On Fri, 01 Oct 2010 13:53:13 +0200 Paul Slootman <p...@debian.org> wrote:
> Package: mdadm > Version: 3.1.4-1+8efb9d1 > Severity: important > > I noticed that when adding internal bitmaps to md devices, > that the chunks used were far too large: That depends on what you mean by 'too large'. I find that the ideal chunk size relates to how much of the array can be resynced in about second or a bit less. I suspect I might be able to do an IO test and see how fast the devices are, but that is messy and error prone. So I simply choose a default of 64M as that seems to be the right ball-park for modern hardware. Smaller chunk sizes increase write overhead for little appreciable gain. > > r...@corky:/home/paul# cat /proc/mdstat > Personalities : [raid1] > md4 : active raid1 sdb5[0] sdc5[1] > 587202560 blocks [2/2] [UU] > bitmap: 2/5 pages [8KB], 65536KB chunk > > md3 : active raid1 sdc3[2](W) sda3[0] sdb3[1](W) > 10485696 blocks [3/3] [UUU] > bitmap: 1/1 pages [4KB], 65536KB chunk > > md2 : active raid1 sdb2[0] sdc2[1] > 4194240 blocks [2/2] [UU] > bitmap: 0/1 pages [0KB], 65536KB chunk > > md1 : active raid1 sdc1[2](W) sda1[0] sdb1[1](W) > 213120 blocks [3/3] [UUU] > bitmap: 1/1 pages [4KB], 65536KB chunk > > unused devices: <none> > > > I mean, using 64M chunks on an md device that's just 208MB is silly. It might also be said that having a bitmap on a 208MB array is a bit silly as it would only take about 5 seconds to resync it without a bitmap. In reality neither are 'silly', but may be non-optimal. > > After downgrading to mdadm 3.0.3: You don't need to down grade. If you don't like the default that mdadm chooses for you, you are free to choose your own and specify it with the --bitmap-chunk option. NeilBrown -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org