Re: table spaces

On 13 Mar 2013, at 13:26, Shaun Thomas <sthomas@optionshouse.com> wrote:

> On 03/12/2013 05:49 PM, Gregg Jaskiewicz wrote:
>
>> So out of 6 disks then having 4 in Raid 1+0 configuration and other
>> two in mirror for WAL. That's another option then for me to test.
>
> That is an option, but it's not necessarily a good one. If all you have are six disks, you are probably better off
justdoing a big RAID-10 for everything. 
>
> As Alban mentioned, you would do best to stay away from RAID-5 as that will basically obliterate your write speeds,
whichis especially critical for WAL files. 
>
> There is one caveat to all of this, of course. What you plan to use the server for has a critical impact on these
decisions.
>
> What it boils down to, is that six spindles is not very many. You don't really have enough to dedicate any to your
WALfiles without drastically cutting the available IOPS for your regular data. In addition, with any of these setups,
you'llwant a hot spare in case of drive failure. If you have six disks available, you really only have five. That's not
aneven number, so one is wasted in RAID-10, and you may find in your RAID-5 tests that it doesn't work well for
long-termuse. 
I do have spare, but there was no point mentioning them.


>
> The truth is that you can get by for quite a while on this no matter how you split up those six spindles. Until you
needa lot of heavy disk reads or writes. Then it's going to hurt. So ask yourself what kind of load or usage you
expect,and plan accordingly. 
>
> I have to tell you though, we had a server with twelve spindles three years ago, and it barely kept up with our
transactionload. We had two hot spares, a RAID-1, and 8-disks in a RAID-10. Several pgbench tests back then showed that
ourRAID-10 could only adequately serve 1800TPS directly, and we needed at least 6000. Ultimately, it lead to us
switchingto NVRAM (SSD) for high TPS data, and creating a tablespace on a RAID-10 for archived or low-priority data. 
>
> But we got by on those original 12 spindles for a couple years. If your data needs are less, you can probably do OK
withsix. For a while, anyway. :) 

The problem I have is that this isn't just for a single installation. The product can be installed (and will be) in few
placesin the future. We just provide software. But I need tools to plan these things depending on load per installation
basis,taking into account that it practically be different for each customer, not to mention variations in hardware. 

Is that SSD mixed in with other disks?