Re: Growing SCSI defects list

[Chuck Swiger <cswiger%mac.com@localhost>]

On Oct 5, 2009, at 2:17 PM, raymond.meyer%rambler.ru@localhost wrote:
> One of my scsi disks has a constantly increasing "grown defects list",
> i.e. every month or so, the list grows. At the moment:
>
> Primary defects list has 2565 defects
> Grown defects list has 174 defects <- this list keeps increasing
>
> Is this normal, or does this mean my disk is failing and needs  
> replacing asap?
A small # of defects, which grows slowly over time is normal.  If the  
# is increasing faster than once a week or so, it's a sign that the  
drive is on it's way out.  smartmontools from pkgsrc can do more  
precise testing and give you better insight.
> Also, how does it detect and manage new defects? What happens to the
> data on defective sectors, does it silently get truncated to 0s, or is
> there some kind of failsafe way to remap the sectors and keep the data
> intact? If so, what is the limit on how many defects can be fixed this
> way, before I start losing data?
Every sector has checksums, and in most drives these are actually  
using Reed-Solomon ECC or variants thereof, which usually allows the  
drive to recover the data even if some bits are corrupted-- this is  
SMART ID 195 aka Hardware_ECC_Recovered.  Depending on the drive  
firmware, if it notices a problem, it will attempt to reassign the bad  
sector to a list of spares which are kept around for this purpose; but  
some drives will only do this when a sector is written to.  You can  
also use the SCSI "reassign block" command to do this process manually  
if you know from error logs that a particular sector is being reported  
as bad.
You'd have to contact your drive's manufacturer to determine how many  
spare sectors they've allocated with their default formatting, and  
this is likely a tunable value by adjusting the SCSI mode pages and  
doing a "format unit".  Doing a reformat or a write of all-zeros is a  
pretty good way of forcing the drive to reallocate all bad sectors  
(although you'd need to backup and restore your data).
Some random links:

http://www.storagereview.com/guide2000/ref/hdd/geom/formatDefect.html
http://en.wikipedia.org/wiki/Self-Monitoring,_Analysis,_and_Reporting_Technology

Regards,
--
-Chuck