Re: RAID performance - new kernel results - 5x SSD RAID5

[Phil Turmel <philip@turmel.org>]

On 03/02/2013 04:15 AM, Stan Hoeppner wrote:
> On 3/1/2013 10:06 AM, Adam Goryachev wrote:

>> 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.

>> Is it worth reducing the chunk size from 64k down to 16k or even smaller?
> 
> 64KB chunks should be fine here.  Any gains with a smaller chunk would
> be small, and would pale in comparison to the amount of PITA required to
> redo the array and everything currently sitting atop it.  Remember you'd
> have to destroy it and start over.  You can't change chunk size of an
> existing array.

Actually, you can.  For levels 0 and 4,5,6.

HTH,

Phil