[linux-audio-user] AMD64 question: update

Pieter Palmers pieterp at joow.be
Thu Jul 6 10:05:53 EDT 2006


Paul Winkler wrote:
> On Thu, Jul 06, 2006 at 01:43:40PM +0200, Pieter Palmers wrote:
>> Modern harddisks use a lot of write caching on the controller to achieve 
>> decent performance. So when power goes down when there is data in the 
>> write cache, it is lost. The file system however 'thinks' that data has 
>> been written correctly. This hence results in file system corruption.
> 
> FS corruption is no fun (I once spent two days recovering data
> after bad RAM corrupted an ext2 fs... I ended up with every file I had
> in lost+found).  But the particular failure I mentioned was drive
> hardware, no doubt about it. Lots of low-level IDE errors in
> /var/log/messages.  Couldn't fsck it, couldn't get any raw
> data out of it with "dd if=/dev/hdb", nothing.
> I didn't have any warning, either... no funny noises, no
> problems or errors the last time I mounted it.  *shrug*
> 

Of course I'm not suggesting that there are no hardware failures, and 
I'm really not questioning your judgment.

Just sharing my personal experience (own hardware and that of others), 
which is that once you prune out the hard disk failures caused by power 
outage, there are not much left. And in (almost) all of the power outage 
caused failures you can re-use the hard disk perfectly after a low level 
format. And that is not due to bad sector relocation, but simply because 
the CRC errors are cleared.

Problems caused by the interruption of a write operation by the disk are 
not distinguishable from 'real' hardware failures by the OS. The hard 
disk's firmware treats the sector where this interruption occurred as a 
bad sector, and reports it like that to the OS. So you also get a lot of 
messages in /var/log/messages. This phenomenon occurs on the harddisk 
itself, past the operating system, filesystem and ide controller.

However, you should be able to get some raw data out of it using dd (or 
ddrescue), as not all sectors are marked as 'bad'. That makes it 
slightly less bad as a total hard disk crash.

Pieter

PS: another tip: The sysadmins at work once told me that their 
experience is that harddisks tend to fail when they are shut down. What 
they dread the most is having to power down a server, even cleanly.


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