Re: Intel SSDs that may not suck

This can be resolved by partitioning the disk with a larger write spare area so that the cells don't have to by recycled so often. There is a lot of "misinformation" about SSD's, there are some great articles on anandtech that really explain how the technology works and some of the differences between the controllers as well. If you do the reading you can find a solution that will work for you, SSD's are probably one of the best technologies to come along for us in a long time that gives us such a performance jump in the IO world. We have gone from completely IO bound to CPU bound, it's really worth spending the time to investigate and understand how this can impact your system.

http://www.anandtech.com/show/2614
http://www.anandtech.com/show/2738
http://www.anandtech.com/show/4244/intel-ssd-320-review
http://www.anandtech.com/tag/storage
http://www.anandtech.com/show/3849/micron-announces-realssd-p300-slc-ssd-for-enterprise

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Jeff
Sent: Tuesday, March 29, 2011 9:33 AM
To: Jeff
Cc: Merlin Moncure; Andy; pgsql-performance(at)postgresql(dot)org; Greg Smith; Brian Ristuccia
Subject: Re: [PERFORM] Intel SSDs that may not suck

On Mar 29, 2011, at 10:16 AM, Jeff wrote:

> Now that all sounds awful and horrible until you get to overall
> performance, especially with reads - you are looking at 20k random
> reads per second with a few disks. Adding in writes does kick it down
> a noch, but you're still looking at 10k+ iops. That is the current
> trade off.
>

We've been doing a burn in for about 4 days now on an array of 8 x25m's behind a p812 controller: here's a sample of what it is currently doing (I have 10 threads randomly seeking, reading, and 10% of the time writing (then fsync'ing) out, using my pgiosim tool which I need to update on pgfoundry)

10:25:24 AM dev104-2 7652.21 109734.51 12375.22 15.96
8.22 1.07 0.12 88.32
10:25:25 AM dev104-2 7318.52 104948.15 11696.30 15.94
8.62 1.17 0.13 92.50
10:25:26 AM dev104-2 7871.56 112572.48 13034.86 15.96
8.60 1.09 0.12 91.38
10:25:27 AM dev104-2 7869.72 111955.96 13592.66 15.95
8.65 1.10 0.12 91.65
10:25:28 AM dev104-2 7859.41 111920.79 13560.40 15.97
9.32 1.19 0.13 98.91
10:25:29 AM dev104-2 7285.19 104133.33 12000.00 15.94
8.08 1.11 0.13 92.59
10:25:30 AM dev104-2 8017.27 114581.82 13250.91 15.94
8.48 1.06 0.11 90.36
10:25:31 AM dev104-2 8392.45 120030.19 13924.53 15.96
8.90 1.06 0.11 94.34
10:25:32 AM dev104-2 10173.86 145836.36 16409.09 15.95
10.72 1.05 0.11 113.52
10:25:33 AM dev104-2 7007.14 100107.94 11688.89 15.95
7.39 1.06 0.11 79.29
10:25:34 AM dev104-2 8043.27 115076.92 13192.31 15.95
9.09 1.13 0.12 96.15
10:25:35 AM dev104-2 7409.09 104290.91 13774.55 15.94
8.62 1.16 0.12 90.55

the 2nd to last column is svctime. first column after dev104-2 is TPS. if I kill the writes off, tps rises quite a bit:
10:26:34 AM dev104-2 22659.41 361528.71 0.00 15.95
10.57 0.42 0.04 99.01
10:26:35 AM dev104-2 22479.41 359184.31 7.84 15.98
9.61 0.52 0.04 98.04
10:26:36 AM dev104-2 21734.29 347230.48 0.00 15.98
9.30 0.43 0.04 95.33
10:26:37 AM dev104-2 21551.46 344023.30 116.50 15.97
9.56 0.44 0.05 97.09
10:26:38 AM dev104-2 21964.42 350592.31 0.00 15.96
10.25 0.42 0.04 96.15
10:26:39 AM dev104-2 22512.75 359294.12 7.84 15.96
10.23 0.50 0.04 98.04
10:26:40 AM dev104-2 22373.53 357725.49 0.00 15.99
9.52 0.43 0.04 98.04
10:26:41 AM dev104-2 21436.79 342596.23 0.00 15.98
9.17 0.43 0.04 94.34
10:26:42 AM dev104-2 22525.49 359749.02 39.22 15.97
10.18 0.45 0.04 98.04

now to demonstrate "write stalls" on the problemtic box:
10:30:49 AM dev104-3 0.00 0.00 0.00 0.00
0.38 0.00 0.00 35.85
10:30:50 AM dev104-3 3.03 8.08 258.59 88.00
2.43 635.00 333.33 101.01
10:30:51 AM dev104-3 4.00 0.00 128.00 32.00
0.67 391.75 92.75 37.10
10:30:52 AM dev104-3 10.89 0.00 95.05 8.73
1.45 133.55 12.27 13.37
10:30:53 AM dev104-3 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00
10:30:54 AM dev104-3 155.00 0.00 1488.00 9.60
10.88 70.23 2.92 45.20
10:30:55 AM dev104-3 10.00 0.00 536.00 53.60
1.66 100.20 45.80 45.80
10:30:56 AM dev104-3 46.53 0.00 411.88 8.85
3.01 78.51 4.30 20.00
10:30:57 AM dev104-3 11.00 0.00 96.00 8.73
0.79 72.91 27.00 29.70
10:30:58 AM dev104-3 12.00 0.00 96.00 8.00
0.79 65.42 11.17 13.40
10:30:59 AM dev104-3 7.84 7.84 62.75 9.00
0.67 85.38 32.00 25.10
10:31:00 AM dev104-3 8.00 0.00 224.00 28.00
0.82 102.00 47.12 37.70
10:31:01 AM dev104-3 20.00 0.00 184.00 9.20
0.24 11.80 1.10 2.20
10:31:02 AM dev104-3 4.95 0.00 39.60 8.00
0.23 46.00 13.00 6.44
10:31:03 AM dev104-3 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00

that was from a simple dd, not random writes. (since it is in production, I can't really do the random write test as easily)

theoretically, a nice rotation of disks would remove that problem.
annoying, but it is the price you need to pay

--
Jeff Trout <jeff(at)jefftrout(dot)com>
http://www.stuarthamm.net/
http://www.dellsmartexitin.com/

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