Unable to set custom 'dev_loss_tmo' value in RHEL7

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本文解析了Red Hat Enterprise Linux 7.2中dm-multipath的dev_loss_tmo参数设置问题，详细介绍了如何通过调整no_path_retry和polling_interval值来有效控制路径丢失超时时间，确保系统稳定性和性能。

https://access.redhat.com/solutions/2487951

SOLUTION 已验证 - 已更新 2017年五月4日19:30 -

English

环境

Red Hat Enterprise Linux 7.2

问题

The value of dev_loss_tmo is set to 20 but still getting the 50 in the output of below command:

Raw

$ less /etc/multipath.conf
[...]
defaults {
        user_friendly_names     yes
        fast_io_fail_tmo        10
        dev_loss_tmo            20   ###
}
[...]

$ for f in /sys/class/fc_remote_ports/rport-*/dev_loss_tmo; do d=$(dirname $f); echo $(basename $d):$(cat $d/node_name):$(cat $f); done
rport-7:0-2:0x50020e8020144afa:50   <---
rport-7:0-3:0x50020e8020144af2:50   <---
rport-8:0-2:0x50020e8020144afb:50   <---
rport-8:0-3:0x50020e8020144af3:50   <---

How is the value of dev_loss_tmo calculated in RHEL 7 ?

决议

For dm-multipath to allow setting dev_loss_tmo of 20, set the no_path_retry to 10 and polling_interval value to 2:

Raw

defaults {
            user_friendly_names     yes
            fast_io_fail_tmo        10
            dev_loss_tmo            20
            polling_interval        2   ###
}
devices {
        device {
                vendor                  "HITACHI"
                product                 "DF.*"
                path_grouping_policy    multibus
                path_checker            tur
                features                "1 queue_if_no_path"
                no_path_retry           10    ###
             }
 }

Then reload the multipathd service:

Raw
```
$ systemctl reload multipathd
```

根源

device-mapper-multipath in RHEL7 refuses to delete the paths sooner than no_path_retry value. This is because once those paths are gone, no retries could be done on affected paths. So, with above new changes, the minimum allowed value of the dev_loss_tmo calculated as below:

Raw
```
Minimum allowed 'dev_loss_tmo' by dm-multipath =  'no_path_retry' * 'polling_interval'   <----- Minimum value of the 'dev_loss_tmo'
```