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storage block size question

mokkan
mokkan asked
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Last Modified: 2012-03-31
I have a question,  if we set block size to  128 kb  in storage and if we want to change it to different size, would it cause any problem ? Also, any performance issue?
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Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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Commented:
where are you making this change in the VM?

on the datastore?
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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any block size change in the future, requires reformatting, and changing the block size.

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Commented:
In the storage ( we are running EMC),  I'm talking  genrally it doesn't need to be a VM or physical host.  There will be any corruption or anything ? Or just performance will go down?

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What are the things happen when you change the block size
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Block size is normally matched to your OS and VMs.

If you select the incorrect block size performance will suffer.

To change the block size, will require reformatting the entire LUN.

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Commented:
ah ok, would be loose data?
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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when running on incorrect block size unlikely, but performance may suffer.

converting to another block size, yes you would lose data!

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Commented:
Thank you,  you are talking about the block size which is written into the cache before it commit into the drive right?
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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yes
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I think mokkan is asking about the stripe element size of the storage box.

Commented:
Contact the vendor for the exact details for the best performance.  As every storage box has different settings in use for optimal performance.
And remember that from vsphere 5 there is a standard uniform blocksize of 1MB. Based on that it can be interesting to work to this setting combined with the storage settings.

Good luck

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Commented:
Thank you guys, just to confirm before I play with block size. Would I loose data, if I change the block size ?
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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Block sizes are usually selected at creation.
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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Commented:
if you are going to experiment with block size changes ensure you have full backups before you proceed.

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Commented:
Why I'm asking is that we run storwize 7000 servers and if we want to bring external storage and if it has different block size,  how do I handle it it?  Can we setup block size for each raid? or is it system wise setup?
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
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strip sizes can be changed per RAID array

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strip size and block size are same or different?
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If you want to change block size, you can use LUN migration on your EMC array to move the affected LUNs.

Here's how you could do it:
Create a new LUN with the block size you want. It has to be the same size or bigger than the existing LUN.
Right click on the LUN you want to move, select Migrate.... A wizard pops up. Work your way through the wizrad - select the new LUN with the new block size. When you click Finish, the array will move the LUN with zero downtime and completely transparently to the attached hosts.

Easy! The only caveat is that you need to have enough spare space on your array to move stuff around.

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Commented:
thanks a lot.  Can you clear the concept of block  size and raid strip size?  What is the difference between strip size and block size?
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On an EMC array, block size is what's written
To each disk. Stripe size is what's written to the raid group (only true for stripes raid of course), so stripe size for a 5 disk raid 5 group with a block size of 64k = 64k x 4 = 128k. Note that you don't count the parity drive when calculating stripe size.

As a very good rule, don't muck about with block
Sizes on a CLARiiON or VNX array unless:
A) you know *exactly* what you're doing as you will affect a number of optimisations the array uses to improve performance
B) you have measured the stripes being written to the array. You can use Navisphere/Unisphere Analyzer to get this information.

Re good thing of course is that you can move workloads around really easily on the array so you can experiment. I strongly recommend getting the EMC reference architectures from Powerlink before you go to much farther as a lot if this sort o performance testin and optimisations already been tried by EMC engineers.

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