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Is it considered safe to run Windows OS level scheduled tasks for disk defragment against SQL volumes?

I've inherited the management of a SQL Server from a previous DBA, and they had a Windows Scheduled Task setup to defragment the SQL Server data volumes. I'm wondering if I can safely disable those jobs.

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Windows-Verified Disk Defragmentation solutions won't cause corruption in your databases, however in most scenarios performing physical defragmentation of SQL Server data and log files is unnecessary.

I say "most scenarios" since most production SQL Servers will have a dedicated drive setup for SQL Server data and log files. However, if your SQL Server data and log files exist on the system drive, or some other physically attached spinning rust disk, then you might want to consider defragmentation if the volume is heavily fragmented, and you can prove that fragmentation is having a negative effect on SQL Server performance.

If you have a locally attached SSD for storage of SQL Server data and log files, or you are using a SAN or other RAID array, then defragmentation of the physical files will have zero effect on performance. SSDs are random access, and so are almost totally unaffected by file fragmentation. RAID arrays spread stripes of each file across disks by design, and who knows what's happening at the SAN level depending on how advanced it is - for instance it might be doing data-de-duplication.

The short answer is, it's almost never necessary to defragment drives containing SQL Server data and log files.

This Microsoft Support page has further details about defragmenting SQL Server volumes.

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