Re: Need help with 8.4 Performance Testing - Mailing list pgsql-performance

From Gregory Stark
Subject Re: Need help with 8.4 Performance Testing
Date
Msg-id 87y6yohmsy.fsf@oxford.xeocode.com
Whole thread Raw
In response to Re: Need help with 8.4 Performance Testing  (Matthew Wakeling <matthew@flymine.org>)
List pgsql-performance
Matthew Wakeling <matthew@flymine.org> writes:

> On Tue, 9 Dec 2008, Scott Marlowe wrote:
>> I wonder how many hard drives it would take to be CPU bound on random
>> access patterns?  About 40 to 60?  And probably 15k / SAS drives to
>> boot.  Cause that's what we're looking at in the next few years where
>> I work.
>
> There's a problem with that thinking. That is, in order to exercise many
> spindles, you will need to have just as many (if not more) concurrent requests.
> And if you have many concurrent requests, then you can spread them over
> multiple CPUs. So it's more a case of "How many hard drives PER CPU". It also
> becomes a matter of whether Postgres can scale that well.

Well:

$ units
2445 units, 71 prefixes, 33 nonlinear units
You have: 8192 byte/5ms
You want: MB/s
    * 1.6384
    / 0.61035156

At 1.6MB/s per drive if find Postgres is cpu-bound doing sequential scans at
1GB/s you'll need about 640 drives to keep one cpu satisfied doing random I/O
-- assuming you have perfect read-ahead and the read-ahead itself doesn't add
cpu overhead. Both of which are false of course, but at least in theory that's
what it'll take.


--
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning

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