Monitoring Storage Volumes

Volumes: Introduction

System Monitor can monitor the current usage of storage space on disk volumes (file systems, partitions) currently connected to your computer. The total amount of storage space used by selected volumes is displayed in the menu bar in this case.

If you like to monitor used storage space of the system, perform the following steps:

1. Ensure the control window of System Monitor is open.
2. Select the item Information Sources > Volumes in the sidebar.
3. Make sure the check mark Monitor this item is set at the right hand side of the window.

Only those disk volumes will become part of the statistics which you have selected for monitoring (see below).

The data shown is calculated live and updated about once per minute. The disks will not be checked more often in order not to thwart transfers of actual user data. In addition, System Monitor respects your personal energy saver settings for hard drives. When the program has to assume that drives are currently in sleep mode, it will automatically avoid retrieving usage data for volumes, in order not to wake up the drives. If you open this menu purposefully, however, all affected disks will be woken up in order to display up-to-date information.

Different definitions of free and used space

System Monitor is capable of working with different definitions for free and used storage space. You can switch between these different display modes using the pop-up button at Definition of “free”.

• True physical storage space is the standard mode of operation which is automatically used on older operating systems. The amount of free storage space shown by System Monitor is the actual amount directly available on the file system. The value will match the number of free bytes shown by Apple’s Disk Utility.
• Space granted to applications is usually less than the true free space. The number of bytes considered free in this mode is the amount of storage space that “well-behaved” apps running on the graphical user interface can typically access. macOS does not offer all of the system’s true free space to apps in order to keep a final reserve for itself and for emergency purposes. Otherwise, the operating system could crash when it runs out of free space on the start partition.
• Space after purge operation considers storage space that is seen by the operating system as being purgeable. When there is really critical need for storage space, macOS can decide to remove files which are known to be replaceable. Examples for replaceable files are local Time Machine snapshots (which can be kept in addition to the true Time Machine backups as additional service for cases where your backup device is offline), or iTunes movies you have purchased and already seen, so they could be downloaded later again if necessary.

The definition of “free” that you choose here becomes effective for all volumes shown by System Monitor in the menu bar and menu items. Note that the values for used space will change accordingly because the true amount of storage will never change of course.

If you enable the status bar of the macOS Finder, the amount of free space shown in these bars of Finder windows will match the definition space after purge operation. In some dialogs, Apple differentiates between the terms free space and available space.

Volumes: Menu Bar

The box Menu Bar controls in which form storage space should be displayed. System Monitor offers a variety of icons and functions that you can combine in any order. The following elements can be used:

• The label VOL (volume) in small, vertical print to designate storage values and to separate them from other icons in the menu bar.
• The label VOL in large, horizontal print.
• The display of used (U) and free (F) storage space as plain text in two lines using small print.
• The display of used storage space as text in percent.
• The display of used storage space as bar chart.
• The display of used storage space as pie chart.
• A spacer to put more room between items.

Within the box, the upper bar simulates as a sample how this monitor section should be presented in the menu bar. You can grab the individual items by mouse and move them into a different order. You can remove an element by pulling it out of the bar. The lower bar contains the stock of items available. If you like to add a certain item to the menu bar, use the mouse to drag it from the lower to the upper bar. Pressing the button Default causes your current settings to be deleted, replacing it by a default suggestion for the menu bar.

Spacers are shown with dotted rectangles to make it easier for you to recognize and to move them. In the actual display, the rectangle will become invisible, so the spacer does its job providing a blank area within the menu bar.

If you mistakenly have removed all items of this monitoring section from the menu bar, a three-star-icon (⁂, asterism) will appear as a placeholder. This ensures that you still can open the associated menu.

You cannot drag items directly into the real menu bar.

Volumes: Menu Items

At least the following items will be shown for each selected volume as part of the menu associated with this monitor section:

• The name of the file system or, more exactly, its Macintosh volume name.
• The current value of used storage space in bytes.
• The current value of free storage space in bytes.
• A colored bar, indicating the degree of usage.

A check mark next to the name of a volume indicates that its storage space has been considered in the total sum shown in the menu bar. If the check mark is missing, the Menu only option (see below) has been activated.

Selecting the volumes to monitor

The table Include the following volumes allows you to control which file systems should be monitored by the app and which shouldn’t. Set a check mark next to each volume which should be monitored and remove the check marks for volumes where no monitoring should be performed. The selection can also be applied to all entries automatically, using the buttons All or None. The lines in the table can be dragged to a different sort order if desired. This order will then be used for the corresponding sections in the menu.

Note: To avoid confusion, volumes which are hidden in the Finder are not offered for display by System Monitor either. Volumes are hidden if macOS mounts them with the “no browse” option.

If you set the check mark Menu only for a volume, it will still be monitored and appears in the open menu, but will no longer be included for the grand total shown in the menu bar. This is helpful to exclude Time Machine backup disks in APFS format, for example. They are cleaned automatically and cannot store other data, so they are not of interest for storage statistics.

External and ejectable volumes will also appear in the table automatically. If you like to monitor a disk which is not always connected to your computer, this will be possible. Perform the following steps:

1. Ensure that the volume to monitor is connected to your computer.
2. Check the corresponding entry in the table.

If this volume is being unmounted later, it will be shown using gray print in the table and crossed out in the menu. It will keep its position among the other listed volumes. The monitoring will automatically continue as soon as the volume becomes available again.

In order to make sure that neither you nor System Monitor confuse disk volumes, you should take care that all volumes used have different names.

Disk volumes for which the feature Ignore ownership on this volume is enabled in the Finder, are usually considered “temporarily connected only” both by macOS and System Monitor. The corresponding entries disappear from the configuration table when ejecting the disk. If storage monitoring had been activated in System Monitor, but should be disabled, you’ll have to connect the volume momentarily again.

Disk volumes which are only virtual APFS snapshots are usually used only temporarily either, with exception of the sealed operating system volume. Such volumes won’t be offered in the table.

Understanding storage space sharing used by volumes in APFS containers

If volumes uses the modern APFS (Apple File System) instead of the classic HFS+ (Mac OS Extended), this will be indicated by an additional marker APFS at the right hand side of the table. APFS supports advanced features where several volumes can share each others storage space. This may affect your choice which volumes you would like to monitor.

To see how APFS volumes are actually organized on your computer, click the button APFS overview below the table. An additional window will show you the hierarchy of APFS objects found on your Mac.

• APFS containers are the physical sections on disk drives or SSDs that mark the “zones” on storage media where APFS is active.
• Physical disks are one level below in the hierarchy, because an APFS container can be spread onto multiple physical storage units. In the default case, an APFS container is located on a single disk. However, it may also use multiple disks of a software RAID system, or it could be placed onto a Fusion drive, a composite of an SSD with a mechanical disk.
• APFS volume groups are an option to collect multiple volumes within the same container into a single unit. APFS volume groups are capable of providing a feature called firmlink which means that a file can appear multiple times on volumes of the same group, but it is actually only stored once.
• APFS volumes appear as separate entities that simulate classic disk drives. APFS volumes don’t need partitions. They can be added or removed at runtime, without stopping the operating system. Volumes within the same container share the same physical storage space, so each volume has virtually its entire container available. This means however, that the same used or free space may be counted multiple times. So a container of 1 TB that hosts 4 volumes provides virtually 4 TB, although only 1 TB is actually available. To avoid concurrency between volumes of the same container, it is possible for a volume to define a reserved space, a minimum of physical storage that is guaranteed to always be available for that volume, or a quota, a maximum of physical space that may be consumed even if more is available in the container.