Re: Rearchitecting for storage

On Thu, Jul 18, 2019 at 10:09 PM Matthew Pounsett <matt@conundrum.com> wrote:
> That would likely keep the extra storage requirements small, but still non-zero.  Presumably the upgrade would be
unnecessaryif it could be done without rewriting files.  Is there any rule of thumb for making sure one has enough
spaceavailable for the upgrade?   I suppose that would come down to what exactly needs to get rewritten, in what order,
etc.,but the pg_upgrade docs don't seem to have that detail.  For example, since we've got an ~18TB table (including
itsindices), if that needs to be rewritten then we're still looking at requiring significant extra storage.  Recent
experiencesuggests postgres won't necessarily do things in the most storage-efficient way.. we just had a reindex on
thatdatabase fail (in --single-user) because 17TB was insufficient free storage for the db to grow into. 
>

I've done a test on a virtual machine of mine, with the following
three databases: one 0f 4.9 GB, one of 500 MB, one of 50 MB. I know
this is not even close to your environment, however upgrading with
pg_upgrade from 10.9 to 11.4 _without_ the link option ask for 85% of
space.

On a machine with a single database of 8.9 GB and a space occupation,
as reported by df, of 64% (mean 46% available) I was able to upgrade
from 10.9 to 11.4 without the link option. Space occupation increased
of 90%.
Using the link option on the same cluster required 1.1% of extra space
(around 100 MB).
Of course, these are poor-man results, but give you an advice on the
space required by pg_ugprade (which seems to be less than 100% or 2x).

Hope this helps.
Luca