On Fri, Oct 23, 2015 at 08:23:38AM +1100, Neil Brown wrote: > Simon Guinot <simon.guinot@xxxxxxxxxxxx> writes: > > > Hi Neil, > > > > I'd like to have your advice about destroying an array created by open > > at close time if not configured, rather than waiting for a ioctl or a > > sysfs configuration. This would allow to get rid of the inactive md > > devices created by an "accidental" open. > > > > On the Linux distribution embedded in LaCie NAS, we are able to > > observe the following scenario: > > > > 1. A RAID array is stopped with a command such as mdadm --stop /dev/mdx. > > 2. The node /dev/mdx is still available because not removed by mdadm at > > stop. > > 3. /dev/mdx is opened by a process such as udev or mdadm --monitor. > > 4. An inactive RAID array mdx is created and a "add" uevent is > > broadcasted to userland. It is let to userland to understand that > > this event must be discarded. > > > > You have to admit that this behaviour is at best awkward :) > > No argument there. > > > > > > I read the commit d3374825ce57 > > "md: make devices disappear when they are no longer needed" in which > > you express some concerns about an infinite loop due to udev always > > opening newly created devices. Is that still actual ? > > > > In your opinion, how could we get rid of an inactive RAID array created > > by open ? Maybe we could switch the hold_active flag from UNTIL_IOCTL to > > 0 after some delay (enough to prevent udev from looping) ? In addition, > > maybe we could remove the device node from mdadm --stop ? Or maybe > > something else :) > > > > If you are interested by any of this solutions or one of yours, I'll > > be happy to work on it. > > By far the best solution here is to used named md devices. These are > relatively recent and I wouldn't be surprised if you weren't aware of > them. > > md devices 9,0 to 9,511 (those are major,minor numbers) are "numeric" md > devices. They have in-kernel names md%d which appear in /proc/mdstat > and /sys/block/ > > If you create a block-special-device node with these numbers, that will > create the md device if it doesn't already exist. > > md devices 9,512 to 9,$BIGNUM are "named" md devices. These have > in-kernel names like md_whatever-you-like. > If you create a block-special-device with device number 9,512 and try to > open it you will get -ENODEV. > To create these you > echo whatever-you-like > /sys/module/md_mod/parameters/new_array > > A number 512 or greater will be allocated as the minor number. > > These arrays behave as you would want them to. They are only created > when explicitly requested and they disappear when stopped. > > mdadm will create this sort of array if you add > CREATE names=yes > to mdadm.conf and don't use numeric device names. > i.e. if you ask for /dev/md0, you will still get 9,0. > But if you ask for /dev/md/home, you will get 9,512 where as > with names=no (the default) you would probably get 9,127. Thanks for describing the usage of the named md devices. We will look to convert our userland from numeric to named md devices. > > A timeout for dropping idle numeric md devices might make sense but it > would need to be several seconds at least as udev can sometimes get very > backlogged and would wouldn't want to add to that. Would 5 minutes be > soon enough to meet your need? No unfortunately, this will not. In our case, I think we need to remove the inactive numeric md device at close or quickly after. Considering that for a numeric md device we need to add the gendisk at probe and that we can't destroy it at close if inactive (due to the udev issue), then I don't think there is a solution to our problem. But I was kind of hoping you had an idea :) Thanks. Simon
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