Re: [PATCH v4 20/20] multipath.rules: find_multipaths "smart" logic

["Benjamin Marzinski" <bmarzins@redhat.com>]

On Wed, Apr 04, 2018 at 06:16:27PM +0200, Martin Wilck wrote:
> When the first path to a device appears, we don't know if more paths are going
> to follow. find_multipath "smart" logic attempts to solve this dilemma by
> waiting for additional paths for a configurable time before giving up
> and releasing single paths to upper layers.
> 
> These rules apply only if both find_multipaths is set to "smart" in
> multipath.conf. In this mode, multipath -u sets DM_MULTIPATH_DEVICE_PATH=2 if
> there's no clear evidence wheteher a given device should be a multipath member
> (not blacklisted, not listed as "failed", not in WWIDs file, not member of an
> existing map, only one path seen yet). In this case, "multipath -u" also sets the
> variable FIND_MULTIPATHS_WAIT_UNIL to a relative time stamp (we need to use
> relative "monotonic" time stamps because this may be triggered early during
> boot, before system "calendar" time is correctly initialized).
> 
> In the DM_MULTIPATH_DEVICE_PATH=2 case, pretend that the path is multipath
> member, disallow further processing by systemd (allowing multipathd some time
> to grab the path), and set a systemd timer to check again after the given
> timeout. If the path is still not multipathed by then, pass it on to systemd
> for further processing.
> 


Like I've mentioned, I think multipath -u should be called with a
special option on add events or if FIND_MULTIPATHS_WAIT_UNTIL exists and
is non-zero (or perhaps we should IMPORT DM_MULTIPATH_DEVICE_PATH, and
check if it's 2 or import SYSTEMD_READY and check if it's 0), that smart
mode could use to allow maybes and new claims on existing paths because
another path appeared.

> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  multipath/multipath.rules | 64 ++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 60 insertions(+), 4 deletions(-)
> 
> diff --git a/multipath/multipath.rules b/multipath/multipath.rules
> index aab64dc..77b9f16 100644
> --- a/multipath/multipath.rules
> +++ b/multipath/multipath.rules
> @@ -19,9 +19,65 @@ LABEL="test_dev"
>  ENV{MPATH_SBIN_PATH}="/sbin"
>  TEST!="$env{MPATH_SBIN_PATH}/multipath", ENV{MPATH_SBIN_PATH}="/usr/sbin"
>  
> -# multipath -u sets DM_MULTIPATH_DEVICE_PATH
> -ENV{DM_MULTIPATH_DEVICE_PATH}!="1", IMPORT{program}="$env{MPATH_SBIN_PATH}/multipath -u %k"
> -ENV{DM_MULTIPATH_DEVICE_PATH}=="1", ENV{ID_FS_TYPE}="mpath_member", \
> -	ENV{SYSTEMD_READY}="0"
> +# FIND_MULTIPATHS_WAIT_UNTIL is the timeout (in seconds after the
> +# epoch).
> +IMPORT{db}="FIND_MULTIPATHS_WAIT_UNTIL"
> +ENV{.SAVED_FM_WAIT_UNTIL}="$env{FIND_MULTIPATHS_WAIT_UNTIL}"
> +
> +# multipath -u sets DM_MULTIPATH_DEVICE_PATH and,
> +# if "find_multipaths smart", also FIND_MULTIPATHS_WAIT_UNTIL.
> +ENV{DM_MULTIPATH_DEVICE_PATH}!="1", \
> +	IMPORT{program}="$env{MPATH_SBIN_PATH}/multipath -u %k"
> +
> +# case 1: this is definitely multipath
> +ENV{DM_MULTIPATH_DEVICE_PATH}=="1", \
> +	ENV{ID_FS_TYPE}="mpath_member", ENV{SYSTEMD_READY}="0", \
> +	GOTO="stop_wait"
> +
> +# case 2: this is definitely not multipath, or timeout has expired
> +ENV{DM_MULTIPATH_DEVICE_PATH}!="2", \
> +	GOTO="stop_wait"
> +
> +# Code below here is only run in "smart" mode.
> +# multipath -u has indicated this is "maybe" multipath.
> +
> +# This shouldn't happen, just in case.
> +ENV{FIND_MULTIPATHS_WAIT_UNTIL}!="?*", GOTO="end_mpath"
> +
> +# Be careful not to start the timer twice.
> +ACTION!="add", GOTO="pretend_mpath"
> +ENV{.SAVED_FM_WAIT_UNTIL}=="?*", GOTO="pretend_mpath"
> +
> +# At this point, we are seeing this path for the first time, and it's "maybe" multipath.
> +
> +# The actual start command for the timer.
> +#
> +# The purpose of this command is only to make sure we will receive another
> +# uevent eventually. *Any* uevent may cause waiting to finish if it either ends
> +# in case 1-3 above, or if it arrives after FIND_MULTIPATHS_WAIT_UNTIL.
> +#
> +# Note that this will try to activate multipathd if it isn't running yet.
> +# If that fails, the unit starts and expires nonetheless. If multipathd
> +# startup needs to wait for other services, this wait time will add up with
> +# the --on-active timeout.
> +#
> +# We must trigger an "add" event because LVM2 will only act on those.
> +
> +RUN+="/usr/bin/systemd-run --unit=cancel-multipath-wait-$kernel --description 'cancel waiting for multipath siblings of $kernel' --no-block --timer-property DefaultDependencies=no --timer-property Conflicts=shutdown.target --timer-property Before=shutdown.target --timer-property AccuracySec=500ms --property DefaultDependencies=no --property Conflicts=shutdown.target --property Before=shutdown.target --property Wants=multipathd.service --property After=multipathd.service --on-active=$env{FIND_MULTIPATHS_WAIT_UNTIL} /usr/bin/udevadm trigger --action=add $sys$devpath"
> +
> +LABEL="pretend_mpath"
> +ENV{DM_MULTIPATH_DEVICE_PATH}="1"
> +ENV{SYSTEMD_READY}="0"
> +GOTO="end_mpath"
> +
> +LABEL="stop_wait"
> +# If timeout hasn't expired but we're not in "maybe" state any more, stop timer
> +# Do this only once, and only if the timer has been started before
> +IMPORT{db}="FIND_MULTIPATHS_WAIT_CANCELLED"
> +ENV{FIND_MULTIPATHS_WAIT_CANCELLED}!="?*", \
> +	ENV{FIND_MULTIPATHS_WAIT_UNTIL}=="?*", \
> +	ENV{FIND_MULTIPATHS_WAIT_UNTIL}!="0", \
> +	ENV{FIND_MULTIPATHS_WAIT_CANCELLED}="1", \
> +	RUN+="/usr/bin/systemctl stop cancel-multipath-wait-$kernel.timer"
>  
>  LABEL="end_mpath"
> -- 
> 2.16.1