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Re: Calculating optimal disk partitions

From: Chris Murphy
Subject: Re: Calculating optimal disk partitions
Date: Wed, 8 May 2013 17:46:06 -0400

On May 8, 2013, at 2:37 PM, address@hidden wrote:

> On Wednesday, May 8, 2013 13:17, "Chris Murphy" <address@hidden> said:
> Thanks for responding Chris!
>> On a 512byte physical and logical sector hard drive, the messages can be 
>> ignored.
>> Alignment isn't an issue.
> Wouldn't the generation of an error message, when no error condition exists, 
> be considered a bug? At the very least I would think it would be undesirable.

It's an older version of parted. I'd try it with 3.1. In any case, it's 
difficult because while 512B physical/logical sector drives should default to 
an alignment value of 1 sector, this probably isn't good for most SSDs which 
claim a 512B physical/logical sector size. The way it works now by default is 
to align on 1MB boundaries regardless of the physical sector size.

It's possible in your case the warning is about another partition which isn't 
aligned, rather than the new one you're trying to create. If that's the case, 
the error message is misleading.

> What about a 4096 physical sector drive?

Alignment on a 4Kn AF HDD is also a non-factor. There's nothing to align. But I 
still think the default is 1MB alignment with new versions of parted. As a data 
drive, they work. For boot drives, even recent firmware, let alone older 
firmware, don't like them at all.

For 512e AF HDD, alignment is a considerable factor depending on the firmware 
implementation. It's best to align.

>> For SSDs which effectively lie about their physical sector size, the 
>> consequences
>> of miss alignment are variable the firmware.
> If mis-alignment can negatively impact performance, I would expect a 
> documented procedure on avoiding it. Or is my expectation out of line? I 
> certainly wouldn't mind helping document the procedure if I could get a clear 
> understanding on how what the procedure is.

It would be nice if it worked this way, but in reality solid state storage is 
in flux on both a physical level, as well as a firmware (the thing most 
responsible for the logical abstraction from the physical drive). Firmware 
makes a huge, huge difference in SSD behavior.

Chris Murphy

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