Power failure: how to make sure that RPi is nicely shutdown

[Number8]

Hello to all,
I'd like to report a solution I setup for 3 RPI running Domoticz. The idea is to hook up an inexpensive solution as an alternative to a UPS when this is not really needed. What does this HAT: when a main power failure condition arises, after 10 seconds it triggers a shutdown. When the power resumes (including during shutdown sequence), it will send a hardware signal to wake up RPi. We all know that having to proper shutdown sequence avoid the high probability to corrupt the SD card. I have a forth Domotiz running on a NUC Intel gear, that is acting as the master. This one is connected to a large UPS. However, I cannot guarantee there will be enough juice time before main power supply resumes. I'm currently setting up a 4th Domoticz whose sole role will be to manage power down sequence of all equipment connected to UPS's.

So far I bought 5 unit to Thomas Giegling http://safe-power.appspot.com/. Thomas is a very nice fellow, very prompt to answer questions.

[grasvezel]

I realize it is not exactly an answer to your question, but in my experience the risk of SD card corruption when running Domoticz on a Pi is significantly reduced if you put the Domoticz logfile on a ramdisk.

[lost]

I realize it is not exactly an answer to your question, but in my experience the risk of SD card corruption when running Domoticz on a Pi is significantly reduced if you put the Domoticz logfile on a ramdisk.

Never screwed a journaling FS like Ext4 with a power fail. That's designed for this! If issues occurs, that's IMO most probably with some uSD integrated storage controller firmware that are not robust enough/buggy.

What may happen as a boot blocker, indeed, is having an initial boot loader (doing the job of a PC BIOS) with minimal Ext4 support (i.e only a simple reader) that cannot handle journal replay if needed. If journal-replay need was concerning blocks allocated to files needed at early boot, you then have an unbootable system. This may occur if power fail occurs when apt upgrading for instance.

Anyway, on a PI, you can extract the uSD. So in such case, just plug the uSD in a Linux PC with full Ext support will do this replay at mount & this will be printed in dmesg/syslog if this needed to be done. If so, just unmount the SD and set it back to PI, should just work again!

Not having log files for post mortem debug because this was setup in a tmpfs is IMO not helpful.

Another reason to do this could be flash wear... But there is some 'knobs' in file-system or virtual memory management to be able to allow better merge writes, delay commits. IMO, that's a better compromise.

For instance, I add this in my /etc/sysctl.conf

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vm.dirty_ratio=60
vm.dirty_expire_centisecs=120000

Can be tested/tuned using sysfs before setting values enforced at boot.

With a good uSD (i.e. branded A2 for applicative use in smartphones, so tuned for OS system continuous use and not for temporary storage with mostly sequential writes), you may also speed up SD interface (from 50MHz default up to 100 max):

Add this (83MHz, maybe a bit conservative setting) in /boot/config.txt

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dtparam=sd_overclock=83

So less writes & less time to handle writes that's less probability to have a power fail in the middle of one. Combined with journaling feature of Ext4 (don't listen to those that advice using a non journaling FS, they did not understand why they were designed at all), that'll be OK.

[grasvezel]

Good tips, thanks for that. I wasn't aware of the incomplete ext4 implementation used at boot. Still, writing stuff to SD that you will not need after the next reboot seems pointless to me.

[manjh]

I realize it is not exactly an answer to your question, but in my experience the risk of SD card corruption when running Domoticz on a Pi is significantly reduced if you put the Domoticz logfile on a ramdisk.

Interesting! I run Domoticz on a Pi 4 8GB, so plenty of RAM to play around with.
Is there a description somewhere about how to set this up?

[EddyG]

I use this on all my Pi's https://github.com/azlux/log2ram
Put Domoticz log also in /var/log
Put /tmp in RAM with in fstab

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tmpfs                                       /tmp            tmpfs   nodev,noatime,mode=0777,size=256M 0       0

And use Industrial grade SD-cards.
I have a Raspberry Pi 2 (with Jessie) running continuously for more that 3 years now, without any problem/error or crash.

[manjh]

I use this on all my Pi's https://github.com/azlux/log2ram
Put Domoticz log also in /var/log
Put /tmp in RAM with in fstab

Code: Select all

tmpfs                                       /tmp            tmpfs   nodev,noatime,mode=0777,size=256M 0       0

And use Industrial grade SD-cards.
I have a Raspberry Pi 2 (with Jessie) running continuously for more that 3 years now, without any problem/error or crash.

I'll have a look at this as soon as I am back home next week.
Also, I am considering using an SSD as boot device, this way there is no need for an SD card any longer, and it is quite a bit faster. I think I have a 32GB SSD lying around somewhere, this should be sufficient.