Re: Using high speed swap to improve performance?

On Sun, Apr 4, 2010 at 4:52 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Apr 2, 2010 at 3:15 PM, Christiaan Willemsen
> <cwillemsen(at)technocon(dot)com> wrote:
>> About a year ago we setup a machine with sixteen 15k disk spindles on
>> Solaris using ZFS. Now that Oracle has taken Sun, and is closing up Solaris,
>> we want to move away (we are more familiar with Linux anyway).
>>
>> So the plan is to move to Linux and put the data on a SAN using iSCSI (two
>> or four network interfaces). This however leaves us with with 16 very nice
>> disks dooing nothing. Sound like a wast of time. If we were to use Solaris,
>> ZFS would have a solution: use it as L2ARC. But there is no Linux filesystem
>> with those features (ZFS on fuse it not really an option).
>>
>> So I was thinking: Why not make a big fat array using 14 disks (raid 1, 10
>> or 5), and make this a big and fast swap disk. Latency will be lower than
>> the SAN can provide, and throughput will also be better, and it will relief
>> the SAN from a lot of read iops.
>>
>> So I could create a 1TB swap disk, and put it onto the OS next to the 64GB
>> of memory. Then I can set Postgres to use more than the RAM size so it will
>> start swapping. It would appear to postgres that the complete database will
>> fit into memory. The question is: will this do any good? And if so: what
>> will happen?
>
> I suspect it will result in lousy performance because neither PG nor
> the OS will understand that some of that "memory" is actually disk.
> But if you end up testing it post the results back here for
> posterity...

Err, the OS will understand it, but PG will not.

...Robert