hopefully simple query regarding dd

tech-lists tech-lists at zyxst.net
Thu Apr 13 10:08:33 UTC 2017

On 13/04/2017 10:36, Steve O'Hara-Smith wrote:
> It should be safe and work, assuming that what you want to do is
> read and rewrite every block. You should probably think very hard about
> what you want it to do in the case of read and/or write errors.
> 	I'm not sure why you'd want to do it though, discs are not
> particularly volatile storage even on timescales of years.
> 	Of course if it ever gets a read error that passes the CRC
> (aka silent corruption) it will cement the error in stone for you by
> writing it back.
> 	If you are really concerned about long term data retention then I
> suggest ZFS with plenty of redundancy (at least two drives redundancy) and a
> regular scrub.


The context is this:

old netbook, slow cpu (boo!) low power (yay!) .. was thinking of making
it a server on either the local or external LAN.

AMD C-70 APU with Radeon(tm) HD Graphics (998.20-MHz K8-class CPU)
real memory  = 4294967296 (4096 MB)
avail memory = 3781300224 (3606 MB)
<WDC WD3200BPVT-22JJ5T0 01.01A01> ATA8-ACS SATA 2.x device
ada0: Serial Number WD-WX51A3312496
ada0: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada0: Command Queueing enabled
ada0: 305245MB (625142448 512 byte sectors)
ada0: quirks=0x1<4K>

Brand new freebsd-12 install as of yesterday. I think ZFS might just
kill it though.

Before installing, I ran a dd if=/dev/zero of=/dev/ada0 bs=1m
conv=noerror from the shell option in the installer because if there
were blocks giving errors, I wanted to re-map them somewhere safe.

After installing, the first thing I did was to install and run smartd to
see where I was with the disk. I knew previously the disk was likely to
have errors.

Here's the relevant stats:

9 Power_On_Hours          -O--CK   085   085   000    -    11379
191 G-Sense_Error_Rate      -O--CK   001   001   000    -    759
  5 Reallocated_Sector_Ct   PO--CK   183   183   140    -    143
197 Current_Pending_Sector  -O--CK   001   001   000    -    65216
198 Offline_Uncorrectable   ----CK   100   253   000    -    0
196 Reallocated_Event_Count -O--CK   196   196   000    -    4
194 Temperature_Celsius     -O---K   104   089   000    -    39

Basically, I want that Current_Pending_Sector number to go to 0. I guess
if it doesn't, then it's either replace the disk or it's time to dispose
of the netbook. Right now it's rebuilding 12 with witness/invariants
disabled, so I can't really do the dd thing I earlier asked about right
now. I just needed to know if it could be run without having to reinstall.


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