Tuning Last Access Time

Provides an overview of the Last Access Time feature and its tuning.

What is Last Access Time?

Last Access Time (atime) is file metadata that is updated whenever a file is read. You can use atime for file management and governance decisions such as:
  • Deleting files that have not been accessed for a while
  • Tiering files (to warm or cold tier) that have not been accessed for a while
  • Migrating files that have not been accessed frequently
  • Purging files that have not been accessed for a time

Considerations When Enabling Last Access Time

  • atime update can be enabled only on Standard/Erasure Coding/Object Tiering volumes. It cannot be enabled on mirrored volumes. If you convert the mirror volume to a Read/Write volume, atime is disabled by default. You can enable atime with the volume modify command.
  • atime is applicable only for files. The atime of directories is NEVER updated.
  • While the read operation is audited, the atime operation is not audited as it is an internal operation.
  • The volume offload operation does not update atime but a file read from a backend/frontend volume updates atime.
  • The file recall operation also updates atime.
  • The file read operation on the EC/Tiered backend volume updates atime on the frontend volume.
  • At the time of mirroring, the atime update frequency (atimeUpdateInterval) is propagated from the source volume to the mirror volume. However, any subsequent changes made to this frequency on the source volume, are not automatically propagated to the mirror volume.
  • The time when you enabled atime updates (atimeTrackingStartTime) is updated to the current time in the following cases:
    • Just started tracking atime, which means that the atime update frequency was previously zero
    • If the value of atime update frequency is decreased
    • If the value of atime update frequency is increased and atime has not been tracked for the duration of the new frequency value

Exceptions to Last Access Time Updates

atime is never updated when:
  • Only the meta data of the file is being read
  • The file is read from the client cache
  • The file is read from a snapshot
  • The atimeUpdateInterval has not been exceeded:

    For example, assume that for a volume the atimeUpdateInterval is set to 1 day. A file is created at 11AM and the file is read at 10:55AM the next day. If the read finishes at 10:58AM, atime will not be updated as the atimeUpdateInterval did not cross a day.

    For another example, assume that for a volume the atimeUpdateInterval is set to 1day. A file is created at 11AM and the file is read at 10:55AM the next day. If the read completes at 11:10AM, the atime will still not be updated though the read completed after 24 hours, because read was triggered at 10:55AM. atime will only be updated when the file is next read.

Upgrade Considerations

When a cluster is upgraded to HPE Data Fabric 6.2, atime is not enabled on the old volumes. You need to enable atime manually using the volume modify command.

When a cluster along with a few clients are upgraded to HPE Data Fabric 6.2, while the remaining clients are not upgraded, the older clients can not update atime on files. Only the upgraded clients can trigger an atime update. However, the older clients can see the updated atime value (updated by the upgraded clients).

Enabling the Last Access Time Feature

The Last Access Time feature is not automatically enabled irrespective of whether you perform a fresh installation or an upgrade. To enable and activate the Last Access Time feature, run: 
maprcli cluster feature enable -name mfs.feature.update.atime

Enabling Last Access Time on Volumes

For performance reasons, the atime feature is disabled on volumes by default. You can enable atime updates at the volume level when creating or modifying volumes.

To set the frequency of atime updates, use the atimeUpdateInterval parameter when creating or modifying volumes. The value is in days. The default value of 0 indicates that atime is never updated.

For example, a value of 2 indicates that the atime is updated Once every 2 days (48 hours) with the first read on the file. atime will not be updated on further reads on the file till the 48 hours have passed.

Viewing the Last Access Time Value

To view the atime value of a specific volume, use the volume info command.

Last Access Time Example

The following command creates a volume and sets the atime to 2 days:
maprcli volume create -name stdvol1 -path /stdvol1 -atimeUpdateInterval 2d
To view the Last Access Time frequency, run:
maprcli volume info -name stdvol1 -json | grep atime
            "atimeUpdateInterval":"2",
            "atimeTrackingStartTime":"2021-03-14 22:45:25 GMT-0700",

Here, the frequency is set to 2 days. The time when atime was enabled on the volume is also displayed.