Re: Renaming MD devices (metadata=1.1)

[Gilbert Kowarzyk <kowarzyk@grm.polymtl.ca>]

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Awesome!

Thanks for the info!

Gilbert


On 12-01-11 6:04 PM, NeilBrown wrote:
> On Wed, 11 Jan 2012 11:01:33 -0500 Gilbert Kowarzyk
> <kowarzyk@grm.polymtl.ca> wrote:
> 
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> 
>> Hello,
> 
>>>> Why do we have these choices, and when should each be used?
>>> 
>>> 1.0 (at the end) is best for RAID1 when booting from the device
>>> as the boot-load just doesn't see the RAID at all.
>>> 
>>> 1.1 (at the start) is generally good because various different 
>>> things have metadata at the start, so there is no chance of 
>>> confusion about what is using that devices - each possible
>>> users will overwrite the metadata of the others. It is also
>>> easier to make the devices larger if you don't need to move the
>>> metadata.
>>> 
>>> 1.2 (4K from the start) has most of the benefits for 1.1 but
>>> works for booting with newer boot loaders - they still want
>>> sector 0 for a boot sector but understand the md superblock.
> 
>> I see. Makes also loads of sense.
> 
>> I had two more quick questions:
> 
>> 1.- when would one want to turn off bitmaps? I turned them on,
>> but I haven't found why one may want them off (or for what they
>> should be turned off).
> 
> There can be a performance impact - writes can be a bit slower with
> bitmaps. If that is more important to you that resync speed after a
> crash you might turn them off. To change the chunk-size of the
> bitmap you need to turn them off, then on again. If you want to
> reshape the array you currently need to turn the bitmap off first
> (because the bitmap doesn't automatically resize). You turn the
> bitmap off with mdadm --grow /dev/mdXX --bitmap=none
> 
> 
>> 2.- if I ever have a mismatch_cnt different than zero, how would
>> I go about finding which drive has the correct info, which was
>> the one that got corrupted, and how to obtain the correct one?
> 
> "correct" is not a well defined concept here.
> 
>> In the past I've changed the "CHECK" to "REPAIR" in the
>> configuration script that checks the RAID arrays periodically 
>> (/etc/sysconfig/raid-check), but from what I understood it's a
>> bit of luck which one the automatic script will choose out of the
>> two drives (assuming raid1). Should I let it be automatic (with
>> "repair"), and if not (if I have to check which one is correct
>> manually), where could I find information to read about how to do
>> this (I am assuming the answer may be long)?
> 
> This:  http://neil.brown.name/blog/20100211050355 touches on the
> topic ... and is long.
> 
> 
> NeilBrown
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