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mokkan
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Basic storage buffer and queue question

I have a  basic question regarding storage, When data arrive to control unit,  it should keep it in queue and send it to necessary lun to write data.

Inside the lun we need to worry about storage pool, raid  and disks.

My question is where are the places I need to worry about cache and buffer queue?  I know this question is more general, but I'm trying to get the view.
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andyalder

8/22/2022 - Mon
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Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

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mokkan

ASKER
Thank you very much. If want to turn of for specific LUN, can you turn off? Or do we need to do it  per storage pool?

queue/buffer on the initiator mean, which initiator? Sorry, you mean by host level?
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Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

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mokkan

ASKER
I'm working on storwize 7000 storage and they were telling me that I can't turn off or on cache on specific LUN?
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

yes, thats quite normal for a SAN.

Most cache settings are global for ALL LUNs.
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David

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andyalder

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mokkan

ASKER
Hi Andyalder,

The reason I wanted to turn of the cache is that I can get better rate on write, when I turn off the cache I got  around 680MB/S write, but when i turn of I got 490 MB/S. I wanted to see which option would provide better write. What is the disadvantage of turning off cache?
andyalder

The disadvantage of disabling cache is that it slows writes down, which seems to be the opposite of what you are seeing. How are you measuring it and where are you turning it off?
mokkan

ASKER
I turn it off on specific LUN and writing data to it and it is increased by closed to 200 MB/S.
I'm using IBM storwize 7000
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David

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mokkan

ASKER
I  just used DD command to write data which sequential write.  One more question, if I'm writing data as 32KB block into raw device and my raid stripe size is 256KB, where does conversion happens? Or it waits for another 8 blocks of data and write all together? Sorry for asking too much questions.
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David

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mokkan

ASKER
I'm using dd command like this. I'm writing in loop.

#!/bin/bash -x
for (( c=1; c<=20; c++ ))
do
  `time dd if=/dev/zero  of=/opt/testing123    bs=8k count=30000000`
  `time rm  /opt/testing123`
  `sleep 40`
done
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David

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andyalder

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mokkan

ASKER
Thank you very much. I'm doing it now using vdbench and will provide you the update.
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mokkan

ASKER
Thanks a lot guys, almost got whats happening.  If  I understand correctly, if I`m writing 32KB to raw device, in the storage my raid stripe size is 256KB. How is it going to write in storage? It waits for another 8 blocks (32 X8), and assemble together? And then write? Or it all depends on the storage? what is the general behavior in storage?
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andyalder

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mokkan

ASKER
Thanks a lot all of you. Your answers are very good.
andyalder

Did you get the same anomalous rsult turning the cache off when you threw random test data at it?
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mokkan

ASKER
It was at least  150 MB/S  was increased on SSD drives when turn of the cache.
andyalder

First time you mention SSD is now? You think we would guess you had SSD drives in it rather than spinning disks covered in rust? It's a completely different ballgame when you have NAND flash.