Message-ID: <519D1232.4090701@optionshouse.com>
Date: Wed, 22 May 2013 13:45:06 -0500
From: Shaun Thomas <sthomas@optionshouse.com>
Reply-To: <sthomas@optionshouse.com>
Organization: Peak6
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To: David Boreham <david_list@boreham.org>
CC: <pgsql-performance@postgresql.org>
Subject: Re: Reliability with RAID 10 SSD and Streaming Replication
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On 05/22/2013 12:31 PM, David Boreham wrote:

>> Device:     r/s    w/s  rMB/s  wMB/s avgrq-sz avgqu-sz  await svctm %util
>> sdd     2702.80  19.40  19.67   0.16    14.91   273.68  71.74 0.37 100.00
>> sdd     2707.60  13.00  19.53   0.10    14.78   276.61  90.34 0.37 100.00
>>
>> That's an Intel 710 being crushed by a random read database server
>> workload, unable to deliver even 3000 IOPS / 20MB/s.  I have hours of
>> data like this from several servers.
>
> This is interesting. Do you know what it is about the workload that
> leads to the unusually low rps ?

That read rate and that throughput suggest 8k reads. The queue size is 
270+, which is pretty high for a single device, even when it's an SSD. 
Some SSDs seem to break down on queue sizes over 4, and 15 sectors 
spread across a read queue of 270 is pretty hash. The drive tested here 
basically fell over on servicing a huge diverse read queue, which 
suggests a firmware issue.

Often this is because the device was optimized for sequential reads and 
post lower IOPS than is theoretically possible so they can advertise 
higher numbers alongside consumer-grade disks. They're Greg's disks 
though. :)

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com

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