Can any data be lost on ext4 with full journalling mode when electricity crashes? I know it's possible on filesystems like JFS and XFS because they don't journal data (only metadata). When ext4 in "journal" mode journalles both - data and metadata. Does it give 100% guarantee that no data can ever be lost in case of problems with electricity?

There is no guarantee; there can't be a guarantee. There are too many variables.

Example:
1) is the computer plugged into a wall socket without surge protection, or is it plugged into a surge protector.
2) when power is interrupted, is it simply a loss of power or is there a power spike along with the loss of power.
3) if there is a spike without surge protector, how many (if any) components of the computer are damaged?
4) if there is a spike with surge protector, did the surge protector provide full protection, or did a portion of the spike reach the computer and cause damage?
And so on.

I live in a rural location. I have frequent power interruptions and occasional spikes. Most interruptions are short (one or more seconds) to a few hours (as during winter ice storms and power lines down). I have a surge protector plugged into the wall, and another on top of my desk plugged into the first protector. The one on my desk has plugins on the back for my peripherals. When storms move through the area, I shutdown and turn off the switches on the surge protector.

I have had to replace the first surge protector twice due to power spikes, but I have yet to have damage to the computer. Nor have I had data loss or apps suddenly becoming disfunctional.

I've used ext3 since shortly after it was introduced. If my understanding of ext4 is correct, journaling for ext3 and ext4 is the same.

So, the bottom line: can any data be lost using ext4? The answer is yes, with qualifications. See 1 - 4 above.

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Hi,

Do a search and you will find loads of debates for this issue. Power fail for data consistency is more dependent on the applications and not just the filesystem. That is one of the reasons to have a GOOD UPS.

But what about journaling data along with metadata in ext4? I mean - does it do its job (to prevent file loss)?

What makes you assume that the write of data and meta-data to a
journal will always complete before the power failure? Ext4
isn't clairvoyant either ... "ooh, power will fail in 15ms; I
better flush all journal writes!"

Hi,

@bigrigdriver

By switching off the bar will only interrupt the power.
More machines are damaged because of arc over from the strike. Passive Surge protection is fine but do not rely on it solely. It would be better to remove the power plug during a storm.

I personally use UPS, EMF/RFI circuit filters along with active surge on branch circuits that have Electronics. Even those are test periodically. Your protection devices are most likely passive and probably have Go-No-Go indicator. Good for one time protection for a passive device when the device ratings are exceeded. Most likely MOV devices in a Pi filter configuration. You have to be sure to have a good earth ground or these will only work in common mode. Too many people place too much on a bar surge protector. You can get a single outlet protector that is active and throw away if damaged. Then again costs.

Unplug the devices, it would give better protection during storms or power fluctuations.

What if power crashes 5 seconds later after you do "$ sync"? Will all the data be alright?

Hi,

Arbitrarily depends on the local hardware. How much was in the buffer or cache that was to be written. Modern storage systems for a desktop would probably move the data to the container before the power loss. But your asking about 'sync' and will the process complete in five seconds. System & data dependent!

Always rely on timed saving of your work or have the means to keep power beyond loss of local source to insure integrity.

Hopefully recovery methods will provide the means for a system to present data after such incidents.