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What happens to dd command on failed disk?

Tony Williams
Regular Advisor

What happens to dd command on failed disk?

We had a disk that failed at a remote site. We called HP out to have the disk replaced and executed a "dd" command to light up the disk so the CE could see which disk to replace. The "dd" command did not return after 15 minutes and would not end with sigkill. We had the CE pull the bad disk and the dd command still did not end. The dd command only ended when the new disk was inserted.

I have a few questions. Can you tell me why the "dd" or any disk related command hangs on a bad disk?

Also where is the command hung at, the SCSI controller, or the disk logic board?

If we waited longer would the command eventually end?

Did we create a problem by removing the disk with the dd command still active?

Thanks
3 REPLIES
James R. Ferguson
Acclaimed Contributor

Re: What happens to dd command on failed disk?

Hi Tony:

> Can you tell me why the "dd" or any disk related command hangs on a bad disk?

The process was waiting on an I/O to complete but the physical disk would not respond to the I/O request.

> Also where is the command hung at, the SCSI controller, or the disk logic board?

Most likely, at the disk level.

> If we waited longer would the command eventually end?

Given what you stated, I'd think that this would have been unlikely.

> Did we create a problem by removing the disk with the dd command still active?

No, not at all. Your 'dd' was only a read request to the disk.

Regards!

...JRF...



Ganesan R
Honored Contributor

Re: What happens to dd command on failed disk?

Hi,

Can you tell me why the "dd" or any disk related command hangs on a bad disk?

Because it didn't get any response from the disk. Unlike other commands dd will not comeback if the request was unsuccessful

Also where is the command hung at, the SCSI controller, or the disk logic board?

disk level. dd is working on disk block level. Also you are specifying the disk as a input.

If we waited longer would the command eventually end?

No.

Did we create a problem by removing the disk with the dd command still active?

After some time dd process go into hung state after unsuccessful tries to the i/o operation.
Best wishes,

Ganesh.
Dennis Handly
Acclaimed Contributor

Re: What happens to dd command on failed disk?

>Did we create a problem by removing the disk with the dd command still active?

If it was hung before the disk was removed, then no. And especially if you didn't have to reboot.