Re: 64-bit kdev_t - just for playing

[Badari Pulavarty <pbadari@us.ibm.com>]

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On Wednesday 02 April 2003 04:18 am, Roman Zippel wrote:
> Hi,
>
> On Mon, 31 Mar 2003, Badari Pulavarty wrote:
> > I have been playing with supporting 4000 disks on IA32 machines.
> > There are bunch of issues we need to resolve before we could
> > do that.
> >
> > I am using scsi_debug to simulate 4000 disks. (Ofcourse, I had
> > to hack "sd" to support more than 256 disks).
>
> Could you please post your changes to sd.c and anything relevant to it?
> Thanks.
>
> bye, Roman

Roman,

Here is the patch for sd to allow more than 256 disks.
There are few issues with the patch that need to be resolved.

1) With the patch I get 16 bits for minor. Since 4 bits are used for
partition, we get 12 bits to represent disks. So each major can support
2^12 = 4096 disks. Disks 0 - 4095 are mapped to major=8, 
disks 4096 - 8191 to major = 65 and so on..

This means ..

(i) I need to create nodes in /dev/ to match new <major, minor> for 
these disks.  Currently "mknod" is broken due to glibc issues with dev_t.

(ii) We need to worry about backward compatibility. For example:
17th disk used to have <65, 0>. Now its major, minor is <8, 256>.
So /dev/ entires need to be re-created to match these, everytime
you reboot 2.4/2.5 etc. Greg KH udev might fix this for us. 

2) Do we still need 16 majors for disks ?

We could change my patch to work around major/minor assignment
issues and maintain backward compatibility. 

Thanks,
Badari




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--- linux/drivers/scsi/sd.c	Thu Mar 20 15:06:00 2003
+++ linux.new/drivers/scsi/sd.c	Fri Mar 21 11:50:54 2003
@@ -56,7 +56,9 @@
  * Remaining dev_t-handling stuff
  */
 #define SD_MAJORS	16
-#define SD_DISKS	(SD_MAJORS << 4)
+#define SD_DISKS_PER_MAJOR_SHIFT	(KDEV_MINOR_BITS - 4)
+#define SD_DISKS_PER_MAJOR	(1 << SD_DISKS_PER_MAJOR_SHIFT)
+#define SD_DISKS	(SD_MAJORS << SD_DISKS_PER_MAJOR_SHIFT)
 
 /*
  * Time out in seconds for disks and Magneto-opticals (which are slower).
@@ -1328,17 +1330,23 @@ static int sd_attach(struct scsi_device 
 	sdkp->index = index;
 
 	gd->de = sdp->de;
-	gd->major = sd_major(index >> 4);
-	gd->first_minor = (index & 15) << 4;
+	gd->major = sd_major(index >> SD_DISKS_PER_MAJOR_SHIFT);
+	gd->first_minor = (index & (SD_DISKS_PER_MAJOR - 1)) << 4;
 	gd->minors = 16;
 	gd->fops = &sd_fops;
 
-	if (index >= 26) {
+	if (index < 26) {
+		sprintf(gd->disk_name, "sd%c", 'a' + index % 26);
+	} else if (index < (26*27)) {
 		sprintf(gd->disk_name, "sd%c%c",
 			'a' + index/26-1,'a' + index % 26);
 	} else {
-		sprintf(gd->disk_name, "sd%c", 'a' + index % 26);
-	}
+		const unsigned int m1 = (index/ 26 - 1) / 26 - 1;
+		const unsigned int m2 = (index / 26 - 1) % 26;
+		const unsigned int m3 = index % 26;
+		sprintf(gd->disk_name, "sd%c%c%c", 
+			'a' + m1, 'a' + m2, 'a' + m3);
+	}	
 
 	sd_init_onedisk(sdkp, gd);