Bad Blocks... Should I RMA?

[Ian Smith]

In freebsd-questions Digest, Vol 285, Issue 3, Message 28
On Mon, 16 Nov 2009 23:16:27 +0100 Roland Smith <rsmith at xs4all.nl> wrote:
 > On Mon, Nov 16, 2009 at 09:43:31PM +0000, Bruce Cran wrote:
 > > On Mon, 16 Nov 2009 19:23:58 +0100
 > > Roland Smith <rsmith at xs4all.nl> wrote:
 > > 
 > > > Install the smartmontools port, and check the drive with 
 > > > 'smartctl -a /dev/ad4'. If you see a non-zero Reallocated_Sector_Ct,
 > > > RMA it immediately, as it is about to fail. If see other errors
 > > > reported, RMA it.
 > > > 
 > > > (S)ATA disk have spare sectors available. If a sector fails, it is
 > > > replaced by one of the spares by the firmware. If you see a non-zero
 > > > Reallocated_Sector_Ct, it means that the drive has run out of spares.
 > > > This is bad news.
 > > 
 > > Surely it's the other way around - if you see a value of zero in the
 > > "value" column the drive has run out of spare sectors and it's time to
 > > RMA the drive?
 > 
 > I was talking about the _RAW_VALUE column. There seems to be some differences
 > in interpretation between vendors as to what the VALUE column means. Most of
 > the advice I've seen over the years says to look at the RAW_VALUE.
 > 
 > See http://en.wikipedia.org/wiki/S.M.A.R.T. as well.

Mmm, but as that article - which really only mentions the 'normalised' 
values smartctl presents in passing - points out, there can be quite a 
lot of variation between different manufacturers as to what RAW_VALUE 
actually represents for various attributes, whereas the usage of VALUE 
WORST THRESH values is much more consistent, and what the vendor is 
actually presenting as the SMART good/fair/fail analysis to the world.

For instance, I've got two Fujitsu 5400rpm 2.5" drives in two laptops, 
one MHV2040AH with near 19,000 hours on it, and a much newer MHV2120AH, 
40 and 120GB respectively.  Nice quiet low-power laptop drives, fwiw.

Both show as (more recently) being in the smartctl database, and both 
show _exactly_ the same values for this one:

  5 Reallocated_Sector_Ct   0x0033   100   100   024    Pre-fail  Always  -  8589934592000

Now if that were a number of 512-byte sectors, it'd be 4096000 GB! :)
but both drives are 100% ok, as the VALUE / WORST figures show.

 > > From what I've seen the 'raw' column appears to count
 > > the number of sectors the drive has remapped using the spares buffer.
 > > If it gets into the hundreds it's probably time to think about RMA'ing
 > > the drive
 > 
 > Yes, the raw value is the number of sectors allocated from the spares. I
 > originally thought it was the number of reallocations _beyond_ the
 > spares. That's a misunderstanding on my part.

Again, may depend on the drive make/model.  With the same make/model you 
can of course usefully compare raw values, but be careful about drawing 
inferences for different drives, or you may be RMA'ing needlessly ..

 > Nevertheless this attribute (along with several) is marked on the Wikipedia
 > page for smart as a "Potential indicator of imminent electromechanical
 > failure". You can find the same attributes marked as critical when perusing
 > mailing list archives.
 > 
 > For me, my data is worth much more than the harddisk it is on. Some of it is
 > literally irreplacable. So my policy is to go look for a replacement harddisk
 > as soon as the RAW_VALUEs of any of these critical indicators start going up
 > from zero. And store any data at least on two harddisks, whether in a mirror
 > or in a cron+rsync setup.

That'd be the case for the disks you tend to use.  I was first going to 
reply to Bruce's message when I spotted yours, but you've dropped the 
last bit of his quote, that I was about to wholeheartedly agree with :)

 : If it gets into the hundreds it's probably time to think about RMA'ing
 : the drive - if you trust that the 'raw' column is reporting what you
 : think it is (you should really only base your decision on the value,
 : worst and threshold columns).

cheers, Ian