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  • Blog Post: 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...
  • Blog Post: NUL or NULL?

    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...
  • Blog Post: 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 the box...
  • Blog Post: “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...
  • Blog Post: 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...
  • Blog Post: 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...
  • Blog Post: 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...
  • Blog Post: 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...
  • Blog Post: 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...
  • Blog Post: ‘No Disks suitable for cluster disks’: one more possible cause

    Today in the lab I was working on a test 3-node cluster. I was using Hyper-V for this. We normally use the iSCSI Software Target for testing, so I had set that up on a central VM and then published the target to all the VMs. The disks were seen in the disk management of Node1, and I brought them online...
  • Blog Post: DB Mirroring Tips

      Here are some essential things to consider when trying to tune a DB mirroring setup. They revolve round the following concepts: The effect of Virtual Log Files (VLFs) on recovery and therefore failover times The interplay (or lack thereof) between mirroring and some other SQL Server engine features...
  • Blog Post: Rolling upgrade of 1 instance from 2-node, 2-instance sql failover cluster to slipstreamed sql 2008 SP1

    Now that SQL Server 2008 SP1 is released (download link here ), it is time to test a very interesting capability: Service Pack slipstreaming. That means that you can save some time by doing an 'integrated' installation of SQL 2008 which includes the SP1 binaries and avoid having to apply SP1 later on...
  • Blog Post: SQL Server 2000 / 2005 Failover Cluster Pre-installation Checklist

    Here's a quick checklist before you install an instance of SQL Server on a failover cluster; or any service pack or hotfix for an instance. Following this list should eliminate 90% of the most common causes of setup failure. Please note that this checklist does not cover all the 'normal' requirements...
  • Blog Post: Howto: Have a Clustered instance of SQL Server listen on multiple virtual IP addresses

    Recently a colleague asked me if there were any changes specific to Windows Server 2008 which allow SQL Server (clustered instance) to listen on multiple subnets. There was only one change in Windows 2008 to my knowledge (which I have detailed in this post later on) but that had nothing specific to do...
  • Blog Post: Concurrency of Full, Differential and Log backups on the same database

    Overview Many a time, we are asked by customers about the effect of overlapping backup schedules: for example, whether they can backup the transaction log at the same time that a long-running complete backup of the database is taking place. Books Online does talk about the concurrency, but does not cover...
  • Blog Post: Database Mirroring Technical Paper

    One of the most exciting new features in SQL Server 2005 is DB Mirroring. Now that Service Pack 1 is released, we officially support this feature. A really useful resource to understand the options available in using this feature and their pros / cons is the " Database Mirroring Best Practices...
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