Chapter 8. Advanced Disk Cache Topics

Performance is one of the biggest concerns for Squid administrators. As the load placed on Squid increases , disk I/O is typically the primary bottleneck. The reason for this performance limitation is due to the importance that Unix filesystems place on consistency after a system crash.

By default, Squid uses a relatively simple storage scheme ( ufs ). All disk I/O is performed by the main Squid process. With traditional Unix filesystems, certain operations always block the calling process. For example, calling open ( ) on the Unix Fast Filesystem (UFS) causes the operating system to allocate and initialize certain on-disk data structures. The system call doesn't return until these I/O operations complete, which may take longer than you'd like if the disks are already busy with other tasks .

Under heavy load, these filesystem operations can block the Squid process for small, but significant, amounts of time. The point at which the filesystem becomes a bottleneck depends on many different factors, including:

• The number of disk drives

• The rotational speed and seek time of your hard drives

• The type of disk drive interface (ATA, SCSI)

• Filesystem tuning options

• The number of files and percentage of free space