Error message “Copying cluster-resource …fssres.dll to C:Windowssystem32fssres.dll failed…” during SQL Server 2014 setup

First and foremost, let me reproduce the complete error message I refer to above: “Copying cluster-resource C:\Program Files\Microsoft SQL Server\MSSQL12.SQL14\MSSQL\Binn\fssres.dll to C:\Windows\system32\fssres.dll failed because the destination file exists and is read-only” Investigation I received the above error message while setting up a SQL Server 2014 instance side by side with an existing copy of SQL…


Potential data latency with AlwaysOn Availability Groups and Memory Optimized Tables

Today I was testing a scenario with a readable secondary in an Availability Group (AG). The database in the AG contained a memory-optimized table and I was testing read-only query support on the secondary. The mystery While changes for on-disk tables are replicated near-instantaneously (of course in my test setup there was no other overhead)…



Ever since the old MS-DOS days, the Windows OS family has supported a special ‘null device’ – NUL. Notice the single ‘L’. This is a ‘reserved’ device identifier which you can use as a path. The special thing about this null device (NUL) is that it essentially swallows whatever is sent to it. NUL and…


Strange issue when enabling AlwaysOn AG feature in SQL 2014

Today I ran into an issue, which eventually turned out to be a really silly thing, but I think it is important that I share the details. I was trying to install SQL 2014 side-by-side with an already existing SQL 2012 AG setup. Everything went fine till the customary step when I tried to ‘tick…


“Could not add cluster access…” during SQL 2012 Failover Cluster Instance setup

Today a lab setup of SQL 2012 SP1 + CU7 (I was using the /UpdateSource flag to have a patched setup + roll in the updated bits in one go) failed sometime before creating the SQL resource in cluster. The error message from the detail.txt file showed: Could not add cluster access for SID ‘S-1-5-80-2358576813-3362504073-1364548504-537691503-200193738’….


SQL Server and Storage / SAN replication (mirroring) tips

Some customers have adopted block level replication of the storage disks (LUNs) as their DR solution for SQL Server databases. Remote mirroring of storage disks sounds great – transparent to the application, replicates every block as-is; so very little reconfiguration required on the other side, scales fairly linearly regardless of the number of databases and…


AlwaysOn Availability Group Forced Failover: Under the Hood

In this walkthrough I will take you through one aspect of FCI + AG configuration: forced failover. We will simulate failure and force failover. And in the process we will have some … Data Loss. Does that not sound horrible! Relax! You should not worry too much , because in this case what we are…


AlwaysOn Availability Groups Connectivity Cheat Sheet

Today we were discussing the AlwaysOn connectivity options, and the various combinations therein of Readable Secondary setting and the connection strings became very complex. So I have summarized all the possible outcomes in the pivot tables below. For convenience, we have two tables. The first table covers the cases when the Primary Replica is set…


AlwaysOn Availability Groups Listener: connectivity details

Today in some discussions with my colleagues we were looking at the AlwaysOn Listener which allows SQL Server client applications to be abstracted from the details of the AlwaysOn Availability Group implementation. On the surface, The Listener essentially is a cluster resource pair – a Virtual IP and a Virtual Network Name. But on deeper…


Error message: The target database ('xyz') is in an availability group and currently does not allow read only connections

Today while poking around the AlwaysOn Availability Groups feature I came across the above error when testing the read-only access routing configuration. That configuration allows connections with declared ReadOnly intent to be routed to one of the read-only secondary replicas. The configuration looked like this: 3-node Windows 2008 R2 Core cluster 3 instances of SQL…