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I don’t remember the last time I actually lost any data when a machine loses power. Am I just lucky or is there more to the story for this comment: > What happ

I don’t remember the last time I actually lost any data when a machine loses power. Am I just lucky or is there more to the story for this comment: > What happens if you unplug your computer before the operating system writes the data to disk? Well, you will lose the data. It’s as simple as that.

scbrg 3y ago
It absolutely happens. My guess is that your machines just don't actually lose power very often. Any normal desk- or laptop won't. When you hit the power button, it doesn't just cut the power; the system is told to shut down so that it can neatly terminate processes and unmount filesystems before halting. Takes a few seconds. At work I'm writing software for industrial machines. It's essentially a Linux system in kiosk mode running on semi custom hardware. We actually do have a power button that simply shuts off power (much to my dismay), and we have data loss all the time. We just make sure that whenever there's actually important data that needs to be stored (very seldom) we make it blatantly obvious to the user that now is a very bad time to push the power button, and sync before those warnings are removed. But we've had boxes that were rendered unbootable because someone decided to push the power button the moment an apt-get upgrade completed and a lot of updated files were still living in memory only.
sltkr 3y ago
There are several reasons why it's actually really unlikely to lose important data when you lose power on modern systems. Firstly, modern filesystems are designed to be robust to random power loss, so it's unlikely that you will corrupt your filesystem in a way that cannot be recovered from. It does mean that some unflushed data would not be written, but it will only affect files that were opened for writing at or shortly before your system lost power. Some filesystems (including ext4) can offer quite strong consistency guarantees if you configure them correctly, though usually stronger consistency means lower performance in some scenarios. Secondly, the Linux regularly flushes dirty pages to disk anyway (15 seconds on my system, see `cat /proc/sys/vm/dirty_expire_centisecs` for yours) so you cannot lose more than a few seconds of data in the worst case. This is particularly important for the case of "oh, I forgot to unmount the USB stick before pulling it out of my laptop". Thirdly, most applications guard against data loss by using some form of write-ahead logging and explicit syncing. For example, if you edit a file in vim and then exit with `wq!` it will call fsync() before exiting, which causes the contents of the file to be flushed to disk immediately. In theory it's possibly that you lose power in the middle of the fsync() call, but it's extremely unlikely. (Of course it would be just as likely that you lose power right before issuing the write-and-quit command.) The sort of data that is not explicitly synced is where it's not really important to keep the latest information. Think about log files that are appended to continuously, or for example downloads in browsers. Do you really care if you have to restart an incomplete download after a sudden power loss? Probably not.

scbrg 3y ago

It absolutely happens. My guess is that your machines just don't actually lose power very often. Any normal desk- or laptop won't. When you hit the power button, it doesn't just cut the power; the system is told to shut down so that it can neatly terminate processes and unmount filesystems before halting. Takes a few seconds. At work I'm writing software for industrial machines. It's essentially a Linux system in kiosk mode running on semi custom hardware. We actually do have a power button that simply shuts off power (much to my dismay), and we have data loss all the time. We just make sure that whenever there's actually important data that needs to be stored (very seldom) we make it blatantly obvious to the user that now is a very bad time to push the power button, and sync before those warnings are removed. But we've had boxes that were rendered unbootable because someone decided to push the power button the moment an apt-get upgrade completed and a lot of updated files were still living in memory only.

sltkr 3y ago

There are several reasons why it's actually really unlikely to lose important data when you lose power on modern systems. Firstly, modern filesystems are designed to be robust to random power loss, so it's unlikely that you will corrupt your filesystem in a way that cannot be recovered from. It does mean that some unflushed data would not be written, but it will only affect files that were opened for writing at or shortly before your system lost power. Some filesystems (including ext4) can offer quite strong consistency guarantees if you configure them correctly, though usually stronger consistency means lower performance in some scenarios. Secondly, the Linux regularly flushes dirty pages to disk anyway (15 seconds on my system, see `cat /proc/sys/vm/dirty_expire_centisecs` for yours) so you cannot lose more than a few seconds of data in the worst case. This is particularly important for the case of "oh, I forgot to unmount the USB stick before pulling it out of my laptop". Thirdly, most applications guard against data loss by using some form of write-ahead logging and explicit syncing. For example, if you edit a file in vim and then exit with `wq!` it will call fsync() before exiting, which causes the contents of the file to be flushed to disk immediately. In theory it's possibly that you lose power in the middle of the fsync() call, but it's extremely unlikely. (Of course it would be just as likely that you lose power right before issuing the write-and-quit command.) The sort of data that is not explicitly synced is where it's not really important to keep the latest information. Think about log files that are appended to continuously, or for example downloads in browsers. Do you really care if you have to restart an incomplete download after a sudden power loss? Probably not.