If your Xsan 4 or 4.1 volume shows less available free space than you expect

If your free space is fragmented, you can alter the ABMFreeLimit variable to change how it displays.

When your Xsan volume free space is heavily fragmented, the cvlog contains entries like these when the volume activates:

[0919 21:17:51] 0xb8eb1000 (Warning) FSM Alloc: Stripe Group "Video-1" 140158 free blocks in 20272 fragments ignored.
[0919 21:17:53] 0xb8f33000 (Warning) FSM Alloc: Stripe Group "Video-2" 551704 free blocks in 77104 fragments ignored.
[0919 21:17:53] 0xb8fb5000 (Warning) FSM Alloc: Stripe Group "Video-3" 561540 free blocks in 78636 fragments ignored.
[0919 21:17:53] 0xb9037000 (Warning) FSM Alloc: Stripe Group "Video-4" 586677 free blocks in 80952 fragments ignored.
[0919 21:17:53] 0xb8e2f000 (Warning) FSM Alloc: Stripe Group "Video-5" 525412 free blocks in 73209 fragments ignored.
[0919 21:17:53] 0xb90b9000 (Warning) FSM Alloc: Stripe Group "Video-6" 280006 free blocks in 36786 fragments ignored.

The product of the Filesystem Block Size and the total ignored free blocks should equal the amount of missing space.

Change the ABMFreeLimit variable

You can set the Xsan volume's ABMFreeLimit variable to yes to restore missing space. This change instructs the File System Manager (FSM) to use an older method to process the Allocation Bit Map. It can also increase the volume startup time and amount of memory that the volume's FSM process uses. If you want to change the ABMFreeLimit variable, follow these steps, replacing VolumeName with the name of your Xsan volume.

1. In the volume configuration on your primary metadata controller (MDC), set true as the abmFreeLimit key's value.

sudo /usr/libexec/PlistBuddy -c 'add :config:globals:abmFreeLimit string true' /Library/Preferences/Xsan/VolumeName.cfgp

2. Update the Xsan configuration stored in LDAP.

sudo xsanctl pushConfigUpdate

3. Restart the Xsan volume.

sudo cvadmin -e 'stop VolumeName'
sudo cvadmin -e 'start VolumeName'
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