A question I get routinely asked relates to Windows disk partition alignment with XIV. If you don't know what I am talking about, take some time to read these very useful pages from our friends at Microsoft. Once you have had a look, come on back and read my perspective.
Disk Partition Alignment (Sector Alignment): Make the Case: Save Hundreds of Thousands of Dollars
Back already? Hopefully you now know that disk partition alignment is all about starting an IO at a logical block address that best matches how the underlying hardware stores your data. So now your wondering, what does this have to do with XIV? Well XIV has two concepts that relate to this: cache and partitions.
XIV cache (the server memory used to speed up reads and writes) is organised into 4 KB blocks (which is nice and small).
So the XIV cache does not care about disk alignment.
But when it comes to writing and read from disk, the XIV writes data into chunks of consecutive logical block addresses (LBAs) that we call partitions. These partitions are 1 MiB in size. What does that concept mean? It means the magical number for XIV is 1024 KB or 1 MB. (actually KiB and MiB, but for the sake of ease, I will stick to the naming used by Microsoft. Given this number is fairly large (other hardware often aligns to 32KB, 64KB or 256KB), for XIV this reduces the potential impact of misaligned partitions. Which is good.
Correct Windows Disk Alignment could give up to a 7% performance improvement when using an offset of 1024 KiB. (1 MiB). I need to be clear, that's not a guaranteedimprovement of 7%. It's a maximum possible improvement. Your particular server will see an improvement somewhere between 0% and 7%. It depends on your workload patterns. The more small and random your workload, the more useful setting the 1024 KB offset will be. The more sequential your workload, the less useful it will be, as only the first and last parts of an I/O could potentially be misaligned. This mis-alignment could equate to a tiny percentage of extra work for the XIV. Sadly there is no metric you can display to detect how much impact misalignment is actually having.
So should you do it? The good news is that new volumes created under Windows 2008 prefer the 1 MB boundary. So a fresh install should already be using the correct values. The bad news is that volumes created under earlier Windows Operating Systems (Such as Windows 2000 and 2003) will almost certainly be misaligned, and correcting the alignment is destructive to the data in the partition.
How to check alignment at the host? Here is an example:
I start diskpart:
Microsoft DiskPart version 6.1.7600
Copyright (C) 1999-2008 Microsoft Corporation.
On computer: ANTHONYV-PC
I list my disks. In this example I have two disks installed in my laptop. I select disk 0:
DISKPART> list disk
Disk ### Status Size Free Dyn Gpt
-------- ------------- ------- ------- --- ---
Disk 0 Online 238 GB 5724 MB
Disk 1 Online 232 GB 1024 KB
DISKPART> select disk 0
Disk 0 is now the selected disk.
Now I list the partitions and see the offset for each one.
DISKPART> list partition
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Primary 100 MB 1024 KB
Partition 2 Primary 232 GB 101 MB
Partition 1 has an offset of 1024 KB, which is 1 MB, which is perfect for XIV. Partition 2 has an offset of 101 MB, which is still on the 1MB boundary (it was pushed there by the combination of the size of the first partition (100 MB) and its offset (1 MB). So this is perfect.
For an example of how to create a partition with the correct offset, check out this how-to document, that also provides some good follow on reading:
What about other IBM products?
The IBM SVC and Storwize V7000 prefers 64 KB (or larger) offsets as documented here:
Why? Because the SVC and Storwize V7000 use a concept of grains, where each grain is usually 64KB or 256KB in size.
The DS8000 (regardless of model), also prefers 64 KB offsets. The DS8000 use the concept of logical tracks where each logical track is 64KB.
The DS3000/DS4000/DS5000 range allow the user to set the segment size of a logical volume on creation. The setting that you define should match the segment size defined for the logical drive being used. In the example below, it is 64 KB.
What about VMWare?
The answers are no different. Misalignment can indeed make a difference to client performance. Check this link from NetApp and this document from VMware:
For an EMC perspective, check out this link from someone I respect a great deal, Chad Sakac:
I searched around looking for an image to highlight the theme of alignment. I found this image in the IBM archives for the IBM Mass Storage Facility announced back in 1974. I am sure this product had some interesting alignment challenges.