RE: troubleshooting slow I/O performance.

From: Reen, Elizabeth <"Reen,>
Date: Wed, 9 May 2018 20:15:40 +0000
Message-ID: <>

                You assume that there is a budget to buy disks.  There may not be one.


From: [] On Behalf Of Mladen Gogala Sent: Wednesday, May 09, 2018 1:09 PM
To: Stefan Koehler; Subject: Re: troubleshooting slow I/O performance.

Hi Stefan,

My understanding of the facts is the following:

  • SLOB established the fact that the average single block read takes 10ms to complete.
  • 10ms is not fast enough.

From those two facts I conclude that the OP needs faster disks. It's as simple as that. The OP has also said that he has some flash disk groups which are much faster. Please let me know if my understanding of the facts is incorrect. Also, what insight can the OP gain from the rather strenuous exercise with blktrace and how can it help him?

He used SLOB and has his results. My understanding is that SLOB results are taken as facts. So, we can take an average of 10ms for a single block read as a fact. If that is fast enough, all is well, nothing needs to be done. If not, the only way to fix things are faster disks. Did I go wrong somewhere?

On 05/09/2018 03:41 AM, Stefan Koehler wrote:

First of all we don't know how the 10 ms (I guess/assume average) are formed. Are most of the IOs in this latency bucket or is the majority of IO much faster and the OP just got a few very bad outliers that lead to this average.


Mladen Gogala

Database Consultant

Tel: (347) 321-1217
-- Received on Wed May 09 2018 - 22:15:40 CEST

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