[linux-lvm] about the lying nature of thin

[Mark Mielke]

Lots of interesting ideas in this thread.

But the practical of things is that there is a need for thin volumes that
are over provisioned. Call it a lie if you must, but I want to have
multiple snapshots, and not be forced to have 10X the storage, just so that
I can *guarantee* that I will have the technical capability to fully
allocate every snapshot without running out of space. This is for my
requirements, where I am not being naive or irresponsible. I'm not
representing the situation to myself. I know exactly what to expect, and I
know that it isn't only important to monitor, but it is also important to
understand the usage patterns. For example, in some of our use cases, files
will only normally be extended or created as new, at which point the
overhead of a snapshot is close to zero.

If people find this model unacceptable, then I think they should not use
thin volumes. It's a technology choice.

We have many systems like this beyond LVM... For example, the NetApp FAS
devices we have are set up with this type of model, and IT normally
allocates 10% or more for "snapshots", and when we get this wrong, it does
hurt in various ways, usually requiring that the snapshots get dumped, and
that we figure out why the monitoring failed. Normally, IT adds to the
aggregate as it passes a threshold. In the particular case that is
important for me - we have a fixed size local SSD for maximum performance,
and we still want to take frequent snapshots (and prune them behind),
similar to what we do on NetApp, but all in the context of local storage. I
don't use the word "lie" to IT in these cases. It's a partnership, and
attempt to make the most use of the storage and the technology.

There was some discussion about how data is presented to the higher layers.
I didn't follow the suggestion exactly (communicating layout information?),
but I did have these thoughts:

   1. When the storage runs out, it clearly communicates layout information
   to the caller in the form of a boolean "does it work or not?"
   2. There are other ways that information does get communicated, such as
   if a device becomes read only. For example, an iSCSI LUN.

I didn't follow communication of specific layout information as this didn't
really make sense to me when it comes to dynamic allocation. But, if the
intent is to provide early warning of the likelihood of failure, compared
to waiting to the very last minute where it has already failed, it seems
like early warning would be useful. I did have a question about the
performance of this type of communication, however, as I wouldn't want the
host to be constantly polling the storage to recalculate the up-to-date
storage space available.
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