hopefully simple query regarding dd

[Arthur Chance]

On 13/04/2017 11:18, Polytropon wrote:
> On Thu, 13 Apr 2017 11:08:23 +0100, tech-lists 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.
>>
>> AMD C-70 APU with Radeon(tm) HD Graphics (998.20-MHz K8-class CPU)
>> real memory  = 4294967296 (4096 MB)
> 
> 4 GB RAM and _old_? ;-)
> 
> 
> 
>> Brand new freebsd-12 install as of yesterday. I think ZFS might just
>> kill it though.
> 
> Having 8+ GB RAMM for ZFS is probably a good choice, but if
> speed isn't a primary concern (maybe because it's limited
> by network) 4 GB RAM will of course work.

I'm running ZFS on a small, low power machine with 4GB quite happily and
have run it on 2GB before now. It'll never match a dual Xeon 64GB server
system but for light loads it's perfectly sufficient and you get the
advantage of total file system flexibility. (The current small machine
has 62 ZFS file systems in total for its various jails.) The one thing
you do need to do is turn down vfs.zfs.arc_max to about half of the
available memory in /boot/loader.conf.

>> 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.
> 
> The disk does this internally. When bad blocks start "bubbling
> up", it indicates that the disk ran out of spare blocks for
> automatic remapping. In this case, the disk is usually starting
> to die, suggesting to replace it.
> 
> 
> 
>> 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.
> 
> Probably replacing the disk is the better choice when the
> netbook itself isn't faulty.

Agreed. Backblaze's articles on SMART monitoring say that attributes 5,
197 & 198 being above 0 is a major warning sign of impending failure.

https://www.backblaze.com/blog/hard-drive-smart-stats

https://www.backblaze.com/blog/what-smart-stats-indicate-hard-drive-failures

>> 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.
> 
> Usually yes. Rebuilding world is also a convenient stress test
> both for the disk and the machine (CPU and RAM).

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