homersssearchNovember 18, 2018

ZFS Health Check and Status

monitor for degraded or over capacity volumes and disk errors

ZFS on FreeBSD is one of the best file systems we have ever used. But, once we setup the machine we asked the same question every other ZFS admin has asked themselves. How do we tell when there is a problem with our ZFS volumes? To monitor the ZFS filesystems we wrote the following shell script.

The ZFS Health shell script will use standard zpool arguments to look at the state of the volumes and drives. We currently check for the following default conditions:

If any problems are found an email is sent to "root" with the current status of the volumes. You can then take action as needed.

We usually try to avoid any graphical user interfaces (GUI) and appreciate the flexibility a command line program gives over a GUI solution. Using some simple shell scripting we can find out the health of the RAID, email ourselves about problems and work with failed drives.

The ZFS Health Check script

You are welcome to copy and paste the following script. We call the script "zfs_health.sh", but you can use any name you wish. Take a look at the section for "scrubExpire" and set the script to either use FreeBSD or Ubuntu's version of the date script format. We commented every method so take a look at the script before using it and make any changes you feel necessary for your environment.

#! /bin/sh
# Calomel.org
#     https://calomel.org/zfs_health_check_script.html
#     FreeBSD ZFS Health Check script
#     zfs_health.sh @ Version 0.18

# Check health of ZFS volumes and drives. On any faults send email.

# 99 problems but ZFS aint one

# Health - Check if all zfs volumes are in good condition. We are looking for
# any keyword signifying a degraded or broken array.

condition=$(/sbin/zpool status | egrep -i '(DEGRADED|FAULTED|OFFLINE|UNAVAIL|REMOVED|FAIL|DESTROYED|corrupt|cannot|unrecover)')
if [ "${condition}" ]; then
        emailSubject="`hostname` - ZFS pool - HEALTH fault"

# Capacity - Make sure the pool capacity is below 80% for best performance. The
# percentage really depends on how large your volume is. If you have a 128GB
# SSD then 80% is reasonable. If you have a 60TB raid-z2 array then you can
# probably set the warning closer to 95%.
# ZFS uses a copy-on-write scheme. The file system writes new data to
# sequential free blocks first and when the uberblock has been updated the new
# inode pointers become valid. This method is true only when the pool has
# enough free sequential blocks. If the pool is at capacity and space limited,
# ZFS will be have to randomly write blocks. This means ZFS can not create an
# optimal set of sequential writes and write performance is severely impacted.


if [ ${problems} -eq 0 ]; then
   capacity=$(/sbin/zpool list -H -o capacity | cut -d'%' -f1)
   for line in ${capacity}
       if [ $line -ge $maxCapacity ]; then
         emailSubject="`hostname` - ZFS pool - Capacity Exceeded"

# Errors - Check the columns for READ, WRITE and CKSUM (checksum) drive errors
# on all volumes and all drives using "zpool status". If any non-zero errors
# are reported an email will be sent out. You should then look to replace the
# faulty drive and run "zpool scrub" on the affected volume after resilvering.

if [ ${problems} -eq 0 ]; then
   errors=$(/sbin/zpool status | grep ONLINE | grep -v state | awk '{print $3 $4 $5}' | grep -v 000)
   if [ "${errors}" ]; then
        emailSubject="`hostname` - ZFS pool - Drive Errors"

# Scrub Expired - Check if all volumes have been scrubbed in at least the last
# 8 days. The general guide is to scrub volumes on desktop quality drives once
# a week and volumes on enterprise class drives once a month. You can always
# use cron to schedual "zpool scrub" in off hours. We scrub our volumes every
# Sunday morning for example.
# Scrubbing traverses all the data in the pool once and verifies all blocks can
# be read. Scrubbing proceeds as fast as the devices allows, though the
# priority of any I/O remains below that of normal calls. This operation might
# negatively impact performance, but the file system will remain usable and
# responsive while scrubbing occurs. To initiate an explicit scrub, use the
# "zpool scrub" command.
# The scrubExpire variable is in seconds. So for 8 days we calculate 8 days
# times 24 hours times 3600 seconds to equal 691200 seconds.


if [ ${problems} -eq 0 ]; then
   currentDate=$(date +%s)
   zfsVolumes=$(/sbin/zpool list -H -o name)

  for volume in ${zfsVolumes}
    if [ $(/sbin/zpool status $volume | egrep -c "none requested") -ge 1 ]; then
        printf "ERROR: You need to run \"zpool scrub $volume\" before this script can monitor the scrub expiration time."
    if [ $(/sbin/zpool status $volume | egrep -c "scrub in progress|resilver") -ge 1 ]; then

    ### Ubuntu with GNU supported date format
    #scrubRawDate=$(/sbin/zpool status $volume | grep scrub | awk '{print $11" "$12" " $13" " $14" "$15}')
    #scrubDate=$(date -d "$scrubRawDate" +%s)

    ### FreeBSD 11.2 with *nix supported date format
     scrubRawDate=$(/sbin/zpool status $volume | grep scrub | awk '{print $15 $12 $13}')
     scrubDate=$(date -j -f '%Y%b%e-%H%M%S' $scrubRawDate'-000000' +%s)

    ### FreeBSD 12.0 with *nix supported date format
    #scrubRawDate=$(/sbin/zpool status $volume | grep scrub | awk '{print $17 $14 $15}')
    #scrubDate=$(date -j -f '%Y%b%e-%H%M%S' $scrubRawDate'-000000' +%s)

     if [ $(($currentDate - $scrubDate)) -ge $scrubExpire ]; then
        emailSubject="`hostname` - ZFS pool - Scrub Time Expired. Scrub Needed on Volume(s)"

# Email - On any problems send email with drive status information and
# capacities including a helpful subject line. Also use logger to write the
# email subject to the local logs. This is also the place you may want to put
# any other notifications like playing a sound file, beeping the internal 
# speaker, paging someone or updating Nagios or even BigBrother.

if [ "$problems" -ne 0 ]; then
  printf '%s\n' "$emailSubject" "" "`/sbin/zpool list`" "" "`/sbin/zpool status`" | /usr/bin/mail -s "$emailSubject" root@localhost
  logger $emailSubject

### EOF ###

What does an email from the ZFS health script look like ?

The shell script will check the status of ZFS volumes, also called virtual drives, and if the volume is degraded or reporting errors an email is sent. The output of the email will show the capacity of the volumes fist and the full "zpool status" of all the volumes below. The email subject will give a clear indication as to the cause of the email report so you know what to look for.

For example, this email was sent out because the raid is fine, but the three(3) drive mirrored "tank" volume is at 97% capacity. By default the script is setup to send email if any volume is over 80% full. You can change the script to report on any upper limit you think is good for your environment.

Date: Wed, 04 Mar 2033 11:02:11 -0500
From: calomel@localhost
To: root@localhost
Subject: calomel.org - ZFS pool - Capacity Exceeded

calomel.org - ZFS pool - Capacity Exceeded

tank      3.6T    3.4T   108G    97%   1.00x   ONLINE   -
root      119G   14.1G   105G    11%   1.00x   ONLINE   -

  pool: tank
 state: ONLINE
  scan: scrub repaired 0 in 1h51m with 0 errors on Sun Mar  3 04:21:58 2033

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            ada3    ONLINE       0     0     0
            ada2    ONLINE       0     0     0
            ada1    ONLINE       0     0     0

errors: No known data errors

  pool: root
 state: ONLINE
  scan: scrub repaired 0 in 0h0m with 0 errors on Sun Mar  3 01:20:38 2033

        NAME         STATE     READ WRITE CKSUM
        root         ONLINE       0     0     0
         gpt/root    ONLINE       0     0     0

errors: No known data errors

Use cron to run the ZFS health check script

We run the script as an unprivileged user in a cron job. This way when the raid has an issue we get notification. The following cron job will run the script a few times a day when we know we should have time to respond to the error. As long as the raid is having an issue we will get emails. We see this as a reminder to check on the raid if it is not fixed in a reasonable amount of time. Cron will check the health of the ZFS volumes at 7am, 11am and 4pm every day.

#minute (0-59)
#|   hour (0-23)
#|   |     day of the month (1-31)
#|   |     |   month of the year (1-12 or Jan-Dec)
#|   |     |   |   day of the week (0-6 with 0=Sun or Sun-Sat)
#|   |     |   |   |   commands
#|   |     |   |   |   |
# zfs health check and report 
00 7,11,16 *   *   *   /home/calomel/zfs_health.sh

Want more speed out of FreeBSD ? Check out our FreeBSD Network Tuning guide where we enhance 1 gigabit and 10 gigabit network configurations.


Any tips to speed up and optimize ZFS performance ?

ZFS on FreeBSD 9.1 is incredibly fast by default. The internal auto optimization is efficient and the developers should be applauded for their work. For efficiency, we recommend installing at least 8 gigabytes of system memory to cache frequently requested data in RAM and then use the zfs-stats script from ports to monitor the ARC cache hit ratio. Spinning disks are very slow and if all the cached data does not fit in RAM then add a Samsung 840 Pro SSD drive as L2ARC cache or add a ZFS Intent Log (ZIL) drive to reduce spinning disk I/O. If your disk controller is too slow (we are looking at you on-board SATA ports) consider an LSI MegaRaid card. Our LSI MegaCLI Scripts and Commands guide has a ZFS setup at the bottom of the page showing our production ZFS RAID reading at 660MB/sec. Also make sure compression and deduplication are off. In any applications, like Nginx, disable sendfile and mmap to avoid redundant data caching.

That said, we do make some changes to ZFS. Set the checksum method to fletcher4, enable lzjb compression and turn access time stamps off (atime). The following will set these options on the "tank" zfs volume. Use "zfs get all tank" to verify.

zfs set atime=off tank
zfs set checksum=fletcher4 tank
zfs set compression=lzjb tank   ### for ZFS v5 pool v28
zfs set compression=lz4 tank    ### for ZFS v5 pool v5000

zpool set autoexpand=on tank
zpool set autoreplace=on tank
zpool set listsnapshots=on tank

We DO NOT EVER recommend turning off checksums. Checksums (i.e. checksum=fletcher4) are used for data to metadata integrity verification and self-healing (scrub, mirror, raidz, raid-z2, raid-z3) of the volume. If you are turning off checksums you will get a slight speed boost, but the integrity of your data is reduced substantially. For more tuning tips and insights check out the ZFS Evil Tuning Guide.

How can I make rolling ZFS snapshots ?

ZFS snapshots are read-only copies of a file system or volume. Snapshots are created in seconds and they initially consume no additional disk space within the pool. As data is added and deleted the active volume changes and the snapshot consumes disk space by continuing to reference the old backup copy of the data. ZFS snapshots are a very efficient way to make multiple time stamped copies of an entire file system without consuming double the space.

In FreeBSD ports or through packages you can install "zfSnap". zfSnap is a simple shell script to make rolling zfs snapshots using cron. The main advantage of zfSnap is it's written in pure /bin/sh so it doesn't require any additional software to run and all information about the snapshot is in the snapshot name itself.

Once you have zfSnap installed just setup a cron job as root. In this example we take a snapshot of the root file system and the tank raid array every Sunday at 6am. The snapshot is set to expire in one month. After a backup is made the script then deletes any snapshots which have since expired. The entire processes on both volumes normally completes in less then a second. Note you can also execute this same command before working on the system. Before upgrading packages we make a quick snapshot with an expiration time of 2 days (--2d) in case we make a mistake.

#minute (0-59)
#|   hour (0-23)
#|   |    day of the month (1-31)
#|   |    |   month of the year (1-12)
#|   |    |   |   day of the week (0-6 with 0=Sun)
#|   |    |   |   |   commands
#|   |    |   |   |   |
#### zfs snapshots
00   6    *   *   0   /usr/local/sbin/zfSnap -d -s -S -a 1m root tank

You can look at all the snapshots using zfs list. Here we see an example of the zfSnap file naming syntax. We have the name of the volume, "@" sign, the date/time and at the end is "1m" which stands for the expiration time of one month. It is not shown here, but after a month you should see four(4) complete snapshots of your data for each separate volume (root and tank).

calomel@RAID:  zfs list -t filesystem,snapshot
NAME                            USED  AVAIL  REFER  MOUNTPOINT
tank                           80.0G  17.7T  80.0G  /tank
tank@2033-03-03_06.00.00--1m       0      -  80.0G  -
root                           7.53G   286G  6.49G  /
root@2033-02-24_06.00.00--1m   2.01M      -  6.33G  -
root@2033-03-03_06.00.00--1m   2.36M      -  6.49G  -

That is about it. You now have a cron job creating rolling snapshots of your system. If you have a problem where a file is deleted or missing you can go to your snapshot to recover the file. zfSnap is simple insurance against data loss.

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