Sitemap use for very large forum-based community site

[CommManager]

I work on a very large site with two main types of content, static landing pages for products, and a forum & blogs (user created) under each product. Site has maybe 500k - 1 million pages. We do not have a sitemap at this time.
Currently our SEO discoverability in general is good, Google is indexing new forum threads within 1-5 days roughly. Some of the "static" landing pages for our smaller, less visited products however do not have great SEO.
Question is, could our SEO be improved by creating a sitemap, and if so, how could it be implemented? I see a few ways to go about it:

1. Sitemap includes "static" product category landing pages only - i.e., the product home pages, the forum landing pages, and blog list pages. This would probably end up being 100-200 URLs.
2. Sitemap contains the above but is also dynamically updated with new threads & blog posts.

Option 2 seems like it would mean the sitemap is unmanageably long (hundreds of thousands of forum URLs). Would a crawler even parse something that size? Or with Option 1, could it cause our organically ranked pages to change ranking due to Google re-prioritizing the pages within the sitemap?
Not a lot of information out there on this topic, appreciate any input. Thanks in advance.

[GFD_Chris]

Agreed, you'll likely want to go with option #2. Dynamic sitemaps are a must when you're dealing with large sites like this. We advise them on all of our clients with larger sites. If your forum content is important for search then these are definitely important to include as the content likely changes often and might be naturally deeper in the architecture.

In general, I'd think of sitemaps from a discoverability perspective instead of a ranking one. The primary goal is to give Googlebot an avenue to crawl your sites content regardless of internal linking structure.

[Martijn_Scheijbeler]

Hi

Go with option 2, there is no scaling issue here. I have worked with and for sites that have a high multiplier on the number of sitemaps and pages that they're submitting, in some cases up to 100M pages. In all cases, Google was totally fine in crawling and processing the data that was there. As long as you follow the guidelines (max 50K URLs in a sitemap) you're fine as you're just providing another file that usually doesn't exceed about 50MB (depending on if you also add images to the sitemap). If you have an engineering team build the right infrastructure you can easily deal with thousands of these files and run them automated every day/week.

My main focus on big sites is also to streamline their sitemaps to have sitemaps with just the last 50.000 pages and the same for the last 50.000 pages that were updated. This way you're able to also monitor the indexation level of these pages. If you are able to, for example, combine the data from log file analysis you can say: we added 50K pages and Google in the last days were able to crawl X percentage of that.

Hope this gives you some extra insights.

Martijn.