Upgrade large farm with Service Pack detaching all content databases?

Question

Hi,
Whenever we upgrade one of our farms (with a Cumulative Update, Language Pack, or Service Pack), the corresponding PSConfig usually yields in error due to the huge time taken to upgrade all the content databases at the same time.
We have heared about the possibility of detaching all content databases previous to the update (just CONTENT databases), then proceed with the installation and PSConfig, and finally a gradual upgrade by attaching one by one the content databases.
After searching online I have not found a relevant or precise official documentation detailing whether this procedure is recommended or viable, and have some additional questions:
- Can all he content database be disconnected (including the ones containig myprofiles, top level sites...) during the upgrade?
Thanks in advance

Answer 1

In my understanding, the patching involves (at least) two steps:
Patching binaries on all servers of the farm
Upgrading the database schema of Service Applications DBs, Configuration & Admin DBs, and content databases.
If during the upgrade the database is not attached to the farm, SharePoint does not know about it, and does nothing with itm just like if it was physically detached from the SQL Server instance. But when, after patching, you reattach the database, SharePoint
detects the difference in the schema version and the patched farm version, and upgrades the database accordingly. As far as I understand, this is automatically done by
Mount-SPContentDatabase:
"The Mount-SPContentDatabase cmdlet attaches an existing content database to the farm. If the database being mounted requires an upgrade, this cmdlet will cause the database to be upgraded.
The default behavior of this cmdlet causes an upgrade of the schema of
the database and initiates upgraded builds for all site collections within the specified content database if required. To prevent initiation of upgraded builds of site collections, use the
NoB2BSiteUpgrade parameter. This cmdlet does not trigger version-to-version upgrade of any site collections."
If you decide not to upgrade, you can later run Upgrade-SPContentdatabase.
The point is, that if you keep attached all the content databases, the PSConfig needs to do a massive upgrade of all the content databases before finishing, thus considering that the farm is ok. On the detach/attach approach, the first PSConfig ends quite
soon so your farm is up and running sooner (although, that's for sure, without the data being available until attached and upgraded - but you can gradually start giving service instead of waiting for the whole process).
So far, so good. But what I was trying to ask in my question, is whether there is a detailed procedure to follow for this approach, or if there is any particular constraint regarding this approach (like "databases containing top level site collections
should not be detached", or "pay attention to the my site host web application", or "that patching method is not supported").
Thanks.