[gpfsug-discuss] Checking a file-system for errors

[Uwe Falke]

If you do both, you are on the safe side. I am not sure wether mmfsck 
reads both replica of the metadata (if it it does, than one could spare 
the mmrestripefs -c WRT metadata, but I don't think so), if not, one could 
still have luckily checked using valid metadata where maybe one (or more) 
MD block has (have) an invalid replica which might come up another time 
...
But the mmfsrestripefs -c is not only ensuring the sanity of the FS but 
also of the data stored within (which is not necessarily the same). 

Mostly, however,  filesystem checks are only done if fs issues are 
indicated by errors in the logs. Do you have reason to assume your fs has 
probs?
 
Mit freundlichen Grüßen / Kind regards

 
Dr. Uwe Falke
 
IT Specialist
High Performance Computing Services / Integrated Technology Services / 
Data Center Services
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From:   "Simon Thompson (IT Research Support)" <S.J.Thompson at bham.ac.uk>
To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:   10/11/2017 12:32 PM
Subject:        Re: [gpfsug-discuss] Checking a file-system for errors
Sent by:        gpfsug-discuss-bounces at spectrumscale.org



OK thanks,

So if I run mmfsck in online mode and it says:
"File system is clean.
Exit status 0:10:0."

Then I can assume there is no benefit to running in offline mode?

But it would also be prudent to run "mmrestripefs -c" to be sure my
filesystem is happy?

Thanks

Simon

On 11/10/2017, 11:19, "gpfsug-discuss-bounces at spectrumscale.org on behalf
of UWEFALKE at de.ibm.com" <gpfsug-discuss-bounces at spectrumscale.org on
behalf of UWEFALKE at de.ibm.com> wrote:

>Hm , mmfsck will  return not very reliable results in online mode,
>especially it will report many issues which are just due to the transient
>states in a files system in operation.
>It should however not find less issues than in off-line mode.
>
>mmrestripefs -c does not do any logical checks, it just checks for
>differences of multiple replicas of the same data/metadata.
>File system errors can be caused by such discrepancies (if an odd/corrupt
>replica is used by the GPFS), but can also be caused (probably more
>likely) by logical errors / bugs when metadata were modified in the file
>system. In those cases, all the replicas are identical nevertheless
>corrupt (cannot be found by mmrestripefs)
> 
>So, mmrestripefs -c is like scrubbing for silent data corruption (on its
>own, it cannot decide which is the correct replica!), while mmfsck checks
>the filesystem structure for logical consistency.
>If the contents of the replicas of a data block differ, mmfsck won't see
>any problem (as long as the fs metadata are consistent), but mmrestripefs
>-c will. 
>
> 
>Mit freundlichen Grüßen / Kind regards
>
> 
>Dr. Uwe Falke
> 
>IT Specialist
>High Performance Computing Services / Integrated Technology Services /
>Data Center Services
>--------------------------------------------------------------------------
>-----------------------------------------------------------------
>IBM Deutschland
>Rathausstr. 7
>09111 Chemnitz
>Phone: +49 371 6978 2165
>Mobile: +49 175 575 2877
>E-Mail: uwefalke at de.ibm.com
>--------------------------------------------------------------------------
>-----------------------------------------------------------------
>IBM Deutschland Business & Technology Services GmbH / Geschäftsführung:
>Thomas Wolter, Sven Schooß
>Sitz der Gesellschaft: Ehningen / Registergericht: Amtsgericht Stuttgart,
>HRB 17122 
>
>
>
>
>From:   "Simon Thompson (IT Research Support)" <S.J.Thompson at bham.ac.uk>
>To:     "gpfsug-discuss at spectrumscale.org"
><gpfsug-discuss at spectrumscale.org>
>Date:   10/11/2017 10:47 AM
>Subject:        [gpfsug-discuss] Checking a file-system for errors
>Sent by:        gpfsug-discuss-bounces at spectrumscale.org
>
>
>
>I'm just wondering if anyone could share any views on checking a
>file-system for errors.
>
>For example, we could use mmfsck in online and offline mode. Does online
>mode detect errors (but not fix) things that would be found in offline
>mode?
>
>And then were does mmrestripefs -c fit into this?
>
>"-c
>                  Scans the file system and compares replicas of
>                  metadata and data for conflicts. When conflicts
>                  are found, the -c option attempts to fix
>                  the replicas.
>"
>
>Which sorta sounds like fix things in the file-system, so how does that
>intersect (if at all) with mmfsck?
>
>Thanks
>
>Simon
>
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