Enabling Qwrite Capability

By default, the Sun QFS file systems disable simultaneous reads and writes to the same file. This mode is defined by the UNIX vnode interface standard, which gives exclusive access to only one write while other writers and readers must wait. Qwrite enables simultaneous reads and writes to the same file from different threads.

The Qwrite feature can be used in database applications to enable multiple simultaneous transactions to the same file. Database applications typically manage large files and issue simultaneous reads and writes to the same file. Each system call to a file acquires and releases a read/write lock inside the kernel. This lock prevents overlapped or simultaneous operations to the same file. If the application itself implements file-locking mechanisms, the kernel-locking mechanism impedes performance by unnecessarily serializing I/O.

Qwrite can be enabled in the /etc/vfstab file, in the samfs.cmd file, and as a mount option. The -o qwrite option with the mount command bypasses the file system locking mechanisms except for applications accessing the file system through NFS and lets the application control data access. If qwrite is specified, the file system enables simultaneous reads and writes to the same file from different threads. This option improves I/O performance by queuing multiple requests at the drive level.

The following example uses the mount command to enable Qwrite on a database file system:

# mount -F samfs -o qwrite /db

For more information about this feature, see the description of the qwrite directive on the samfs.cmd(4) man page or the -o qwrite option on the mount_samfs(1M) man page.