If you notice you have many “SQL Timeouts”, in the event logs of your SharePoint 2010 WFE servers, and you are running the SharePoint databases on a mirrored set of SQL servers you may be encountering this problem.
There is a .Net framework article / kb from Microsoft that says this is a known problem with .Net & Mirroring see KB 2605597 which is a hotfix to resolve the problem.
However there is also a further article KB 2600211 that has a section in it that says the KB above is included in the .Net 4.0.3 Update, see note from the web page for the .Net 4.0.3 update.
Don’t let this fool you into thinking that this update contains all of the changes that are in the original KB 2605597, it does not, the original KB has updated DLL’s for .Net 4.0 and earlier versions of .Net back to v2.0, if you apply the .Net 4.0.3 update you only get the .Net System.Data.dll update for v4.0 or later.
AS SharePoint 2010 is built on framework 3.5.1 you need to make sure you install the original KB 2605597 as this contains a new version of the System.Data.dll for framework 2 which is used by .Net v3.5.1 applications.
Hope this saves you some time in installing the rollup KB and not being able to work out why it hasn’t fixed the problem.
A further update to this 22nd Feb 2013, even after applying all the above fixes we still saw occasional SQL timeouts between the SharePoint WFE and the SQL servers, further diagnostic’s from using debugdiag, pointed the problem squarely at the the system.data.dll (Part of .Net Framework 2.0) it suggested we were still suffering from the same problem with the SSPI causing timeouts during the initial connection attempt from WFE to SQL.
Given this Microsoft recommended we deploy a further hotfix to .Net 2.0, KB 2784148 this was released in DEC 2012, and contains amongst other things a further update of the system.data.dll to v2.0.50727.7012.
It appears this latest version of the system.data.dll still hasn’t resolved our timeout issues, next steps time for some BID tracing, to see if that shed’s any more light.
An update on this saga (now August 2013), even though we have been through another two set’s of hotfix’s for our timeout issues, they still haven’t gone away completely. In conjunction with Microsoft they have now identified a SQL server scheduling problem that appears to be affecting SQL 2008 R2 when deployed on HP DLX80 Generation 6 or newer servers, this causes the SQL ring buffer to drop a connection sporadically when running on specific hardware, the infra I have seen this issue on is HP DL380 / DL 580 6th Generation or newer.
I’ll post another update on this subject once Microsoft have worked out how we get around this problem, but if you have SQL timeouts with SharePoint and are using the later generations of HP Servers you could be experiancing the same issue.
Update Microsoft have recently issued a new KB that is suppose to solve (or greatly reduce this problem), seems the underlying problem is with the operating system rather than SQL, so if you are running Windows 2008 R2 SP1 or Windows 7 SP1 this could be affecting you.
Please see the article here which gives instructions on how to apply the hotfix.