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Re: Do integrity checks by file comparison really work?

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Robert Wessel wrote:
> On Sat, 19 Jan 2013 17:12:24 +1300, Ian Collins <ian-news@hotmail.com>
> wrote:
>
>> Robert Wessel wrote:
>>> On Wed, 16 Jan 2013 11:38:54 +0100, Thomas B <thomasb@nospam.it>
>>> wrote:
>>>>
>>>> You wrote: "I've never seen an OS that would share file buffers between
>>>> two files like that", so I suppose this should be the same for files on
>>>> the same volume.
>>>
>>>
>>> Some OS's/filesystems can do a copy with the equivalent of a *nix hard
>>> link, and then do a copy-on-write procedure if the file is ever
>>> modified.  In that case you may have only a single copy of the file.
>>> It can't do that to you across volumes.
>>
>> ZFS is a good example.  If you snapshot or clone a filesystem, no extra
>> space is used until it (a clone) or the source are modified.
>>
>> ZFS also checksums its data, so there is no need to use another tool to
>> validate copes.
>
>
> That's not completely true - a copy can go bad while you're not
> looking.  The ZFS checksums will catch that right away when you do go
> to read it (so actually comparing it to the original is theoretically
> not necessary), a compare after writing a copy could add a level of
> confidence to the backup.  Of course nothing prevents sectors on the
> backup disk from going bad immediately after you do the compare.

Paranoia makes for a good sys-admin!  That's one reason why ZFS has the 
scrub feature which reads all of the data in a pool.

> Another reason to do a compare is if you're paranoid enough to not
> completely trust the backup software/filesystem/I/O subsystem/disk
> drive.  We've all seen cases where the software and/or hardware did
> peculiar things.

There were a number of cases of virtual environments causing trouble 
with ZFS by claiming data was written to disk when it wasn't.

-- 
Ian Collins

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Thread

Do integrity checks by file comparison really work? Thomas B <thomasb@nospam.it> - 2013-01-08 13:04 +0100
  Re: Do integrity checks by file comparison really work? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-13 05:04 -0600
    Re: Do integrity checks by file comparison really work? Mark F <mark53916@gmail.com> - 2013-01-13 09:26 -0500
      Re: Do integrity checks by file comparison really work? Mark F <mark53916@gmail.com> - 2013-01-13 09:40 -0500
    Re: Do integrity checks by file comparison really work? Thomas B <thomasb@nospam.it> - 2013-01-16 11:38 +0100
      Re: Do integrity checks by file comparison really work? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-18 15:07 -0600
        Re: Do integrity checks by file comparison really work? Ian Collins <ian-news@hotmail.com> - 2013-01-19 17:12 +1300
          Re: Do integrity checks by file comparison really work? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-18 22:37 -0600
            Re: Do integrity checks by file comparison really work? Ian Collins <ian-news@hotmail.com> - 2013-01-19 18:36 +1300
        Re: Do integrity checks by file comparison really work? Thomas B <thomasb@nospam.it> - 2013-01-19 14:06 +0100
          Re: Do integrity checks by file comparison really work? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-21 03:40 -0600
            Re: Do integrity checks by file comparison really work? Thomas B <thomasb@nospam.it> - 2013-01-21 17:41 +0100
              Re: Do integrity checks by file comparison really work? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-21 15:28 -0600
  Re: Do integrity checks by file comparison really work? Willem <willem@turtle.stack.nl> - 2013-01-14 18:23 +0000
    Re: Do integrity checks by file comparison really work? Thomas B <thomasb@nospam.it> - 2013-01-16 16:46 +0100
      Re: Do integrity checks by file comparison really work? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-18 15:09 -0600

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