hard drive sector testing on Ubuntu

[Steve Flynn]

On 4 October 2012 10:43, lazer100 <lazer100 at talktalk.net> wrote:

>>Also note that by default, badblocks repeats the test with 4 different
>>patterns.  On a modern >1TB drive, that ... can take a while, so I would
>>usually use -t 0xaa (or -t random) to only do 1 pass.
>
> there is an option to set the number of passes, but the program rejected
> the command line when I tried this!

What command did you issue?

>>Do not even thing of doing this on an SDD.  Probably not a good idea on
>>those hybrid drives either.  Reasons are outside the scope of this quick
>>e-mail, just don't do it.
>
> now I'm curious what the reasons are!

SSD's have a limited number of read/write cycles. Running badblocks
against one for any length of time would do little other than burn up
its life.

> not sure what you mean by a hybrid drive.

A spinning platter drive backed up by a smaller SSD on-board. Heavily
used blocks are cached into the SSD, thereby speeding up access to
them.

> the drive would appear to have removed the bad sectors, but I just wonder
> what happens if a bad sector is part of a file. The drive cannot just
> replace that sector as that part of the file would become junk.
>
> or are the bad sectors removed on a write?

Bad blocks are found on a write. The data is written to the block, and
then read back and compared to the write buffer. If the two match, the
data is good on the drive and the next block is written. if the two
differ, that block cannot be trusted so it's moved into the badblocks
list on the drive and the data is written to another block where the
process repeats. This is why you can hear a failing drive struggling
to write blocks cleanly and the heads zipping about. It's also why
failing drives get slower - they spend more time trying to find a good
place to stash the data.

It's not coincidence that people normally think there's something
wrong with a failing drive because it "sounds funny" and "it's much
slower".

Of course, If a block goes "bad" (maybe from a head touchdown) after
it's already got data on it, then you are indeed looking at a trashed
file.

> if the drive has started to malfunction I'm a bit wary about continuing
> to use the drive,

I would be too. Only way to find out is to run badblocks in write mode
and see how many (if any) blocks are written to the badblocks list. On
big drives, this takes some time, as you have discovered.

> would you say that zeroing a modern drive would then remove the bad sectors?

It would re-write the badblocks list, but as soon as those blocks are
written to again (and they are still bad) they'd be put back onto the
badblocks list.
-- 
Steve

When one person suffers from a delusion it is insanity. When many
people suffer from a delusion it is called religion.