Suddenly bad disk I/O utilization on SQL-server even on weekend where we have few transactions

Question

On friday our MS SQL-server 2012 suddenly showed 100% disk I/O utilization in New Relic (Performance monitor). We hade made no updates what so ever and windows update showed nothing that had happend. The load on the server was low because fridays have low traffic on our website. The disk I/O utilization has been kept high even over the weekend.

The server is a VM-Ware machine with 16 procs and 36 gb of memory. The disk are located in a san.

We have about 5 mb of reads per second and very low on writes on the database server. The server has about 500 I/O operations per second. The CPU is at 25%

The database is stored in 12 files on a separate drive on the server. No long running task are running. The server is defragmentet and all the indexeson the database have been rebuilt.

Perfmon on the sql server shows disk que at peek 5.

Our server guys says that the SAN is running smoothly. But my gues is that something happend on that friday whick keeps our SQL have to wait for file operations.

Any ideas?

Answer 1

The reaseon was Another SQL-server on the same host that accessed the disk a lot

Answer 2

There are lots of reasons this could happen.

Most obvious, backup schedule?

If your traffic was so low on Friday that IIS shut down your Application Pool after the idle timeout, the next hit to that Website will trigger reloading of all data which is cached on application startup.

Since your database server is virtualised, the IO to that server may be also virtualised (as opposed to directly connecting the SQL Server Virtual Machine to the storage on the physical host). In this case, your database server performance may be limited by other machines on the same host saturating the link to the SAN.