On Mon, 6 Nov 2006, Neil Brown wrote:

quoted

hey i have another related question... external bitmaps seem to pose a bit 
of a chicken-and-egg problem.  all of my filesystems are md devices. with 
an external bitmap i need at least one of the arrays to start, then have 
filesystems mounted, then have more arrays start... it just happens to 
work OK if i let debian unstale initramfs try to start all my arrays, 
it'll fail for the ones needing bitmap.  then later /etc/init.d/mdadm-raid 
should start the array.  (well it would if the bitmap= in mdadm.conf 
worked :)

is it possible to put bitmaps on devices instead of files?  mdadm seems to 
want a --force for that (because the device node exists already) and i 
haven't tried forcing it.  although i suppose a 200KB partition would be 
kind of tiny.... but i could place the bitmap right beside the external 
transaction log for the filesystem on the raid5.

Create the root filesystem with --bitmap=internal, and store all the
other bitmaps on that filesystem maybe?

yeah i only have the one external bitmap (it's for a large raid5)... so 
things will work fine once i apply your patch.  thanks.

I don't know if it would work to have a bitmap on a device, but you
can always mkfs the device, mount it, and put a bitmap on a file
there??

yeah this was the first thing i tried after i found mdadm -b /dev/foo 
wasn't accepted...

without modifying startup scripts there's no way to use any filesystem 
other than root... it's just due to ordering of init scripts:

# ls /etc/rcS.d | grep -i 'mount\|raid'
S02mountkernfs.sh
S04mountdevsubfs.sh
S25mdadm-raid
S35mountall.sh
S36mountall-bootclean.sh
S45mountnfs.sh
S46mountnfs-bootclean.sh

i'd need to run another mdadm-raid after the S35mountall, and then another 
mountall.

anyhow, i don't think you need to change anything (except maybe a note in 
the docs somewhere), i'm just bringing it up as part of the experience of 
trying external bitmap.  i suspect that in the wild and crazy direction 
debian and ubuntu are heading (ditching sysvinit for event-driven systems) 
it'll be "easy" to express the boot dependencies.

-dean