[linux-lvm] thin handling of available space

Marek Podmaka marki at marki-online.net
Thu Apr 28 06:46:35 UTC 2016

Hello Xen,

Wednesday, April 27, 2016, 23:28:31, you wrote:

> The real question you should be asking is if it increases the monitoring
> aspect (enhances it) if thin pool data is seen through the lens of the
> filesystems as well.

> Beside the point here perhaps. But. Let's drop the "real sysadmin" 
> ideology. We are humans. We like things to work for us. "Too easy" is 
> not a valid criticism for not having something.

As far as I know (someone correct me) there is no mechanism at all in
kernel for communication from lower fs layers to higher layers -
besides exporting static properties like physical block size. The
other way (from higher layer like fs to lower layers works fine - for
example discard support).

So even if what you are asking might be valid, it isn't as simple as adding
some parameter somewhere and it would magically work. It is about
inventing and standardizing new communication system, which would of
course work only with new versions of all the tools involved.

Anyway, I have no idea what would filesystem itself do with information
that no more space is available. Also this would work only for lvm
thin pools, not for thin provision directly from storage, so it would
be a non-consistent mess. Or you would need another protocol for
exporting thin-pool related dynamic data from storage (via NAS, SAN,
iSCSI and all other protocols) to the target system. And in some
organizations it is not desirable at all to make this kind of
information visible to all target systems / departments.

What you are asking can be done for example directly in "df" (or you
can make a wrapper script), which would not only check the filesystems
themselves, but also the thin part and display the result in whatever
format you want.

Also displaying real thin free space for each fs won't be "correct".
If I see 1 TB free in each filesystem and starting writing, by the
time I finish, those 1 TB might be taken by the other fs. So
information about current free space in thinp is useless for me, as in
1 minute it could be totally different number.

  bYE, Marki

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