Speed it takes to write to disk

Question

I am looking for some T-sql that I can use that will test to see how long it takes to write to disk. I have two disks. G and H. I T-Sql that will tell me which one takes longer to write to. That should be pretty simple right?
Alan

Answer 1

I am looking for some T-sql that I can use that will test to see how long it takes to write to disk. I have two disks. G and H. I T-Sql that will tell me which one takes longer to write to. That should be pretty simple right?
I assume you are interested in the time it takes to write data pages.  This is simple at first glance but you need to consider how the database engine performs writes. 
When you insert, update, or delete data, SQL Server does not immediately write the modified pages to disk.  Instead, data modified only in memory and a record written to the transaction log upon COMMIT or when the log buffer is full.  The
actual writes of modified pages occur asynchronously.  These writes are done by the Lazy Writer and Checkpoint background processes.  You have little control over when those writes occur so it is difficult to measure timing.
Also consider that writes are actually rewrites of existing data pages.  The page to be modified must first be read into memory, where it is modified as described above.  These physical reads are synchronous during query execution and you also
have little control over whether the page is already in memory or not and that will influence query time.
Be aware that transaction log writes are mostly sequential and are on the critical path for performance of insert/update/delete queries.  Data pages are mostly random and therefore take much longer on spinning media due to seek time.  Data file
write performance helps the background processes write dirty pages to disk faster, allowing buffer cache to be reused sooner.
That being said, T-SQL is not the write tool to measure hardware performance and you need to consider what types of write I/O you want to measure (sequential versus random).  I suggest you use a tool like SQLIO for this task.  See
http://www.microsoft.com/en-us/download/details.aspx?id=20163
Dan Guzman, SQL Server MVP, http://www.dbdelta.com