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I'm assuming that your goal here is automatic shutdown when the UPS battery gets low so you don't actually have the NAS see unexpected power loss.
This isn't an answer to your question, but stepping back and getting a big-picture view: do you actually need a clean, automatic shutdown on your Synology server if the power goes out?
I'd assume that the filesystems that the things are set up to run are power-loss safe.
I'd also assume that there isn't server-side state that needs to be cleanly flushed prior to power loss.
Historically, UPSes providing a clean shutdown were important on personal computers for two reasons:
Some filesystems couldn't deal with power loss, could produce a corrupted filesystem. FAT, for example, or HFS on the Mac. That's not much of an issue today, and I can't imagine that a Synology NAS would be doing that unless you're explicitly choosing to use an old filesystem.
Some applications maintain state and when told to shut down, will dump it to disk. So maybe someone's writing a document in Microsoft Word and hasn't saved it for a long time, a few minutes will provide them time to save it (or the application to do an auto-save). Auto-save usually partially-mitigates this. I don't have a Synology system, but AFAIK, they don't run anything like that.
Like, I'd think that the NAS could probably survive a power loss just fine, even with an unclean shutdown.
If you have an attached desktop machine, maybe case #2 would apply, but I'd think that hooking the desktop up to the UPS and having it do a clean shutdown would address the issue -- I mean, the NAS can't force apps on computers using the NAS to dump state out to the NAS, so hooking the NAS up that way won't solve case #2 for any attached computers.
If all you want is more time before the NAS goes down uncleanly, you can just leave the USB and RS-232 connection out of the picture and let the UPS run until the battery is exhausted and then have the NAS go down uncleanly. Hell, that'd be preferable to an automated shutdown, as you'd get a bit more runtime before the thing goes down.
Right now when updates get applied to the NAS, if it gets powered off during the update window that would be really bad and inconvenient require manual intervention.
In memory caching, and the Amy cashing, well I think the file system would almost certainly be in a consistent state, you might lose data in flight if you're not careful.
The real problem, that I need an nas for, is not the loss of some data, it's when the storms hit and there's flooding, the power can go up and down and cycle quite rapidly. And that's really bad for sensitive hardware like hard disks. So I want the NAS to shut off when the power starts getting bad, and not turn on for a really long time but still turn on automatically when things stabilize
Because this device runs a bunch of VMs and containers as well closing down so that all of those rights get flushed is good practice
You sure? I mean, sure, it's possible; there are devices out there that can't deal with power loss during update. But others can: they'll typically have space for two firmware versions, write out the new version into the inactive slot, and only when the new version is committed to persistent storage, atomically activate it.
Last device I worked on functioned that way.
That's the responsibility of the application if they rely on the data to be persistent at some point; they need to be written to deal with the fact that there may be in-flight data that doesn't make it to the disk if they intend to take other actions that depend on that fact; they'll need to call
fsync()or whatever their OS has if they expect the data to be on-drive.Normally, there will always a period where some data being written out is partial: the
write()could complete after handing the data off to the OS's buffer cache. The local drive could complete because data's in its cache. The app could perform multiplewrite()calls, and the first could have completed without the second. With a NAS, the window might be a little bit longer than it otherwise would be, but something like a DBMS will do thefsync(); at any point, it'd be hypothetically possible for the OS to crash or power loss or something to happen.Like I said in the above comment, you'll get that even without a clean shutdown; you'll actually get a bit more time if you don't do a clean shutdown.
Ah, okay, it's not just a file server? Fair enough -- then that brings the case #2 back up again, which I didn't expect to apply to the NAS itself.
I was surprised too. But a lot of the current NAS devices basically operate as hosting devices. It makes sense the hard drives are there the power is there the RAM is there the CPU is there. So for the low intensity containers and VMs you want to run like a Plex server, or DNS server, or tail scale it's all right there