Extreme ZFS slowdowns after several months

I've got a general-purpose server, providing mail, DNS, web, databases, and some other services for a number of users.

It's got a Xeon E3-1275 at 3.40 GHz, 16 GB ECC RAM. Running Linux kernel 4.2.3, with ZFS-on-Linux 0.6.5.3.

The disk layout is 2x Seagate ST32000641AS 2 TB drives and 1x Samsung 840 Pro 256 GB SSD

I've got the 2 HDs in a RAID-1 mirror, and the SSD is acting as a cache and log device, all managed in ZFS.

When I first set up the system, it was amazingly fast. No real benchmarks, just… fast.

Now, I notice extreme slowdowns, especially on the filesystem holding all of the maildirs. Doing a nightly backup takes over 90 minutes for a mere 46 GB of mail. Sometimes, the backup causes such an extreme load that the system is nearly unresponsive for up to 6 hours.

I've run zpool iostat zroot (my pool is named zroot) during these slowdowns, and seen writes on the order of 100-200kbytes/sec. There are no obvious IO errors, the disk doesn't seem to be working particularly hard, but read is almost unusably slow.

The strange thing is that I had the exact same experience on a different machine, with similar spec hardware, though no SSD, running FreeBSD. It worked fine for months, then got slow in the same way.

My going suspicion is this: I use zfs-auto-snapshot to create rolling snapshots of each filesystem. It creates 15-minute, hourly, daily, and monthly snapshots, and keeps a certain number of each around, deleting the oldest. It means that over time, thousands of snapshots have been created and destroyed on each filesystem. It's the only ongoing filesystem-level operation that I can think of with a cumulative effect. I've tried destroying all of the snapshots (but kept the process running, creating new ones), and noticed no change.

Is there a problem with constantly creating and destroying snapshots? I find having them an extremely valuable tool, and have been led to believe that they are (aside from disk space) more or less zero-cost.

Is there something else that may be causing this problem?

EDIT: command output

Output of zpool list:

NAME    SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
zroot  1.81T   282G  1.54T         -    22%    15%  1.00x  ONLINE  -

Output of zfs list:

NAME             USED  AVAIL  REFER  MOUNTPOINT
zroot            282G  1.48T  3.55G  /
zroot/abs       18.4M  1.48T  18.4M  /var/abs
zroot/bkup      6.33G  1.48T  1.07G  /bkup
zroot/home       126G  1.48T   121G  /home
zroot/incoming  43.1G  1.48T  38.4G  /incoming
zroot/mail      49.1G  1.48T  45.3G  /mail
zroot/mailman   2.01G  1.48T  1.66G  /var/lib/mailman
zroot/moin       180M  1.48T   113M  /usr/share/moin
zroot/mysql     21.7G  1.48T  16.1G  /var/lib/mysql
zroot/postgres  9.11G  1.48T  1.06G  /var/lib/postgres
zroot/site       126M  1.48T   125M  /site
zroot/var       17.6G  1.48T  2.97G  legacy

This is not a very busy system, in general. Peaks on the graph below are nightly backups:

I've managed to catch the system during a slowdown (starting around 8 this morning). Some operations are fairly responsive, but the load average is currently 145, and zpool list just hangs. Graph: