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Re: Testing Sandforce SSD

From: Matthew Wakeling <matthew(at)flymine(dot)org>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: Yeb Havinga <yebhavinga(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Testing Sandforce SSD
Date: 2010-07-26 15:42:48
Message-ID: alpine.DEB.2.00.1007261632380.20471@aragorn.flymine.org (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, 26 Jul 2010, Greg Smith wrote:
> Matthew Wakeling wrote:
>> Does your latency graph really have milliseconds as the y axis? If so, this 
>> device is really slow - some requests have a latency of more than a second!
>
> Have you tried that yourself?  If you generate one of those with standard 
> hard drives and a BBWC under Linux, I expect you'll discover those latencies 
> to be >5 seconds long.  I recently saw >100 *seconds* running a large pgbench 
> test due to latency flushing things to disk, on a system with 72GB of RAM. 
> Takes a long time to flush >3GB of random I/O out to disk when the kernel 
> will happily cache that many writes until checkpoint time.

Apologies, I was interpreting the graph as the latency of the device, not 
all the layers in-between as well. There isn't any indication in the email 
with the graph as to what the test conditions or software are. Obviously 
if you factor in checkpoints and the OS writing out everything, then you 
would have to expect some large latency operations. However, if the device 
itself behaved as in the graph, I would be most unhappy and send it back.

Yeb also made the point - there are far too many points on that graph to 
really tell what the average latency is. It'd be instructive to have a few 
figures, like "only x% of requests took longer than y".

Matthew

-- 
 I wouldn't be so paranoid if you weren't all out to get me!!

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