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I'm moving the site from flat files (which had become way out of control) to WordPress, so all urls will change completely and there is no pattern matching possible. I've listed out about the 500 top pages and their new urls and will manually redirect them, but I've 2 things on my mind.
1. How much of an additional load on the server will this cause? There are not huge numbers of visitors but enough - maybe 80,000 uniques per month and growing.
2. I know Google generally deals with this quite well, but a lot of incoming links are on .edu sites and other sites that will probably never update them, or not for years. How long should I leave the redirects in place?
Thanks for any input.
On my own sites, the answer is, "forever."
For this reason, it's a very good idea to think about coming up with a 'friendly URL' architecture that will be portable in the future, no matter what CMS your site is based on. This will prevent ever having to change your URLs again. This can be a huge challenge, but is usually well worth the effort, at least for the top dozen or few-dozen pages of your site.
As for load, the initial load may be noticeable, but realize two things:
1) After the search engines atart updating their links, they won't be causing the redirects to be invoked any more.
Think about the redirect code in the same way as you think about PHP code; You have a few hundred lines of redirect code, but how many thousand lines of PHP code in that CMS? Therefore, the PHP execution time and 'load' will likely swamp the redirect code execution time, and you may not even notice it. However, this fact should not make you feel that any and all optimizations shouldn't be made to the redirect code -- Take every opportunity to make it efficient from the start, and then you just won't have to worry about it.