hopefully simple query regarding dd

Steve O'Hara-Smith steve at sohara.org
Thu Apr 13 10:25:22 UTC 2017

On Thu, 13 Apr 2017 11:08:23 +0100
tech-lists <tech-lists at zyxst.net> wrote:

> 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.
> Hi,
> 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.

	Ah - so forget about redundancy :)

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

	It's not *that* much worse than the box that serves my RAIDZ2 NAS
but there's not much point with only one drive IMHO.

> 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.

	OK so that should have forced all the reallocation the drive needs
to do unless it is dying.

> 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

	Eeek! Is that really claiming nearly 64K sectors waiting to
remap ?

> 196 Reallocated_Event_Count -O--CK   196   196   000    -    4

	But why isn't this *much* higher ? Puzzled.

	Hmm before you do anything drastic do a read test on the whole
drive (dd to /dev/null with a reasonable blocksize), it may just be waiting
for a good read on sectors you've never touched apart from writing nulls.

> now. I just needed to know if it could be run without having to reinstall.

	Yeah you should be safe - at least as safe as anything with those
odd numbers. Do the read test first though.

Steve O'Hara-Smith <steve at sohara.org>

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