On 03/03/2013 10:19 AM, Adam Goryachev wrote:

Phil Turmel [off-list ref] wrote:

quoted

On 03/02/2013 04:15 AM, Stan Hoeppner wrote:

quoted

On 3/1/2013 10:06 AM, Adam Goryachev wrote:

quoted

quoted

15) "Make sure all LVs are aligned to the underlying md device
geometry. This will eliminate any possible alignment issues."
What does this mean? The drive partitions are now aligned properly,
but how does LVM allocate the blocks for each LV, and how do I
ensure it does so optimally? How do I even check this?

I'm not an LVM user so I can't give you command lines.  But what I
can tell you follows, and it is somewhat critical to RMW performance,
more for rust but also for SSD to a lesser degree.

Run "dmsetup table" and look at the start sectors for your volumes:
Fast-Root: 0 314572800 linear 9:3 3072
This volume starts at sector 3072 (1.5MB) on /dev/sda3.  So the volume
alignment within LVM is 512K.

I see this (for the first LV)
vg0-hostname: 0 204808192 linear 147:2 512
So, I'm guessing mine is starting at 512, and is also aligned at 512k ?

Not quite.  Those size and offset numbers are shown in *sectors*, so you
have 256k alignment.  But that's exactly the minimum you need to match
your raid5 chunk size, so you are good for the moment.

Also, pvdisplay tells me the PE Size is 4M, so I'm assuming that regardless of how the LV's are arranged, they will always be 512k aligned?

256k, but yeah.

So, is that enough to be sure that this is not an issue?

It looks to me like you are good on alignment.

Phil