On 05/12/14 00:43, Cejka Rudolf wrote:
> Daniel Pocock wrote (2014/12/04):
>> On 04/12/14 18:35, Kern Sibbald wrote:
>>> On 12/03/2014 08:49 PM, Daniel Pocock wrote:
>>>>
>>>> Does Bacula checksum content on the spool disk before sending it to tape?
>>>>
>>>> To be more explicit, if there is a single bit error on the spool disk,
>>>> will it be noticed before going onto tape or would it only be noticed in
>>>> future when a file is taken off the tape?
>>>
>>> Unless you are running ZFS for the spool disk, the error will only be
>>> noticed when the data is read from the tape.
>>>
>>
>> In that case, it sounds like a good idea to use ZFS or Btrfs with
>> checksums enabled
> 
> Hard drives use error correction/detection codes, so single bit error
> without any error indication is unlikely. Especially in case of spool
> disks, where datas are read shortly after write.
> 

Unfortunately, that is completely untrue.  Disks and IO subsystems do
not provide any guarantees that they will return the exact data that was
written.  That is why modern filesystems have checksums.

The actual corruption of data often occurs in the writing phase, so
whether you read back the sector in 5 minutes or 5 weeks, it will always
come back with some bit changed.  I've seen this more than once
unfortunately.

This looks at some of the issues;

https://blogs.oracle.com/bonwick/entry/zfs_end_to_end_data



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