Scale2014

ZFS 101 (aka ZFS is Cool
and Why You Should be
Using It
Dru Lavigne
Documentation Lead, iXsystems
SCALE, February 23, 2014

Outline
Discuss ZFS features and describe the available
management utilities for the following FreeBSDbased operating systems:
- FreeNAS 9.2.1: open source NAS (Network
Attached Storage)
- PC-BSD 10.0: open source desktop (GUI) or
server (CLI)
Latest versions of these operating systems are on
par with the latest OpenZFS “feature flags”

History of ZFS
Modern filesystem specifically designed to add
features not available in traditional filesystems
Originally developed at Sun with the intent to
open source
After the Oracle acquisition, open source
development continued and the original engineers
founded OpenZFS (open-zfs.org) which is under
active development
OpenZFS uses feature flags instead of versions

What is ZFS?
128-bit COW (Copy on Write) filesystem and
logical volume manager with a maximum pool/file
size of 16 exabytes
In a traditional Unix filesystem, you need to define
the partition size and mount point at filesystem
creation time
In ZFS, you instead feed disks to a “pool” and
create filesystems from the pool as needed

Pool
Root (parent) volume which can be logically subdivided as needed
The number of disks added at a time is known as
a “vdev”
To optimize performance and resilvering time,
number of disks per vdev is limited
As more capacity is needed, add identical vdevs-these will be striped into the pool

RAIDZ
RAIDZ* levels designed to overcome hardware
RAID limitations such as the write-hole and
corrupt data written over time before the controller
provides an alert
Designed for commodity disks so no RAID
controller is needed
Can also be used with a RAID controller, but it
typically should be put into JBOD mode

RAIDZ1
Parity blocks are distributed across all disks
Up to one disk can fail per vdev without losing
pool
Pool can be lost if second disk in a vdev fails
before resilver completes
Optimized for vdev of 3, 5, or 9 disks

RAIDZ2
Double-parity solution similar to RAID6
Up to two disks can fail per vdev without losing
pool, with no restrictions on which disks can fail

RAIDZ3
Triple-parity solution
Up to three disks can fail per vdev without losing
pool, with no restrictions on which disks can fail

ZIL
ZFS Intent Log
Effectively a filesystem journal that stores sync
writes until they are committed to the pool
A dedicated SSD as a secondary log device
(SLOG) can increase synchronous write
performance, will have no effect on asynchronous
writes
FreeNAS includes the zilstat CLI utility to help
determine if system would benefit from a SLOG

ARC and L2ARC
ARC refers to read cache in RAM. Takes time for
ARC to populate with hits; if high misses continue
for cached reads, the system needs to be tuned.
Freenas adds ARC stats to top(1) and includes
arc_summary.py and arcstat.py tools for ARC
monitoring
Optional, secondary ARC can be installed on SSD
or disk in order to increase random read
performance. Always add as much RAM as
possible first.

Datasets
As needed, pool can be divided into additional,
dynamically sized filesystems known as datasets
Permissions and properties such as quotas and
compression can be set on a per-dataset level
A well thought out design can optimize storage for
the type of data being stored

Properties

Dozens of configurable properties such as: atime
(access time), canmount, compression, copies,
dedup, exec, quota, userquota, groupquota,
readonly, recordsize, reservation, setuid, etc.
Descriptions can be found at
http://www.freebsd.org/cgi/man.cgi?query=zfs

Adding Dataset During
PC-BSD Installation

Adding Dataset Using PC-BSD
Disk Manager

Zvols
Pool can also be divided into zvols
Essentially, a virtual, raw block device
Ideal for iSCSI device extents or for hosting
foreign file systems
Regardless of the filesytem the zvol is formatted
with by the iSCSI initiator, the underlying disk
blocks still benefit from all of the features provided
by ZFS

Snapshots
Provide low cost, instantaneous, read-only, pointin-time image of the specified pool, dataset, or
zvol
Snapshots can be recursive (atomic inclusion of
all child datasets)
Initial size is 0 bytes as COW, snapshot increases
in size as changes are written to disk
Can be replicated to another system

Create Snapshot on PC-BSD
Using Warden

Automating Snapshots on PC-BSD
Using Life Preserver

Snapshot Restore
In PC-BSD, the Life Preserver utility provides a
snapshot browser for finding and restoring copies
of earlier versions of files
It can also automate the replication of local
snapshots to another system or to a FreeNAS
system over SSH
A remote snapshot can be used to perform an
operating system restore from a PC-BSD install
media, should the system become unusable

Restoring Data from
a PC-BSD Snapshot

Restoring the OS From
a Remote Snapshot

Scrubs
ZFS was designed to be self-healing; it creates
and verifies checksums as data is written to disk
A scrub verifies the checksum in each disk block
and attempts to correct data as necessary
I/O intensive, so should be scheduled
appropriately
Reading the scrub results can provide an early
indication of possible disk failure

Scrubs
In FreeNAS, a scrub is automatically scheduled to
run every Sunday at midnight whenever a
pool/volume is created (this can be edited)
The results of the last scrub can be viewed from
Volume Status or by typing “zpool status”, and a
scrub can be started now from View Volumes
In PC-BSD, a scrub can be started from Disk
Manager or Life Preserver

Deduplication
ZFS property which avoids writing duplicate data
Can improve storage efficiency at the price of
performance—compression is often the better
choice
Dedup tables must fit into L2ARC, rule of thumb
is at least 5 GB RAM/L2ARC per TB of storage to
be deduplicated

PC-BSD Boot Environments
A snapshot of the dataset the operating system
resides on can be taken before an upgrade or a
system configuration change
This saved “boot environment” is automatically
added to the GRUB boot manager
Should the upgrade or configuration change fail,
simply reboot and select the previous boot
environment from the boot menu

Managing PC-BSD Boot
Environments

Additional Resources
PC-BSD Users Handbook: wiki.pcbsd.org
FreeNAS User Guide: doc.freenas.org
ZFS Best Practices Guide: http://ow.ly/oHtP3
Becoming a ZFS Ninja:
https://blogs.oracle.com/video/entry/becoming_a_
zfs_ninja

Questions

Contact:
dru@freebsd.org
URL to Slides:
http://slideshare.net/dlavigne/scale2014

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