Re: RAID6 chunk size change?

Roman Mamedov <rm@xxxxxxxxxx> · Mon, 24 Jan 2011 00:52:04 +0500

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

> On Sun, Jan 23, 2011 at 10:53 AM, Mark Knecht <markknecht@xxxxxxxxx> wrote:
> > Hi,
> > Â 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:
> > Â Â Â ÂVersion : 1.2
> > ÂCreation Time : Sun Jan 16 17:02:24 2011
> > Â Â Raid Level : raid6
> > Â Â Array Size : 395387904 (377.07 GiB 404.88 GB)
> > ÂUsed Dev Size : 131795968 (125.69 GiB 134.96 GB)
> > Â Raid Devices : 5
> > ÂTotal Devices : 5
> > Â ÂPersistence : Superblock is persistent
> >
> > Â ÂUpdate Time : Sun Jan 23 10:25:41 2011
> > Â Â Â Â ÂState : clean
> > ÂActive Devices : 5
> > Working Devices : 5
> > ÂFailed Devices : 0
> > ÂSpare Devices : 0
> >
> > Â Â Â Â Layout : left-symmetric
> > Â Â Chunk Size : 512K
> >
> > Â Â Â Â Â Name : c2stable:7 Â(local to host c2stable)
> > Â Â Â Â Â UUID : ded5e0c6:1a5a73ad:01949557:bb4e015a
> > Â Â Â Â Events : 17
> >
> > Â ÂNumber Â Major Â Minor Â RaidDevice State
> > Â Â Â 0 Â Â Â 8 Â Â Â Â7 Â Â Â Â0 Â Â Âactive sync Â /dev/sda7
> > Â Â Â 1 Â Â Â 8 Â Â Â 23 Â Â Â Â1 Â Â Âactive sync Â /dev/sdb7
> > Â Â Â 2 Â Â Â 8 Â Â Â 39 Â Â Â Â2 Â Â Âactive sync Â /dev/sdc7
> > Â Â Â 3 Â Â Â 8 Â Â Â 50 Â Â Â Â3 Â Â Âactive sync Â /dev/sdd2
> > Â Â Â 4 Â Â Â 8 Â Â Â 66 Â Â Â Â4 Â Â Âactive sync Â /dev/sde2
> > c2stable ~ #
> >
> > Â I would like to change this RAID to a smaller chunk size. (16K for
> > now) What is the right command to use?
> >
> > Â 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
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