# Identical HDDs, different num heads & cylinders

## lord

I have two identical hdds which I want to install linux on.

One is primary master (hda) and the other is secondary master (hdc). My CD-ROM is secondary slave (hdd).

According to my bios my hdds are 120034mb with 8192kb cache. with CHS=14593/255/63

According to kernel hda has that configuration while hdc has CHS=232581/16/63. Why is that? (Num sectors and size are the same for both drives)

Does the fact that the cdrom is connected to the same ide-bus as hdc make this occur?

I want to raid-1 both drives (with LVM, EVMS) and install linux them. Will this size-differance matter? Will the speed increase/decrease? What otther consequences will this have?

Lots of questions, but it'd be nice to know..  :Smile: 

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## pjp

That is interesting... I'd disconnect the CD just to test.  Are the HDs both the same make/model?

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## lord

I would gladly disconnect the cdrom, but then I wouldnt have anywhere to boot from. Its a fresh box :~

Both hdds are as i mentioned identical.

I've checked /proc/ide/hda/model   and same for hdc. everything is the same...  :Razz: 

same firmware, model, size etc etc

really strange :~

Are there any bootdisks I could download from gentoo's homepage, so i can disconnect the cdrom?

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## pjp

Installation from a boot floppy HOWTO might help.  Or, you could swap the HD on the CD channel for the other HD and see what happens.  Probably easier.

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## coolcash

I am guessing that the drives might be physically different. One might have 1 or 2 platters and the other has 2 or 3. Raid should run fine.

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## lord

 *coolcash wrote:*   

> I am guessing that the drives might be physically different. One might have 1 or 2 platters and the other has 2 or 3. Raid should run fine.

 

Both my posts state that my HDDs are identical, and they still are...

They are identical because they have exactly the same model number, purchased at the same time from the same vendor with the same identifier. Every parameter is the same, including firmware. Even labels on both drives are the same except for serial no's and stuff... 

They are identical.  :Smile: 

kanuslupus:

Thanks, I'll try to move around the drives and see what happens...

But the question is still, are there any performance effects on how the drives are detected (255 vs 16 heads etc). Any ideas?  :Smile: 

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## Dixie_Flatline

OK, I had the same problem with a Dell PowerEdge 500SC and the two identical drives I ordered it with.  If I left them both on the primary channel (the way it came configured), they both showed up with the same geometry.  If I moved one to the secondary channel, they changed.  The BIOS is addressing the primary-channel drives in LBA mode (which rearranges the geometry), but not the secondary.

What I found is that if I partitioned the second drive while it was primary-slave (and showing LBA geometry, i.e. ???/255/63), then moved it to secondary-master, the BIOS recognized the LBA geometry in the partition table and would then show LBA geometry for the drive.  Strange but true.

So what you need to do is partition the second drive, then move it to the secondary channel, and you should be fine.  Good luck!

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## vod

I have the same thing with my two identical deskstars. Maybe that's why i'm getting terrible performance from one of them. They are raid0 too.

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## dbacon

 *lord wrote:*   

> 
> 
> According to my bios my hdds are 120034mb with 8192kb cache. with CHS=14593/255/63
> 
> According to kernel hda has that configuration while hdc has CHS=232581/16/63. Why is that? (Num sectors and size are the same for both drives)
> ...

 

This happened to me when I let windoze initialize one MBR, and linux initialize the other.  They assign arbitrary (yet obviously different) values for CHS.  I waxed the MBR on both, let Linux initialize them both, and they then were detected to be identical.

By 'waxed', I mean 

```
dd if=/dev/zero of=/dev/hda bs=512 count=2
```

Of course, back it up if it's important.

--Dave[/code]

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## dbacon

... and by 'initialize', I mean use 'fdisk' under linux.  It should print a warning that no valid MBR was found, and it will of course create an empty one for you.

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## dbacon

.. and there's more.

The reason why, is that Linux doesn't use BIOS values at all.  In fact, if you could turn off the BIOS for your IDE controller(which some off-board controllers support), Linux still finds the controller and drives.  It then gets the drive geometry to use from the disk label.

This being the case, if 'fdisk' finds a disk w/ no label, it has no BIOS values to write, so it just calculates some, which end up being different than the values in BIOS.  Windows, on the other hand, probably grabs the values from BIOS, and writes them, so they show up the same.

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## asimon

 *dbacon wrote:*   

> ... and by 'initialize', I mean use 'fdisk' under linux.  It should print a warning that no valid MBR was found, and it will of course create an empty one for you.

 

Will a new MBR and new CHS values affect partitions which are already on the dirve?

Cheers,

Andreas

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## perra

This has happened to me too.

I "waxed" them both and restarted Linux before I partitioned them. I think the kernel remember the geometry and you need to reboot to make it start from scratch. That was my experience anyway...

In my case one of the drives was initialized by Windows first, and I guess Windows decided that the drive should look a bit different...   :Smile: 

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## dbacon

 *asimon wrote:*   

> 
> 
> Will a new MBR and new CHS values affect partitions which are already on the dirve?
> 
> Cheers,
> ...

 

Yes, it will.   So back it all up somewhere if it's important.

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## lord

dbacon:

I tried to 'wipe', reboot and then re-partition... It didnt help  :Sad: ((

Any ideas why?

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