Why is a Transformer rated in kVA, not in kW?

I saw a post like this fly by on my Facebook feed today. I lost it when I closed the window, but the question lingered. Why is a Transformer rated in kVA, not in kW? I did not know the answer, but I should know because I have also seen kVA rating used in datacenter contract rather than kW.

Many electricians point out that kVA represents apparent power, and kW is the available amount of work under load. The difference is the power factor representing efficiency, so kW = kVA × pf. In a perfect system, kVA = kW. And electricians explain that power factor is less than 1 due to transmission loss.

But someone also pointed out that for DC (direct current), kVA = kW because the current does not go out of phase (not a great explanation though). That reminded me from college physics class that AC (alternative current) is a sine wave, and its RMS (root mean squared) voltage is smaller than the peak voltage. For sine, rmsV = (√2 / 2) × Vp ≅ 0.707 Vp. RMS voltage is used to calculate work, not the peak voltage.

As a side note, AC in the real world only approximates a sine wave, and UPS modified sine wave can look much worse.

So the answer is this: transformer is rated in kVA because it has to handle peak voltage, but the usable work kW is rated in RMS voltage which would under-specify the design requirements of a transformer.

Here is an afterthought: when physics gets applied in the real world, the electricians learn a simplified version of it stated to them as a matter of fact, which is not wrong, but they do not understand the reasoning behind it. Some people are perfectly happy to learn just enough to get stuff done. But if you want to keep innovating, you have to understand the reasoning behind an existing system, or else you would reinvent the wheel poorly by not learning from past mistakes and making additional mistakes that people before you knew to avoid.

On that topic, I recommend watching this video by SmarterEveryDay - I Was SCARED To Say This To NASA... (But I said it anyway).

ZFS zpool replace "no such device in pool"

Since the last time I built my ZFS pool in 2016 with 4x 1TB SSDs in a thunderbolt 2 enclosure, I ran out of space in 2019 and upgraded the SSDs to 4x 2TB. I didn't have an extra enclosure, so I took a fairly risky approach to yank each SSD out, put in a new SSD, and let ZFS resilver. It only worked because my pool is a raidz1. During resilvering, the pool was in a degraded state due to the lack of redundancy.

And I am just about to run out of space again after 4 years. I also lucked out because the SSDs are heavy discounted right now due to oversupply. The chip makers are trying to reduce production, so the price might rise again.

This time, I bought 4x 4TB M.2 NVMe (non-affiliate B&H link) and put them in a Thunderbolt 3 enclosure (non-affiliate vendor link). This means that I could do a proper replace without risking the degraded state.

But when I try to replace the SSDs, I keep getting the "no such device in pool" error. I tried these variations:

$ sudo zpool replace tank disk2 disk13
cannot replace disk2 with disk13: no such device in pool
$ sudo zpool replace tank disk2 /dev/disk13
cannot replace disk2 with /dev/disk13: no such device in pool
$ sudo zpool replace tank disk2 media-728C3B9F-647B-4C40-9DDF-E140E55D8C89
cannot replace disk2 with media-728C3B9F-647B-4C40-9DDF-E140E55D8C89: no such device in pool
$ sudo zpool replace tank disk2 /var/run/disk/by-id/media-728C3B9F-647B-4C40-9DDF-E140E55D8C89
cannot replace disk2 with /var/run/disk/by-id/media-728C3B9F-647B-4C40-9DDF-E140E55D8C89: no such device in pool

None of the above worked, and it was driving me nuts. When I was younger, I would have pulled my hair out. Now that my hair is more spase, I've learned to take better care of it by showing restraint.

It turned out that "no such device in pool" actually meant it could not find disk2, even though it is able to find disk13 just fine for some reason. This worked:

$ sudo zpool replace tank media-E235653B-0924-2F46-B708-FDEFFAB5CB15 disk13

The colon in the error message makes it appear that "no such device in pool" is about the new disk, and it is confusing why the new disk have to be added to the pool first. The error is actually about ZFS not being able to identify the existing disk in pool that needs replacement. In fact, if it could not find the new disk, the error message is different:

$ sudo zpool replace tank disk2 disk777
cannot open 'disk777': no such device in /dev
must be a full path or shorthand device name

The error message could be improved, for sure. I wonder if baldness of the people in the tech industry could have been prevented if more error messages are helpful.

Another thing I learned is that when upgrading raidz1, it is far more efficient to replace all the disks at once and let ZFS resilver them in parallel. It takes the same amount of time to resilver one disk as to resilver all disks. The reason is that resilvering requires reading all disks in order to check the parity, whether it is resilvering one disk or many. In my case, the process is bottlenecked by the read, not the write.