Hey, all...It's been a while.

I find myself frequently in need of performing disk-to-disk copies from
a bad hard disk to a good hard disk, for system recovery purposes.
Normally, I would use dd for disk-to-disk copying, but that doesn't
work well for damaged disks.

The problem stems from a half-second (or so) delay each time dd tries
to read a bad sector from the damaged disk.  What is the source of this
delay?  I'd like to modify the kernel to remove, or at least lessen, it.

At 15 Jan 2007 05:20:37 -0800 "Michael  Mol" <mikemol@gmail.com> wrote:

> 
> Hey, all...It's been a while.
> 
> I find myself frequently in need of performing disk-to-disk copies from
> a bad hard disk to a good hard disk, for system recovery purposes.
> Normally, I would use dd for disk-to-disk copying, but that doesn't
> work well for damaged disks.
> 
> The problem stems from a half-second (or so) delay each time dd tries
> to read a bad sector from the damaged disk.  What is the source of this
> delay?  I'd like to modify the kernel to remove, or at least lessen, it.

Several sources.  The kernel will issue re-trys if it gets an error
reading a (bad) sector.  This disk itself may recalibrate itself when
encountering an error.

> 
>                                

-- 
Robert Heller             -- 978-544-6933
Deepwoods Software        -- Linux Installation and Administration
http://www.deepsoft.com/  -- Web Hosting, with CGI and Database
heller@deepsoft.com       -- Contract Programming: C/C++, Tcl/Tk
                                                                 

Michael Mol wrote:
> Hey, all...It's been a while.
> 
> I find myself frequently in need of performing disk-to-disk copies from
> a bad hard disk to a good hard disk, for system recovery purposes.
> Normally, I would use dd for disk-to-disk copying, but that doesn't
> work well for damaged disks.
> 
> The problem stems from a half-second (or so) delay each time dd tries
> to read a bad sector from the damaged disk.  What is the source of this
> delay?  I'd like to modify the kernel to remove, or at least lessen, it.
> 

The kernel tries several times to read the bad sectors and, after a 
number of tries recalibrates the controller.  I don't think that it is a 
good idea to change this behavior.

Ciao
Giovanni
-- 
   A computer is like an air conditioner,
   it stops working when you open Windows.
   Registered Linux user #337974  < http://giovanni.homelinux.net/ >

Giovanni wrote:
> Michael Mol wrote:
>>
>> I find myself frequently in need of performing disk-to-disk copies
>> from a bad hard disk to a good hard disk, for system recovery
>> purposes. Normally, I would use dd for disk-to-disk copying, but
>> that doesn't work well for damaged disks.
>>
>> The problem stems from a half-second (or so) delay each time dd
>> tries to read a bad sector from the damaged disk.  What is the
>> source of this delay?  I'd like to modify the kernel to remove, or
>> at least lessen, it.
> 
> The kernel tries several times to read the bad sectors and, after a
> number of tries recalibrates the controller.  I don't think that it
> is a good idea to change this behavior.

It would be somewhat more efficacious to replace the bad disk.

-- 
 "A man who is right every time is not likely to do very much."
                           -- Francis Crick, co-discover of DNA
 "There is nothing more amazing than stupidity in action."
                                             -- Thomas Matthews

Michael Mol wrote:
> Hey, all...It's been a while.
> 
> I find myself frequently in need of performing disk-to-disk copies from
> a bad hard disk to a good hard disk, for system recovery purposes.
> Normally, I would use dd for disk-to-disk copying, but that doesn't
> work well for damaged disks.
> 
> The problem stems from a half-second (or so) delay each time dd tries
> to read a bad sector from the damaged disk.  What is the source of this
> delay?  I'd like to modify the kernel to remove, or at least lessen, it.
> 
IME it is simply retrying it again and again, and then doing a seek to 
track zero to ensure calibration, then another attempt. Then giving up.