Minimizing downtime during a snapshot on a production server

I have a Linode with around 500gb of data on it. I need to upload about 400 gigs of that data to an image service which slows the production server down considerably. Since I have users on the production servers at all hours of the day, I'm trying to minimize downtime as much as possible by doing the uploading from a different source.

My thoughts were I could power off the system to create a snapshot. Then just use the snapshot to upload that data where it needs to go w/o any impact on the production server. If I did that though, I'd have to create a snapshot and I'm assuming I should power down the Linode before doing that. If I power down the Linode on a production server, then there's no maintenance page when my customers go to the site during this period. Furthermore with a 500gb snapshot, I've no idea how long the downtime would actually be.

Optimally, I could do this with the Production server being booted up to minimize impact on the server but I still don't know how long that Snapshot process will take and how much it will impact my server's performance during that process.

Or, if there's a better way (emphasis on not impacting the production customers and minimizing downtime), I would love to hear it.

Since this is a huuuuge undertaking, any help you have would be useful and I'm sure, very insightful.

Thank you