Synology disk mirror SHR or RAID 1

There doesn't seem to be an actual question, but here are some thoughts:

• This is only worthwhile if your time is valueless; otherwise, just buy another 2 TB hard disk, make them a 3-drive RAID 5 set and quit messing about.
• If it were possible to grow software RAID 10 volumes under Linux, that would solve your issue, since you could just add the drive and it would sort it out. Unfortunately this doesn't seem to be the case (at least on the mdadm & md version on my system). But if you didn't care about growing the array, RAID 10 is definitely a better answer than your setup above.
• And finally, there isn't really any way to add the 500 GB drive you mention without a lot of manual data movement. To accomplish it you could (but i wouldn't):
  • create 2 x 125 and 1 x 250 on the new drive (let's assume they're sdd1-3)
  • move sda1 to sdd1: add sdd1 to md0; change number of copies to 3; wait for resync; remove sda1 from md0; change number of copies to 2
  • repeat for sda2 -> sdd2 (md1)
  • remove sda2, resize sda1 to 250 GB
  • create md3 from sda1 and sdd3
  • add md3 to your volume group

Hope that makes sense.

Side note:

• Dear HP, AutoRAID arrays were the best thing since sliced bread. Bring them back.

No there wont be any down time, this really is a standard thing to do now in enterprise, where always up availability is key.

LVM does this expansion in the background and it is pretty much instant, it wont have any effect on services, I do this once or twice a week on my estate and haven't had a problem yet