[gpfsug-discuss] what's on a 'dataOnly' disk?

Yuri L Volobuev volobuev at us.ibm.com
Mon Feb 1 18:28:01 GMT 2016

> What's on a 'dataOnly' GPFS 3.5.x NSD besides data and the NSD disk
> header, if anything?

That's it.  In some cases there may also be a copy of the file system
descriptor, but that doesn't really matter in your case.

> I'm trying to understand some file corruption, and one potential
> explanation would be if a (non-GPFS) server wrote to a LUN used as a
> GPFS dataOnly NSD.
> We are not seeing any 'I/O' or filesystem errors, mmfsck (online) doesn't
> detect any errors, and all NSDs are usable. However, some files seem to
> have changes in content, with no changes in metadata (modify timestamp,
> ownership), including files with the GPFS "immutable" ACL set.

This is all consistent with the content on a dataOnly disk being
overwritten outside of GPFS.

> If an NSD was changed outside of GPFS control, would mmfsck detect
> filesystem errors, or would the GPFS filesystem be consistent, even
> though the content of some of the data blocks was altered?

No.  mmfsck can detect metadata corruption, but has no way to tell whether
a data block has correct content or garbage.

> Is there any metadata or checksum information maintained by GPFS, or any
> means of doing a consistency check of the contents of files that would
> correlate with blocks stored on a particular NSD?

GPFS on top of traditional disks/RAID LUNs doesn't checksum data blocks,
and thus can't tell whether a data block is good or bad.  GPFS Native RAID
has very strong on-disk data checksumming, OTOH.

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