Sql-server – SQL Server Database File Groups on a SAN: Relevant or Not

Unfortunately this is a big ol' "it depends". Complicated further by your having so many applications, probably with wildly different IO profiles, sharing the same instance/server and arrays.

Usually you would want to isolate data and log files as the IO profiles are polar opposite. Typically weighted toward random read for data files and sequential write for logs. The log element is somewhat different in your case as you have multiple log files.

I'd start by getting an understanding of the IO the databases are consuming relative to each other.

    SELECT
        DB_NAME(fs.database_id) AS [Database Name]
      , mf.physical_name
      , io_stall_read_ms
      , num_of_reads
      , CAST(io_stall_read_ms / (1.0 + num_of_reads) AS NUMERIC(10, 1)) AS [avg_read_stall_ms]
      , io_stall_write_ms
      , num_of_writes
      , CAST(io_stall_write_ms / (1.0 + num_of_writes) AS NUMERIC(10, 1)) AS [avg_write_stall_ms]
      , io_stall_read_ms + io_stall_write_ms AS [io_stalls]
      , num_of_reads + num_of_writes AS [total_io]
      , CAST((io_stall_read_ms + io_stall_write_ms) 
        / (1.0 + num_of_reads + num_of_writes) AS NUMERIC(10,1)) AS [avg_io_stall_ms]
    FROM
        sys.dm_io_virtual_file_stats(NULL, NULL) AS fs
    INNER JOIN sys.master_files AS mf
    ON  fs.database_id = mf.database_id
        AND fs.[file_id] = mf.[file_id]
    ORDER BY
            DatabaseName ASC, mf.physical_name
--  num_of_reads DESC
--  num_of_writes DESC  
    OPTION (RECOMPILE) ;

If you can format the output of this into a readable form and add to your question, someone can make a more educated guess at the best use of your 18 disks.

If I was doing this blind, I'd probably start off allocating:

• 6 RAID10 for the busiest database's data files
• 6 RAID10 for the other database's data files
• 4 RAID10 shared Logs
• 2 RAID1 Tempdb (your single disk configuration is a risky single point of failure)

Assuming that:

1. the SAN hosts other arrays and therefore hot spares are available from a pool
2. snapshot isolation isn't in use or isn't placing heavy demands on tempdb

Edit: Following update to question regarding queue lengths:

Queue length counters are oft misunderstood when evaluating SQL Server performance. One of the best analogies I've come across is from a Simon Sabin blog post, Disk Queue Length - a bit like buying Guiness.

In your case, they are so low that you might get by with half the spindles. Would still be interesting to see the other stats from the above query though.

1: yes.

2: depends;) The query execution is still driven by the optimizer. It wont do paralellization for small result sets.

3: Ah - what would that be good given that you are still reading one backup file ;)?

4: Not running the files on one underlying SAN and thus improoving your IO budget?

The most brutal thing I have ever seen like that had nearly 30 files all on separate SAN volumes (count of hard discs going close to 200) and it was done as every LUN had a queue limit in the driver of 255 outstanding requests, which the SAN (with a 32 gigabyte cache) was just not caring about ;) THat thing was pulling in nearly 1.5 gigabyte per second over multiple fiber connections.