Disadvantages of putting a database on the same drive as the Windows OS

Putting a database on the same drive as the Windows OS creates significant performance, reliability, and security disadvantages. This practice is strongly discouraged for any professional or high-performance environment, where separating the two is considered a fundamental best practice.

Performance bottlenecks

I/O contention: Both the OS and the database continuously perform read and write operations. When they share a single drive, they compete for the same I/O resources, causing bottlenecks and slowing down both systems. The OS handles background tasks, virtual memory, and logging, which can conflict with the database's resource-intensive data access.
Mixed I/O patterns: The OS typically handles many small, random I/O requests. Databases, however, have complex I/O patterns, with data files often using random access and log files requiring constant, sequential writes. Combining these different access patterns on one drive is inefficient and can cause delays for both processes.
Impact of fragmentation: On traditional spinning hard disk drives (HDDs), file fragmentation can seriously hurt performance. The constant growth of database files and OS-level operations can cause fragmentation, forcing the drive heads to move around extensively, which increases seek time and slows down all I/O.

Increased risk of data loss and corruption

Single point of failure: A drive failure on a single disk will take down both the OS and the database, making it impossible to recover any data stored locally.
Risk from low disk space: If the database grows uncontrollably and consumes all available disk space, it can cause the entire OS to crash. This can leave the system in an unrecoverable state, potentially corrupting both the OS and the database files.
Inadequate backup strategy: Many organizations keep local backups on the same machine as the database for speed. If a physical disk fails, both the database and its local backups could be lost simultaneously.

Difficulty with maintenance and recovery

Complex disaster recovery: If the OS becomes corrupt, the entire drive may need to be formatted and the OS reinstalled. With the database on the same drive, this process is much riskier and more complex, as the database files must be backed up first and restored later.
Challenges with OS upgrades: Any major OS upgrade or rebuild requires careful planning to avoid damaging the database. Keeping the data on a separate drive makes OS maintenance and upgrades far simpler and safer.

Security vulnerabilities

Reduced security isolation: Placing the database on the same drive as the OS makes it more vulnerable to any security compromises that affect the Windows system. If malware or unauthorized access compromises the OS, the database is immediately exposed.
Complicates security permissions: Proper database security requires careful management of file-level permissions to restrict access to sensitive data files. This becomes more complex and prone to misconfiguration when the database shares the OS's file system, potentially creating unintended vulnerabilities.

Related external articles:

https://www.microsoftpressstore.com/articles/article.aspx?p=3004575

https://learn.microsoft.com/en-us/sql/relational-databases/policy-based-management/place-data-and-log-files-on-separate-drives?view=sql-server-ver15

https://techdocs.broadcom.com/us/en/symantec-security-software/endpoint-security-and-management/it-management-suite/ITMS/Getting-Started/Planning-for-IT-Management-Suite/about-planning-your-sql-server-configuration-v54149252-d780e1396/about-hard-drive-configuration-for-sql-server-v54149126-d780e1482.html