[Novalug] LVM query

[James Ewing Cottrell 3rd]

In general, my guiding principles are:

[1] Use the Latest and Greatest.
[2] Use the Default/Easiest.
[3] ...unless there is a good reason otherwise.

In this case, principles [1] and [2] are in conflict. Metadata 0.90 is 
the default. The 1.x metadata seem to offer more choices, but unless you 
measure your metadisk sizes in Terabytes, it doesn't seem necessary. And 
then one would hope that the appropriate size would be chosen automatically.

You have to read that section carefully and choose for yourself.

My own reading leads me to go with the default unless someone more 
knowledgeable directs me otherwise.

JIM

On 7/24/2010 11:02 AM, Brander Snaxe wrote:
> Thanks for the article! I checked it out and it does have some enlightening information.
>
> Here is another question related to MD RAID along the same lines of alignment:
>
> There is an option in mdadm for metadata ( --metadata=<version>) with multiple metadata versions.  From the manpage:
>
> Options are:
>
>                0, 0.90, default
>                       Use  the  original  0.90  format superblock.  This format
>                       limits arrays to 28 component devices and  limits  compoâ
>                       nent devices of levels 1 and greater to 2 terabytes.
>
>                1, 1.0, 1.1, 1.2
>                       Use  the  new  version-1 format superblock.  This has few
>                       restrictions.   The  different  sub-versions  store   the
>                       superblock  at  different locations on the device, either
>                       at the end (for 1.0), at the start (for 1.1) or  4K  from
>                       the start (for 1.2).
>
>
> So there are different versions. What is the impact of the metadata on alignment? Is it similar to the LVM metadata alignment problem?  Does mdadm automatically put the metadata into an aligned area using other options such as chunk size and stripe size for determining this?
>
> I've read that one can only boot from a version 0.90 metadata. Is this true?
>
> Would it be better to use a version 1.0 where the metadata is at the end of the device for alignment purposes? I realize it may not be possible to boot from this device, but that's okay.
>
> The overarching question is: Does md metadata cause file system alignment problems? If so, how does one adjust for it when creating the md device?
>
> I'm going to start experimenting and try to find answers with a combination of google and my own use of VirtualBox, but thought I'd ask the list for pointers first.
>
> Thanks again!
>
>
> --- On Wed, 7/21/10, Jeff Stoner<leapfrog at freeshell.org>  wrote:
>
>    
>> From: Jeff Stoner<leapfrog at freeshell.org>
>> Subject: Re: [Novalug] LVM query
>> To: "Brander Snaxe"<brandon20va at yahoo.com>
>> Cc: "James Ewing Cottrell 3rd"<JECottrell3 at Comcast.NET>, novalug at calypso.tux.org
>> Date: Wednesday, July 21, 2010, 10:40 AM
>> On Mon, 19 Jul 2010, Brander Snaxe
>> wrote:
>>
>>      
>>> What is the first track problem you mention? Is this
>>>        
>> something to do with LVM alignment? I have read about this
>> before when dealing with filesystem alignment with RAID5
>> stripes. Also it comes into play with LVM. And it always
>> comes into play with SSD drives apparently.
>>      
>>> I assume that for plain LVM or for LVM+RAID (no
>>>        
>> striping) that the mis-alignment problem would be so
>> negligible that worrying about is moot.
>>      
>>> Just curious.
>>>        
>> Here's a write up that gives a bit more detail on
>> alignment, partitioning
>> and RAID: http://insights.oetiker.ch/linux/raidoptimization/
>>
>>
>> --Jeff
>>
>> "You cannot unsay a cruel word." - Unknown
>>
>>      
>>
>>
>>
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