Re: RAID-10 explicitly defined drive pairs?

[NeilBrown <neilb@suse.de>]

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On Fri, 6 Jan 2012 16:08:23 +0100 Jan Kasprzak <kas@fi.muni.cz> wrote:

> John Robinson wrote:
> : On 12/12/2011 11:54, Jan Kasprzak wrote:
> : >	Is there any way how to tell mdadm explicitly how to set up
> : >the pairs of mirrored drives inside a RAID-10 volume?
> : 
> : If you're using RAID10,n2 (the default layout) then adjacent pairs
> : of drives in the create command will be mirrors, so your command
> : line should be something like:
> : 
> : # mdadm --create /dev/mdX -l10 -pn2 -n44 /dev/shelf1drive1
> : /dev/shelf2drive1 /dev/shelf1drive2 ...
> 
> 	OK, this works, thanks!
> 
> : Having said that, if you think there's a real chance of a shelf
> : failing, you probably ought to think about adding more redundancy
> : within the shelves so that you can survive another drive failure or
> : two while you're running on just one shelf.
> 
> 	I am aware of that. I don't think the whole shelf will fail,
> but who knows :-)
> 
> : If you are sticking with RAID10, you can potentially get double the
> : read performance using the far layout - -pf2 - and with the same
> : order of drives you can still survive a shelf failure, though your
> : use of port multipliers may well limit your performance anyway.
> 
> 	On the older hardware I have a majority of writes, so the far
> layout is probably not good for me (reads can be cached pretty well
> at the OS level).
> 
> 	After some experiments with my new hardware, I have discovered
> one more serious problem: I have simulated an enclosure failure,
> so half of the disks forming the RAID-10 volume disappeared.
> After removing them using mdadm --remove, and adding them back,
> iostat reports that they are resynced one disk a time, not all
> just-added disks in parallel.
> 
> 	Is there any way of adding more than one disk to the degraded
> RAID-10 volume, and get the volume restored as fast as the hardware permits?
> Otherwise, it would be better for us to discard RAID-10 altogether,
> and use several independent RAID-1 volumes joined together using LVM
> (which we will probably use on top of the RAID-10 volume anyway).
> 
> 	I have tried mdadm --add /dev/mdN /dev/sd.. /dev/sd.. /dev/sd..,
> but it behaves the same way as issuing mdadm --add one drive at a time.

I would expect that to first recover just the first device added, then
recover all the rest at once.

If you:
  echo frozen > /sys/block/mdN/md/sync_action
  mdadm --add /dev/mdN /dev......
  echo recover > /sys/block/mdN/md/sync_action

it should do them all at once.

I should teach mdadm about this..

NeilBrown


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