Re: blending fast and temp space volumes

On Wed, Feb 21, 2018 at 7:53 AM, Rick Otten <rottenwindfish(at)gmail(dot)com>
wrote:

> Some of my data processes use large quantities of temp space - 5 or 6T
> anyway.
>
> We are running in Google Cloud. In order to get the best performance out
> of all of my queries that might need temp space, I've configured temp space
> on a concatenated local (volatile) SSD volume. In GCE, local SSD's are
> more than 20x faster than SAN SSD's in GCE.
>
> side note: The disadvantage of local SSD is that it won't survive
> "hitting the virtual power button" on an instance, nor can it migrate
> automatically to other hardware. (We have to hit the power button to add
> memory/cpu to the system, and sometimes the power button might get hit by
> accident.) This is OK for temp space. I never have my database come up
> automatically on boot, and I have scripted the entire setup of the temp
> space volume and data structures. I can run that script before starting
> the database. I've done some tests and it seems to work great. I don't
> mind rolling back any transaction that might be in play during a power
> failure.
>
> So here is the problem: The largest local SSD configuration I can get in
> GCE is 3T. Since I have processes that sometimes use more than that, I've
> configured a second temp space volume on regular SAN SSD. My hope was
> that if a query ran out of temp space on one volume it would spill over
> onto the other volume. Unfortunately it doesn't appear to do that
> automatically. When it hits the 3T limit on the one volume, the query
> fails. :-(
>
> So, the obvious solution is to anticipate which processes will need more
> than 3T temp space and then 'set temp_tablespaces' to not use the 3T
> volume. And that is what we'll try next.
>
> Meanwhile, I'd like other processes to "prefer" the fast volume over the
> slow one when the space is available. Ideally I'd like to always use the
> fast volume and have the planner know about the different performance
> characteristics and capacity of the available temp space volumes and then
> choose the best one (speed or size) depending on the query's needs.
>
> I was wondering if there anyone had ideas for how to make that possible.
> I don't think I want to add the SAN disk to the same LVM volume group as
> the local disk, but maybe that would work, since I'm already building it
> with a script anyhow ... Is LVM smart enough to optimize radically
> different disk performances?
>

Couldn't you configure both devices into a single 6T device via RAID0 using
md?

Craig

>
> At the moment it seems like when multiple temp spaces are available, the
> temp spaces are chosen in a 'round robin' or perhaps 'random' fashion. Is
> that true?
>
> I'm meeting with my GCE account rep next week to see if there is any way
> to get more than 3T of local SSD, but I'm skeptical it will be available
> any time soon.
>
> thoughts?
>
>
>
>

--
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Craig A. James
Chief Technology Officer
eMolecules, Inc.
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