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Re: realtime section bugs still around

To: stan@xxxxxxxxxxxxxxxxx
Subject: Re: realtime section bugs still around
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Fri, 03 Aug 2012 06:28:29 -0500
Cc: Jason Newton <nevion@xxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <501A58DB.7070801@xxxxxxxxxxxxxxxxx>
References: <CAGou9MgezsS=2+SngGWBJv5Npsuqacx1VPJwvMuf0FS+XnXt8A@xxxxxxxxxxxxxx> <20120730030333.GE2877@dastard> <CAGou9MheeBWxajd65szNfDB2L+VVoZ7SypEdUKj7np3L0H8fHA@xxxxxxxxxxxxxx> <CAGou9MgMHbTPebFUDXdyLvrUr4T9wo-geNtaPUft9uUmOaju6Q@xxxxxxxxxxxxxx> <50186E51.1020107@xxxxxxxxxxxxxxxxx> <CAGou9MhneejOuhX4c8G06c3Zh7dxF-OtZ+=mT-7fho_u1Q3zWw@xxxxxxxxxxxxxx> <5018A8C7.8050406@xxxxxxxxxxxxxxxxx> <CAGou9Mj=8xcF4YwaSQYp0_BbJW_Ms1sasDDRNxL1oh0oR7zupw@xxxxxxxxxxxxxx> <5019CC34.5050003@xxxxxxxxxxxxxxxxx> <CAGou9Mg4teJU3VHWxDK3-y1MMxN_FQ-Dpk3E1KzX0pNdLEHNOQ@xxxxxxxxxxxxxx> <501A58DB.7070801@xxxxxxxxxxxxxxxxx>
Reply-to: stan@xxxxxxxxxxxxxxxxx
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On 8/2/2012 5:39 AM, Stan Hoeppner wrote:

> We already gave you the biggest cause of your latency, which is garbage
> collection/wear leveling.  You can't see inside the SSDs, but you can
> see the latency jump with either top (%wa) or iostat (await,
> milliseconds).  Run
> 
> iostat -x -d 1 20
> 
> and you get 20 reports 1 second apart.  1s is minimum granularity.  This
> should clearly show the latency spikes caused by the SSDs.  Maybe even
> execute it for 60 seconds and pipe to a file.

The above assumes Linux can see the individual devices.  I've never used
Intel's fakeraid.  If its driver presents a single device to the kernel
instead of both SSD devices, iostat won't show which SSD's garbage
collection is kicking in and/or when.  It would be most beneficial if
you could see the iostat data for both SSD devices as it would tell you
exactly when each drive's GC/leveling kicks in.  If the Intel fakeraid
doesn't allow you to see both devices, you'll need to switch to md/RAID.
 I'm sure that will be problematic as you're very likely booting from
the Intel RAIDed SSD device.

-- 
Stan


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