I have been preparing for the migration of the Dolphin site to UNA for several months. My website with dolphin has about 10,000 users and about 100,000 images.
I have a test site with Una. When I do the migration, the first time an image has to be displayed, the page waits for that image to be generated. This only happens the first time the image has to be generated. The problem occurs when you have to generate many images for the first time, this causes a server error.
If I perform the migration under these conditions, the server will constantly crash until all the images are generated again, which will be very annoying for my users. Is there a way to regenerate the images in bulk?
During Migration only main images are transferring and added to database. When user opens the page with images first time images are transcoded to get proper size and in this moment server may be loaded. So, if the all images were transferred and problems appear only when users open the page with images, then server configuration should be checked, seems like transcoding takes to much resources of your server.
Server's settings looks good, but you may try to check server logs to detect why it is crashed during transcoding.
To decrease loading of the server during transcoding you may try to hide temporary blocks with big amount of images like timeline, it prevents processing of many photos simultaneously and when the most part of the photos are transcoded, blocks can be activated.
Also you may try to change php GD library which uses for photos processing by default to ImageMagick (if it is installed on your server), it works a little bit faster.