[systemd-devel] [survey] BTRFS_IOC_DEVICES_READY return status

[Andrei Borzenkov]

В Wed, 17 Jun 2015 23:02:02 +0200
Lennart Poettering <lennart at poettering.net> пишет:

> On Wed, 17.06.15 21:10, Goffredo Baroncelli (kreijack at libero.it) wrote:
> 
> > > Well, /bin/mount is not a daemon, and it should not be one.
> > 
> > My helper is not a deamon; you was correct the first time: it blocks
> > until all needed/enough devices are appeared.
> > Anyway this should not be different from mounting a nfs
> > filesystem. Even in this case the mount helper blocks until the
> > connection happened. The block time is not negligible, even tough
> > not long as a device timeout ...
> 
> Well, the mount tool doesn't wait for the network to be configured or
> so. It just waits for a response from the server. That's quite a
> difference.
> 
> > > Well, it's not really ugly. I mean, if the state or properties of a
> > > device change, then udev should update its information about it, and
> > > that's done via a retrigger. We do that all the time already, for
> > > example when an existing loopback device gets a backing file assigned
> > > or removed. I am pretty sure that loopback case is very close to what
> > > you want to do here, hence retriggering (either from the kernel side,
> > > or from userspace), appears like an OK thing to do.
> > 
> > What seems strange to me is that in this case the devices don't have changed their status.
> > How this problem is managed in the md/dm raid cases ?
> 
> md has a daemon mdmon to my knowledge.
> 

No, mdmon does something different. What mdadm does is to start timer
when RAID is complete enough to be started in degraded mode. If
notifications for missing devices appear after that, RAID is started
normally. If no notification appears until timer is expired, RAID is
started in degraded mode. 

ACTION=="add|change", IMPORT{program}="BINDIR/mdadm --incremental --export $devnode --offroot ${DEVLINKS}"
ACTION=="add|change", ENV{MD_STARTED}=="*unsafe*", ENV{MD_FOREIGN}=="no", ENV{SYSTEMD_WANTS}+="mdadm-last-resort@$env{MD_DEVICE}.timer"