Windows 2003 GPT disk alignment

Question

Could someone please clear this up for me? I've searched the forums and all the CX best practice docs I can find but still don't have a definitive answer. Searches refer to EMC White Paper 300-004-075 'Aligning GPT Basic and Dynamic disks' which I cannot seem to find. MBR partitions must be aligned. Easy; no questions; been doing it for years. What about GPT partitions? I've read conflicting things (even on this forum). Based on what I've found, a GPT partition does *not* need to be aligned. Here's my reasoning: MBR uses a 63K area for storing the actual partition table. Since the CX uses a 64K element size as the default (which is what we use), an offset of 64K makes total sense. GPT, from what I can tell, uses a 128MB (131,072K) area for storing the partition table. Since this is a multiple of 64K (by a factor of 2048), the GPT partition will automatically be aligned. Is this the correct conclusion or was my train of thought derailed somewhere? In addition, if we *did* offset the GPT partition by 64K, this would have no impact as the partition table would now just be offset by a factor of 2049, or one more 64K chunk of disk. I've been on vacation for two weeks so please help clear up my clouded mind! Thanks! Jeff

kelleg · Accepted Answer

Try this link:http://powerlink.emc.com/km/live1/en_US/Offering_Technical/Technical_Documentation/300-004-075_a01_elccnt_0.pdfglen

StorageDC · Answer

The closest document I could find, and this is the one referred to in the Windows Host Connectivity Guide is 'Using diskpar and diskpart to Align Partitions on Windows Basic and Dynamic Disks'

jmery · Answer

@space: Thanks. I have seen that document. Unfortunately, there's no mention of aligning GPT disks. The only reference to GPT is in the partition table contents (the hex value listing partition table type).

@AranH: Good catch on the 17K offset. I would bet that this *does* affect the overall disk alignment.

I'm searching now to see if there are any tools for manually messing with GPT partitions...

AranH1 · Answer

Well I used to think that GPT disks required the alignment offset, but now after reading up on them a bit more I am not so sure.

The knowledge base article emc104675 states that GPT disks do require alignment and refers to the white paper you mentioned, but I cannot find that white paper either. There is a white paper titled " Using diskpar and diskpart to align partitions on Windows Basic and Dynamic Disks" but there is no mention of GPT disk in he document.

If I look at the partitions on a GPT disk on a host , this is what I see:
DISKPART> list partition

Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Reserved 128 MB 17 KB
Partition 2 Primary 2248 GB 128 MB

So the reserved partition on a GPT disk is, as referenced in the GPT literature from MS, 128 MB. But the offset is 17KB which is not ideal for the stripe alignment right? This is assuming that the reserved partition affects the data partition. This reserved partition is created automatically when the disk is converted from basic to GPT so I don't know if there is a way to set that offset value.

Hmm... Good Question

jmery · Answer

@AranHWith some poking around, I did find that you can, in fact, change the offset for the hidden GPT partition. If our assumption is correct that the 17KB offset *does* impact GPT disk alignment, then the process below would correct that.I'd still like EMC to say, 'Yes, GPT partitions need to be aligned and here's the approved/recommended way to do it.'. I'll see if anyone replies within the next couple of days. If not, I'll put in a phone call A couple of notes on what's below:1. The default unit for diskpart on GPT disks is MB and there doesn't appear to be a way to change this. As a result the smallest offset is 1 MB which is fine since it's evenly divisible by 64. Shops using a different element size would need to do the math to find out where this lines up for them. On an MBR disk, the unit is KB for offset values so this is less of an issue.2. I know the disk is only 1.5TB, but it's likely to be expanded beyond 2TB in the future, hence the need for GPT in the first place The process (as I see it):DISKPART list disk Disk ### Status Size Free Dyn Gpt -------- ---------- ------- ------- --- --- Disk 0 Online 136 GB 0 B Disk 1 Online 136 GB 0 B* Disk 4 Online 1500 GB 1500 GBDISKPART convert gptDiskPart successfully converted the selected disk to GPT format.DISKPART list partition Partition ### Type Size Offset ------------- ---------------- ------- ------- Partition 1 Reserved 128 MB 17 KBDISKPART select partition 1Partition 1 is now the selected partition.DISKPART delete partition overrideDiskPart successfully deleted the selected partition.DISKPART list partitionThere are no partitions on this disk to show.DISKPART create partition msr size=128 offset=1DiskPart succeeded in creating the specified partition.DISKPART list partition Partition ### Type Size Offset ------------- ---------------- ------- -------* Partition 1 Reserved 128 MB 1024 KBMessage was edited by: jmery

AranH1 · Answer

Nice one! I didn't realize you could delete the MSR without impacting the disk. Very cool. That looks good to me. Now let's get some EMC gurus in here to chime in and see if we are crazy or not

jmery · Answer

Yes - much appreciated Glen!

I'm in the same boat as AranH - how did you find this document? I too searched everything I could think of and wasn't able to locate it.

Also looking forward to the answer to AranH's question on aligning the MSR as well

AranH1 · Answer

Thanks Glen! Alright, so where is this document located and how did you find it?

No matter how I searched on Powerlink, the search results never came back with this document.

What about the MSR partition? The document does not talk about the need to align the MSR partition.

kelleg · Answer

Try thisHome > Support > Technical Documentation and Advisories > Hardware/Platforms Documentation > CLARiiON CX3 UltraScale Series Systems > CLARiiON CX3-20 > Technical Notes/Troubleshooting

kelleg · Answer

do a search on powerlink using:Aligning GPT Basic and Dynamic Disks for Microsoft Windows

CLARiiON

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