geometry (void) {
     
         cylinders++;
         
-        state->image_size = ((cylinders * heads_per_cylinder) * sectors_per_track);
-        state->image_size += ((heads_per_cylinder - 1) * sectors_per_track);
+#if     0
         
+        /* This works for PCem and 86box but not for qemu. */
+        state->image_size = cylinders * heads_per_cylinder;
+        state->image_size = (state->image_size * sectors_per_track) * 512;
+        
+#else
+        
+        /**
+         * Ideally we'd have a new size of 41,954,304 for the image as Pcem and
+         * 86box reports 81940 sectors and times by 512 equals 41,953,280 add 512
+         * bytes for the mbr and 512 for the VHD footer however qemu seems to report
+         * different disk geometry compared to PCem and 86box resuling to 81900
+         * sectors being reported instead so we need to make the image way bigger
+         * than it needs to be to accommodate qemu.
+         */
+        state->image_size = (cylinders * heads_per_cylinder) * sectors_per_track;
+        
+        state->image_size = state->image_size + ((heads_per_cylinder - 1) * sectors_per_track);
         state->image_size = (state->image_size + (sectors_per_track - 1)) * 512;
+        
+#endif
     
     }
+    
+    sectors_per_cylinder = heads_per_cylinder * sectors_per_track;
 
 }
 
@@ -251,7 +272,7 @@ int main (int argc, char **argv) {
     struct mbr mbr;
     
     long p, sector = 1;
-    long sectors_per_cylinder, start, end;
+    long start, end;
     
     if (argc && *argv) {
     
@@ -321,7 +342,6 @@ int main (int argc, char **argv) {
     write741_to_byte_array (mbr.signature, generate_signature (), 1);
     
     calculate_geometry ();
-    sectors_per_cylinder = heads_per_cylinder * sectors_per_track;
     
     for (p = 0; p < state->nb_parts; ++p) {