hopefully simple query regarding dd
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.
> 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
> 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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