zfs - reboot after panic

[Kiriakos Georgiou]

Hello,

I've been happily running a FreeBSD 10.1 fileserver (updated regularly via freebsd-update) with ZFS zvols served out via iscsi.  Total RAM is 8G. For 2-3 months I regularly successfully run some heavy duty checksuming via unison (it's kind of like rsync, but does bidirectional syncing).  I recently updated the OS to 10.1-RELEASE-p26, and soon after I started to consistently get kernel panics when I run unison to sync certain files across my VMs:

reboot after panic: I/O to pool 'zdata' appears to be hung on vdev guid ....

First I thought a bug must have been introduced, but after some googling I set my vfs.zfs.arc_max to 4G (50% of my ram) and hammered zfs via a massive unison sync.  I monitored memory via top.  ARC grew quickly to 4096M and stayed there.  Free memory would slowly drop all the way to just under 1G but then it'd suddenly jump to over 2G, then slowly go down to 1G and jump to 2G again so on.  I suspect it was a RAM starvation issue that is now resolved by tuning the ARC max.  I've no idea why it started happening.  I just wanted to share it with you.  Does the "fix" make sense?

Thanks,
Kiriakos