On Fri, 6 Jan 2012 16:08:23 +0100 Jan Kasprzak <kas@xxxxxxxxxx> 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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