On 26/11/13 09:28, Andrew Dunstan wrote:
On 11/25/2013 03:19 PM, Heikki Linnakangas wrote:
On 25.11.2013 22:01, Lee Nguyen wrote:
Hi,
Having attended a few PGCons, I've always heard the remark from a few
presenters and attendees that Postgres shouldn't be run inside a VM.
That
bare metal is the only way to go.
Here at work we were entertaining the idea of running our Postgres
database
on our VM farm alongside our application vm's. We are planning to run a
few Postgres synchronous replication nodes.
Why shouldn't we run Postgres in a VM? What are the downsides? Does
anyone
have any metrics or benchmarks with the latest Postgres?
I've also heard people say that they've seen PostgreSQL to perform
worse in a VM. In the performance testing that we've done in VMware,
though, we haven't seen any big impact. So I guess the answer is that
it depends on the specific configuration of CPU, memory, disks and the
software. Synchronous replication is likely going to be the biggest
bottleneck by far, unless it's mostly read-only. I don't know if
virtualization will have a measurable impact on network latency, which
is what matters for synchronous replication.
So, I'd suggest that you try it yourself, and see how it performs. And
please report back to the list, I'd also love to see some numbers!
Yeah, and there are large numbers of public and/or private cloud-based
offerings out there (from Amazon RDS, Heroku, EnterpriseDB and VMware
among others.) Pretty much all of these are VM based, and can be
suitable for many workloads.
Maybe the advice is a bit out of date.
Agreed.
Possibly years ago the maturity of various virtualization layers was
such that the advice was sound. But these days it seems that provided
some reading is done (so you understand for instance how to make writes
go to the hosting hardware), it should be fine.
We make use of many KVM guest VMs on usually Ubuntu and the IO
performance is pretty indistinguishable from bare metal. In some tests
we did notice that VMs with >8 cpus tended to stop scaling so we are
using more smaller VMs rather than fewer big ones [1].
regards
Mark
[1] This was with Pgbench. Note this was over a year ago, so this effect
may be not present (different kernels and kvm versions), or the magic
number may be higher than 8 now...
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