Disabling access time (atime) updates on ZFS file systems
When running web servers, it is extremely common for website administrators to disable access time updates on file systems that contain content. This alleviates the need for the kernel to update the access time of a file each time it is requested, and less work in the kernel means more cycles are available for serving content.
Prior to the introduction of the Solaris ZFS file system, you could disable access time updates by adding the noatime mount option to the desired entry in /etc/vfstab. Since ZFS stores the file system attributes with the file system, this allows the attributes (e.g., compress content, checksum content, disable atime updates, etc.) to move with the file system. To view the attributes of a ZFS file system, you can run the zfs utility with the “get” option and the file system to retrieve attributes from:
$ zfs get all home/apps
NAME PROPERTY VALUE SOURCE home/apps type filesystem - home/apps creation Sat Jul 8 23:14 2006 - home/apps used 170M - home/apps available 54.9G - home/apps referenced 170M - home/apps compressratio 1.00x - home/apps mounted yes - home/apps quota none default home/apps reservation none default home/apps recordsize 128K default home/apps mountpoint /home/apps default home/apps sharenfs off default home/apps checksum on default home/apps compression off default home/apps atime on default home/apps devices on default home/apps exec on default home/apps readonly off default home/apps zoned off default home/apps snapdir hidden default home/apps aclmode groupmask default home/apps aclinherit secure default
As you can see from the output above, there are numerous attributes available. The attribute to control access time updates is “atime,” and it can be switched off with the zfs “set” option:
$ zfs set atime=off home/apps
Once a file system attribute is modified, the change will immediately take effect, no mounting or unmounting required:
$ zfs get all home/apps
NAME PROPERTY VALUE SOURCE home/apps type filesystem - home/apps creation Sat Jul 8 23:14 2006 - home/apps used 170M - home/apps available 54.9G - home/apps referenced 170M - home/apps compressratio 1.00x - home/apps mounted yes - home/apps quota none default home/apps reservation none default home/apps recordsize 128K default home/apps mountpoint /home/apps default home/apps sharenfs off default home/apps checksum on default home/apps compression off default home/apps atime off local home/apps devices on default home/apps exec on default home/apps setuid on default home/apps readonly off default home/apps zoned off default home/apps snapdir hidden default home/apps aclmode groupmask default home/apps aclinherit secure default
I have only worked with ZFS for two months, and am impressed with that I see so far. Once ZFS has better methods to detect disk failures, the ability to remove devices and expand RAIDZ sets, support for “/,” and the known performance issues are addressed, it will be the final word in Solaris file systems.
