On Sun, 23 Jan 2011 11:10:40 -0800
Mark Knecht <markknecht@gmail.com> wrote:

> On Sun, Jan 23, 2011 at 10:53 AM, Mark Knecht <markknecht@gmail.com> wrote:
> > Hi,
> >   I've got an existing (and well backed up as of a couple of minutes
> > ago) 5-drive RAID6 shown here:
> >
> > c2stable ~ # mdadm --detail /dev/md7
> > /dev/md7:
> >        Version : 1.2
> >  Creation Time : Sun Jan 16 17:02:24 2011
> >     Raid Level : raid6
> >     Array Size : 395387904 (377.07 GiB 404.88 GB)
> >  Used Dev Size : 131795968 (125.69 GiB 134.96 GB)
> >   Raid Devices : 5
> >  Total Devices : 5
> >    Persistence : Superblock is persistent
> >
> >    Update Time : Sun Jan 23 10:25:41 2011
> >          State : clean
> >  Active Devices : 5
> > Working Devices : 5
> >  Failed Devices : 0
> >  Spare Devices : 0
> >
> >         Layout : left-symmetric
> >     Chunk Size : 512K
> >
> >           Name : c2stable:7  (local to host c2stable)
> >           UUID : ded5e0c6:1a5a73ad:01949557:bb4e015a
> >         Events : 17
> >
> >    Number   Major   Minor   RaidDevice State
> >       0       8        7        0      active sync   /dev/sda7
> >       1       8       23        1      active sync   /dev/sdb7
> >       2       8       39        2      active sync   /dev/sdc7
> >       3       8       50        3      active sync   /dev/sdd2
> >       4       8       66        4      active sync   /dev/sde2
> > c2stable ~ #
> >
> >   I would like to change this RAID to a smaller chunk size. (16K for
> > now) What is the right command to use?
> >
> >   From Google and the mdadm man pages I'm coming up with something like:
> >
> > mdadm --grow /dev/md7 --level=6 --chunk=16 --backup-file=/root/backup-md7
> >
> > Maybe I don't need --level because the level isn't changing? Neither
> > is the number of drives.
> >
> > Thanks in advance,
> > Mark
> >
> 
> OK, I found a blog of Neil's in which I had read the top but hadn't
> gone through the comments:
> 
> http://neil.brown.name/blog/20090817000931
> It seems he answers my question further down in the comments.
> 
> Looks like
> 
> mdadm --grow /dev/md7 --chunk=16

I suggest that you reconsider going with 16K, it is a suboptimal chunk size.
The older default of 64K seems to be the sweet spot, at least for RAID5/6:
http://louwrentius.com/blog/2010/05/linux-raid-level-and-chunk-size-the-benchmarks/

-- 
With respect,
Roman