1. Introduction — FreeNAS User Guide 9.3.1 Table of Contents

FreeNAS® is © 2011-2015 iXsystems

FreeNAS® and the FreeNAS® logo are registered trademarks of iXsystems.

FreeBSD is a registered trademark of the FreeBSD Foundation

Written by users of the FreeNAS® network-attached storage operating system.

Version 9.3.1

Copyright © 2011-2015 iXsystems

This Guide covers the installation and use of FreeNAS® 9.3.1.

The FreeNAS® Users Guide is a work in progress and relies on the contributions of many individuals. If you are interested in helping us to improve the Guide, read the instructions in the README. If you use IRC Freenode, you are welcome to join the #freenas channel where you will find other FreeNAS® users.

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Typographic Conventions

The FreeNAS® 9.3.1 Users Guide uses the following typographic conventions:

- Names of graphical elements such as buttons, icons, fields, columns, and boxes are enclosed within quotes. For example: click the “Import CA” button.
- Menu selections are italicized and separated by arrows. For example: System ‣ Information.
- Commands that are mentioned within text are highlighted in bold text. Command examples and command output are contained in green code blocks.
- Volume, dataset, and file names are enclosed in a blue box `/like/this`.
- Keystrokes are formatted in a blue box. For example: press `Enter`.
- bold text: used to emphasize an important point.
- italic text: used to represent device names or text that is input into a GUI field.

# 1. Introduction

FreeNAS® is an embedded open source network-attached storage (NAS) operating system based on FreeBSD and released under a BSD license. A NAS is an operating system that has been optimized for file storage and sharing.

FreeNAS® provides a browser-based, graphical configuration interface. Its built-in networking protocols can be configured to provide storage access to a wide range of operating systems. A plugins system is provided for extending the built-in features by installing additional software.

## 1.1. What’s New Since 9.3-RELEASE

Beginning with version 9.3, FreeNAS® uses a “rolling release” model instead of point releases. The new Update mechanism makes it easy to keep up-to-date with the latest security fixes, bug fixes, and new features. Some updates affect the user interface so this section lists any functional changes that have occurred since 9.3-RELEASE.

Note

the screenshots in this documentation assume that your system is fully updated to the latest STABLE version of FreeNAS® 9.3.1. If a screen on your system looks different than the documentation, make sure that the system is fully up-to-date and apply any outstanding updates if it is not.

- Samba was updated to 4.1.18.
- Netatalk was updated to 3.1.7.
- SSSD was updated to 1.11.7.
- Nut has been updated to 2.7.3 which adds support for several new devices, including the Tripp Lite SMART500RT1U UPS.
- The driver for the Intel X710 10GbE adapter was added.
- The 6Gbps Avago (LSI) HBA driver, mps(4), has been updated to version 20 and an Alert will be issued if there is a version mismatch.
- The 12Gbps Avago (LSI) HBA driver, mpr(4), has been updated to version 9 and an Alert will be issued if there is a version mismatch. The sas3ircu command line utility has also been added. This tool is similar in functionality to the sas2ircu tool which is for MegaRAID HBAs using the mps(4) driver.
- The mrsas(4) Avago MegaRAID driver was added.
- Support for the Mach Xtreme MX-ES/MXUB3 and the Kingston DT100G2 USB drives has been added.
- Support for Avago MegaRAID SAS passthrough has been added.
- Man pages have been added and can be accessed from Shell.
- LZ4 compression is used on the boot pool in order to increase space for boot environments.
- Support for hot spare drive replacement has been added. If you have spare drives in your pool, and a drive fails, FreeNAS® should automatically remove the failed drive from the pool and replace it with the spare.
- An installation of STABLE, as of 201501212031, now creates two boot environments. The system will boot into the default boot environment and users can make their changes and update from this version. The other boot environment, named Initial-Install can be booted into if the system needs to be returned to a pristine, non-configured version of the installation.
- The “Create backup” and “Restore from a backup” options have been added to the FreeNAS® console setup menu shown in Figure 3a.
- The “Microsoft Account” checkbox has been added to Account ‣ Users ‣ Add User.
- The ability to set the boot pool scrub interval has been added to System ‣ Boot.
- The size of and the amount of used space in the boot pool is displayed in System ‣ Boot.
- The “Enable automatic upload of kernel crash dumps and daily telemetry” checkbox has been added to System ‣ Advanced.
- A “Backup” button has been added to System ‣ Advanced.
- The “Periodic Notification User” drop-down menu has been added to System ‣ Advanced.
- The “Performance Test” button has been removed from System ‣ Advanced.
- The system will issue an alert if an update fails and the details of the failure will be written to `/data/update.failed`.
- The “Confirm Passphrase” field has been added to System ‣ CAs ‣ Import CA and System ‣ Certificates ‣ Import Certificate.
- The “Support” tab has been added to System ‣ Support, providing a convenient method for reporting a bug or requesting a new feature.
- The “Rsync Create” checkbox has been renamed to “Validate Remote Path” and the “Delay Updates” checkbox has been added to Tasks ‣ Rsync Tasks ‣ Add Rsync Task.
- The “VLAN ID” field has been added to Network ‣ IPMI.
- A reboot is no longer required when creating Link Aggregations.
- The “Recursively replicate and remove stale snapshot on remote side” option in Storage ‣ Replication Tasks ‣ Add Replication has been divided into two options: “Recursively replicate child dataset’s snapshots” and “Delete snapshots that are no longer available locally after successful replication of new snapshots”.
- The “Initialize remote side for once” option has been removed from Storage ‣ Replication Tasks ‣ Add Replication as the new replication backend handles this automatically when needed.
- The `/usr/local/bin/test_ssh.py` script has been added for testing the SSH connection for a defined replication task.
- The “Encryption Mode” and “Certificate” drop-down menus and the “Allow DNS updates” checkbox have been added to Directory Service ‣ Active Directory.
- A pop-up warning will appear if you go to change Directory Service ‣ Active Directory ‣ Advanced Mode -> Idmap backend as selecting the wrong backend will break Active Directory integration.
- The “Schema” drop-down menu has been added to Directory Service ‣ LDAP.
- The “Kerberos Settings” tab as been added to Directory Service.
- The ability to “Online” a previously offlined disk has been added to Storage ‣ Volumes ‣ Volume Status.
- The “Security” field of Sharing ‣ UNIX (NFS) ‣ Add Unix (NFS) Share ‣ Advanced Mode now only appears when the “Enable NFSv4” checkbox is checked in | Services ‣ NFS.
- The “Periodic Snapshot Task” drop-down menu has been added to Sharing ‣ Windows (CIFS) ‣ Add Windows (CIFS) Share.
- All available VFS objects have been added to Sharing ‣ Windows (CIFS) ‣ Add Windows (CIFS) Share ‣ Advanced Mode ‣ VFS Objects and the “aio_pthread” and “streams_xattr” VFS objects are enabled by default.
- The “Pool Available Size Threshold” field has been renamed to “Pool Available Space Threshold” in Sharing ‣ Block (iSCSI) ‣ Target Global Configuration.
- The “Discovery Auth Method” and “Discovery Auth Group” fields have moved from Sharing ‣ Block (iSCSI) ‣ Target Global Configuration to Sharing ‣ Block (iSCSI) ‣ Portals ‣ Add Portal.
- The “Logical Block Size” field has been moved from Sharing ‣ Block (iSCSI) ‣ Targets ‣ Add Target to Sharing ‣ Block (iSCSI) ‣ Extents ‣ Add Extent.
- The “Serial” field has been moved from Sharing ‣ Block (iSCSI) ‣ Targets ‣ Add Target to Sharing ‣ Block (iSCSI) ‣ Extents ‣ Add Extent.
- The Sharing ‣ Block (iSCSI) ‣ Targets ‣ Add Target screen now supports the creation of multiple iSCSI groups.
- The “Disable Physical Block Size Reporting” and the “Read-only” checkboxes, the “Available Space Threshold” field, and the “LUN RPM” drop-down menu have been added to Sharing ‣ Block (iSCSI) ‣ Extents ‣ Add Extent.
- The “Home share name” field has been added to Services ‣ AFP.
- The “DNS Backend” field has been removed from Services ‣ Domain Controller as BIND is not included in FreeNAS®.
- The “Require Kerberos for NFSv4” checkbox has been added to Services ‣ NFS.
- The “SNMP v3 Support” checkbox, “Username”, “Password”, and “Privacy Passphrase” fields, and “Authentication Type” and “Privacy Protocol” drop-down menus have been added to Services ‣ SNMP so that SNMPv3 can be configured.
- The “Allow Kerberos Authentication” checkbox has been added to Services ‣ SSH.
- The “Host Private Key” has been removed from Services ‣ SSH.
- The “Power Off UPS” checkbox had been added to Services ‣ UPS.
- The MediaBrowser Plugin has been renamed to Emby.
- The Jails ‣ Add Jails button has been renamed to “Add Jail”.
- A “Restart” button is now available when you click the entry for an installed jail.
- The “Mtree” field and “Read-only” checkbox have been added to Jails ‣ Templates ‣ Add Jail Templates.
- The “Mtree” field has been added to the “Edit” options for existing jail templates.
- The -C, -D and -j options have been added to freenas-debug.
- A Support Icon has been added to the top menubar, providing a convenient method for reporting a bug or requesting a new feature.
- The “Help” icon has been replaced by the Guide icon, providing an offline version of the FreeNAS® User Guide (this documentation).
- A warning message now occurs if you stop the iSCSI service when initiators are connected. Type ctladm islist to determine the names of the connected initiators.
- An alert will be generated when a new update becomes available.
- An alert will be generated when a S.M.A.R.T. error occurs.
- An alert will be generated if a Certificate Authority or certificate is invalid or malformed.
- The zfslower.d DTrace script has been added. This script is useful for determining the cause of latency, where a reasonable latency might be 10 ms. If you run dtrace -s zfslower.d 10, it will display all ZFS operations that take longer than 10ms. If no ZFS operations take longer than 10ms but the client is experiencing latency, you know it is not a filesystem issue.

## 1.2. Hardware Recommendations

Since FreeNAS® 9.3.1 is based on FreeBSD 9.3, it supports the same hardware found in the FreeBSD Hardware Compatibility List. Supported processors are listed in section 2.1 amd64. Beginning with version 9.3, FreeNAS® is only available for 64-bit (also known as amd64) processors.

Note

beginning with version 9.3, FreeNAS® boots from a GPT partition. This means that the system BIOS must be able to boot using either the legacy BIOS firmware interface or EFI.

Actual hardware requirements will vary depending upon what you are using your FreeNAS® system for. This section provides some guidelines to get you started. You can also skim through the FreeNAS® Hardware Forum for performance tips from other FreeNAS® users or to post questions regarding the hardware best suited to meet your requirements. This forum post provides some specific recommendations if you are planning on purchasing hardware. Refer to Building, Burn-In, and Testing your FreeNAS system for detailed instructions on how to test new hardware.

### 1.2.1. RAM

The best way to get the most out of your FreeNAS® system is to install as much RAM as possible. The recommended minimum is 8 GB of RAM. The more RAM, the better the performance, and the FreeNAS® Forums provide anecdotal evidence from users on how much performance is gained by adding more RAM.

Depending upon your use case, your system may require more RAM. Here are some general rules of thumb:

- If you plan to use ZFS deduplication, ensure you have at least 5 GB RAM per TB of storage to be deduplicated.
- If you plan to use Active Directory with a lot of users, add an additional 2 GB of RAM for winbind’s internal cache.
- If you plan on Using the phpVirtualBox Template, increase the minimum RAM size by the amount of virtual memory you configure for the virtual machines. For example, if you plan to install two virtual machines, each with 4GB of virtual memory, the system will need at least 16GB of RAM.
- If you plan to use iSCSI, install at least 16GB of RAM, if performance is not critical, or at least 32GB of RAM if performance is a requirement.
- If you are installing FreeNAS® on a headless system, disable the shared memory settings for the video card in the BIOS.

If your system supports it and your budget allows for it, install ECC RAM. While more expensive, ECC RAM is highly recommended as it prevents in-flight corruption of data before the error-correcting properties of ZFS come into play, thus providing consistency for the checksumming and parity calculations performed by ZFS. If you consider your data to be important, use ECC RAM. This Case Study describes the risks associated with memory corruption.

If you don’t have at least 8GB of RAM, you should consider getting more powerful hardware before using FreeNAS® to store your data. Plenty of users expect FreeNAS® to function with less than these requirements, just at reduced performance. The bottom line is that these minimums are based on the feedback of many users. Users that do not meet these requirements and who ask for help in the forums or IRC will likely be ignored because of the abundance of information that FreeNAS® may not behave properly with less than 8GB of RAM.

### 1.2.2. Compact or USB Flash

The FreeNAS® operating system is installed to at least one device that is separate from the storage disks. The device can be a USB stick, compact flash, or SSD. Technically, it can also be installed onto a hard drive, but this is discouraged as that drive will then become unavailable for data storage.

Note

if you will be burning the installation file to a USB stick, you will need two USB slots, each with an inserted USB device, where one USB stick contains the installer and the other USB stick is selected to install into. When performing the installation, be sure to select the correct USB device to install to. In other words, you can not install FreeNAS® into the same USB stick that you boot the installer from. After installation, remove the USB stick containing the installer, and if necessary, configure the BIOS to boot from the remaining USB stick.

When determining the type and size of device to install the operating system to, keep the following points in mind:

- the bare minimum size is 4GB. This provides room for the operating system and two boot environments. Since each update creates a boot environment, the recommended minimum is at least 8GB or 16GB as this provides room for more boot environments.
- if you plan to make your own boot environments, budget about 1GB of storage per boot environment. Consider deleting older boot environments once you are sure that a boot environment is no longer needed. Boot environments can be created and deleted using System ‣ Boot.
- when using a USB stick, it is recommended to use a name brand USB stick as ZFS will quickly find errors on cheap, not well made sticks.
- when using a USB stick, USB 3.0 support is disabled by default as it currently is not compatible with some hardware, including Haswell (Lynx point) chipsets. If you receive a “failed with error 19” message when trying to boot FreeNAS®, make sure that xHCI/USB3 is disabled in the system BIOS. While this will downclock the USB ports to 2.0, the bootup and shutdown times will not be significantly different. To see if USB 3.0 support works with your hardware, follow the instructions in Tunables to create a “Tunable” named xhci_load, set its value to YES, and reboot the system.
- if a reliable boot disk is required, use two identical devices and select them both during the installation. Doing so will create a mirrored boot device.

### 1.2.3. Storage Disks and Controllers

The Disk section of the FreeBSD Hardware List lists the supported disk controllers. In addition, support for 3ware 6gbps RAID controllers has been added along with the CLI utility tw_cli for managing 3ware RAID controllers.

FreeNAS® supports hot pluggable drives. To use this feature, make sure that AHCI is enabled in the BIOS.

If you need reliable disk alerting and immediate reporting of a failed drive, use an HBA such as an Avago MegaRAID controller or a 3Ware twa-compatible controller.

Suggestions for testing disks before adding them to a RAID array can be found in this forum post.

This article provides a good overview of hard drives which are well suited for a NAS.

If you have some money to spend and wish to optimize your disk subsystem, consider your read/write needs, your budget, and your RAID requirements:

- If you have steady, non-contiguous writes, use disks with low seek times. Examples are 10K or 15K SAS drives which cost about $1/GB. An example configuration would be six 600 GB 15K SAS drives in a RAID 10 which would yield 1.8 TB of usable space or eight 600 GB 15K SAS drives in a RAID 10 which would yield 2.4 TB of usable space.
- 7200 RPM SATA disks are designed for single-user sequential I/O and are not a good choice for multi-user writes.

If you have the budget and high performance is a key requirement, consider a Fusion-I/O card which is optimized for massive random access. These cards are expensive and are suited for high-end systems that demand performance. A Fusion-I/O card can be formatted with a filesystem and used as direct storage; when used this way, it does not have the write issues typically associated with a flash device. A Fusion-I/O card can also be used as a cache device when your ZFS dataset size is bigger than your RAM. Due to the increased throughput, systems running these cards typically use multiple 10 GigE network interfaces.

If you will be using ZFS, Disk Space Requirements for ZFS Storage Pools recommends a minimum of 16 GB of disk space. Due to the way that ZFS creates swap, you can not format less than 3 GB of space with ZFS. However, on a drive that is below the minimum recommended size you lose a fair amount of storage space to swap: for example, on a 4 GB drive, 2 GB will be reserved for swap.

If you are new to ZFS and are purchasing hardware, read through ZFS Storage Pools Recommendations first.

ZFS uses dynamic block sizing, meaning that it is capable of striping different sized disks. However, if you care about performance, use disks of the same size. Further, when creating a RAIDZ*, only the size of the smallest disk will be used on each disk.

### 1.2.4. Network Interfaces

The Ethernet section of the FreeBSD Hardware Notes indicates which interfaces are supported by each driver. While many interfaces are supported, FreeNAS® users have seen the best performance from Intel and Chelsio interfaces, so consider these brands if you are purchasing a new NIC. Realteks will perform poorly under CPU load as interfaces with these chipsets do not provide their own processors.

At a minimum, a GigE interface is recommended. While GigE interfaces and switches are affordable for home use, modern disks can easily saturate 110 MB/s. If you require higher network throughput, you can bond multiple GigE cards together using the LACP type of Link Aggregations. However, the switch will need to support LACP which means you will need a more expensive managed switch.

If network performance is a requirement and you have some money to spend, use 10 GigE interfaces and a managed switch. If you are purchasing a managed switch, consider one that supports LACP and jumbo frames as both can be used to increase network throughput. Refer to the 10 Gig Networking Primer for more information.

Note

at this time the following are not supported: InfiniBand, FibreChannel over Ethernet, or wireless interfaces.

If network speed is a requirement, consider both your hardware and the type of shares that you create. On the same hardware, CIFS will be slower than FTP or NFS as Samba is single-threaded. If you will be using CIFS, use a fast CPU.

Wake on LAN (WOL) support is dependent upon the FreeBSD driver for the interface. If the driver supports WOL, it can be enabled using ifconfig(8). To determine if WOL is supported on a particular interface, specify the interface name to the following command. In this example, the capabilities line indicates that WOL is supported for the re0 interface:

    ifconfig -m re0
    re0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
    options=42098<VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,WOL_MAGIC,VLAN_HWTSO>
    capabilities=5399b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,WOL_UCAST,WOL_MCAST, WOL_MAGIC,VLAN_HWFILTER,VLAN_H WTSO>

If you find that WOL support is indicated but not working for a particular interface, create a bug report using the instructions in Support.

orphan: |

## 1.3. ZFS Primer

ZFS is an advanced, modern filesystem that was specifically designed to provide features not available in traditional UNIX filesystems. It was originally developed at Sun with the intent to open source the filesystem so that it could be ported to other operating systems. After the Oracle acquisition of Sun, some of the original ZFS engineers founded OpenZFS in order to provided continued, collaborative development of the open source version. To differentiate itself from Oracle ZFS version numbers, OpenZFS uses feature flags. Feature flags are used to tag features with unique names in order to provide portability between OpenZFS implementations running on different platforms, as long as all of the feature flags enabled on the ZFS pool are supported by both platforms. FreeNAS® uses OpenZFS and each new version of FreeNAS® keeps up-to-date with the latest feature flags and OpenZFS bug fixes.

Here is an overview of the features provided by ZFS:

ZFS is a transactional, Copy-On-Write (COW) filesystem. For each write request, a copy is made of the associated disk block(s) and all changes are made to the copy rather than to the original block(s). Once the write is complete, all block pointers are changed to point to the new copy. This means that ZFS always writes to free space and most writes will be sequential. When ZFS has direct access to disks, it will bundle multiple read and write requests into transactions; most filesystems can not do this as they only have access to disk blocks. A transaction either completes or fails, meaning there will never be a write-hole and a filesystem checker utility is not necessary. Because of the transactional design, as additional storage capacity is added it becomes immediately available for writes; to rebalance the data, one can copy it to re-write the existing data across all available disks. As a 128-bit filesystem, the maximum filesystem or file size is 16 exabytes.

ZFS was designed to be a self-healing filesystem. As ZFS writes data, it creates a checksum for each disk block it writes. As ZFS reads data, it validates the checksum for each disk block it reads. If ZFS identifies a disk block checksum error on a pool that is mirrored or uses RAIDZ*, ZFS will fix the corrupted data with the correct data. Since some disk blocks are rarely read, regular scrubs should be scheduled so that ZFS can read all of the data blocks in order to validate their checksums and correct any corrupted blocks. While multiple disks are required in order to provide redundancy and data correction, ZFS will still provide data corruption detection to a system with one disk. FreeNAS® automatically schedules a monthly scrub for each ZFS pool and the results of the scrub will be displayed in View Volumes. Reading the scrub results can provide an early indication of possible disk failure.

Unlike traditional UNIX filesystems, you do not need to define partition sizes at filesystem creation time. Instead, you feed a certain number of disk(s) at a time (known as a vdev) to a ZFS pool and create filesystems from the pool as needed. As more capacity is needed, identical vdevs can be striped into the pool. In FreeNAS®, Volume Manager can be used to create or extend ZFS pools. Once a pool is created, it can be divided into dynamically-sized datasets or fixed-size zvols as needed. Datasets can be used to optimize storage for the type of data being stored as permissions and properties such as quotas and compression can be set on a per-dataset level. A zvol is essentially a raw, virtual block device which can be used for applications that need raw-device semantics such as iSCSI device extents.

ZFS supports real-time data compression. Compression happens when a block is written to disk, but only if the written data will benefit from compression. When a compressed block is accessed, it is automatically decompressed. Since compression happens at the block level, not the file level, it is transparent to any applications accessing the compressed data. By default, ZFS pools made using FreeNAS® version 9.2.1 or later will use the recommended LZ4 compression algorithm.

ZFS provides low-cost, instantaneous snapshots of the specified pool, dataset, or zvol. Due to COW, the initial size of a snapshot is 0 bytes and the size of the snapshot increases over time as changes to the files in the snapshot are written to disk. Snapshots can be used to provide a copy of data at the point in time the snapshot was created. When a file is deleted, its disk blocks are added to the free list; however, the blocks for that file in any existing snapshots are not added to the free list until all referencing snapshots are removed. This means that snapshots provide a clever way of keeping a history of files, should you need to recover an older copy of a file or a deleted file. For this reason, many administrators take snapshots often (e.g. every 15 minutes), store them for a period of time (e.g. for a month), and store them on another system. Such a strategy allows the administrator to roll the system back to a specific time or, if there is a catastrophic loss, an off-site snapshot can restore the system up to the last snapshot interval (e.g. within 15 minutes of the data loss). Snapshots are stored locally but can also be replicated to a remote ZFS pool. During replication, ZFS does not do a byte-for-byte copy but instead converts a snapshot into a stream of data. This design means that the ZFS pool on the receiving end does not need to be identical and can use a different RAIDZ level, volume size, compression settings, etc.

ZFS boot environments provide a method for recovering from a failed upgrade. Beginning with FreeNAS® version 9.3, a snapshot of the dataset the operating system resides on is automatically taken before an upgrade or a system update. This saved boot environment is automatically added to the GRUB boot loader. Should the upgrade or configuration change fail, simply reboot and select the previous boot environment from the boot menu. Users can also create their own boot environments in System ‣ Boot as needed, for example before making configuration changes. This way, the system can be rebooted into a snapshot of the system that did not include the new configuration changes.

ZFS provides a write cache in RAM as well as a ZFS Intent Log (ZIL). The ZIL is a temporary storage area for synchronous writes until they are written asynchronously to the ZFS pool. If the system has many synchronous writes where the integrity of the write matters, such as from a database server or when using NFS over ESXi, performance can be increased by adding a dedicated log device, or slog, using Volume Manager. More detailed explanations can be found in this forum post and in this blog post. A dedicated log device will have no effect on CIFS, AFP, or iSCSI as these protocols rarely use synchronous writes. When creating a dedicated log device, it is recommended to use a fast SSD with a supercapacitor or a bank of capacitors that can handle writing the contents of the SSD’s RAM to the SSD. The zilstat utility can be run from Shell to help determine if the system would benefit from a dedicated ZIL device. See this website for usage information. If you decide to create a dedicated log device to speed up NFS writes, the SSD can be half the size of system RAM as anything larger than that is unused capacity. The log device does not need to be mirrored on a pool running ZFSv28 or feature flags as the system will revert to using the ZIL if the log device fails and only the data in the device which had not been written to the pool will be lost (typically the last few seconds of writes). You can replace the lost log device in the View Volumes ‣ Volume Status screen. Note that a dedicated log device can not be shared between ZFS pools and that the same device cannot hold both a log and a cache device.

ZFS provides a read cache in RAM, known as the ARC, to reduce read latency. FreeNAS® adds ARC stats to top(1) and includes the arc_summary.py and arcstat.py tools for monitoring the efficiency of the ARC. If an SSD is dedicated as a cache device, it is known as an L2ARC and ZFS uses it to store more reads which can increase random read performance. However, adding an L2ARC is not a substitute for insufficient RAM as L2ARC needs RAM in order to function. If you do not have enough RAM for a good sized ARC, you will not be increasing performance, and in most cases you will actually hurt performance and could potentially cause system instability. RAM is always faster than disks, so always add as much RAM as possible before determining if the system would benefit from a L2ARC device. If you have a lot of applications that do large amounts of random reads, on a dataset small enough to fit into the L2ARC, read performance may be increased by adding a dedicated cache device using Volume Manager. SSD cache devices only help if your active data is larger than system RAM, but small enough that a significant percentage of it will fit on the SSD. As a general rule of thumb, an L2ARC should not be added to a system with less than 64 GB of RAM and the size of an L2ARC should not exceed 5x the amount of RAM. In some cases, it may be more efficient to have two separate pools: one on SSDs for active data and another on hard drives for rarely used content. After adding an L2ARC, monitor its effectiveness using tools such as arcstat. If you need to increase the size of an existing L2ARC, you can stripe another cache device using Volume Manager. The GUI will always stripe L2ARC, not mirror it, as the contents of L2ARC are recreated at boot. Losing an L2ARC device will not affect the integrity of the pool, but may have an impact on read performance, depending upon the workload and the ratio of dataset size to cache size. Note that a dedicated L2ARC device can not be shared between ZFS pools.

ZFS was designed to provide redundancy while addressing some of the inherent limitations of hardware RAID such as the write-hole and corrupt data written over time before the hardware controller provides an alert. ZFS provides three levels of redundancy, known as RAIDZ*, where the number after the RAIDZ indicates how many disks per vdev can be lost without losing data. ZFS also supports mirrors, with no restrictions on the number of disks in the mirror. ZFS was designed for commodity disks so no RAID controller is needed. While ZFS can also be used with a RAID controller, it is recommended that the controller be put into JBOD mode so that ZFS has full control of the disks. When determining the type of ZFS redundancy to use, consider whether your goal is to maximize disk space or performance:

- RAIDZ1 maximizes disk space and generally performs well when data is written and read in large chunks (128K or more).
- RAIDZ2 offers better data availability and significantly better mean time to data loss (MTTDL) than RAIDZ1.
- A mirror consumes more disk space but generally performs better with small random reads. For better performance, a mirror is strongly favored over any RAIDZ, particularly for large, uncacheable, random read loads.
- Using more than 12 disks per vdev is not recommended. The recommended number of disks per vdev is between 3 and 9. If you have more disks, use multiple vdevs.
- Some older ZFS documentation recommends that a certain number of disks is needed for each type of RAIDZ in order to achieve optimal performance. On systems using LZ4 compression, which is the default for FreeNAS® 9.2.1 and higher, this is no longer true. See ZFS RAIDZ stripe width, or: How I Learned to Stop Worrying and Love RAIDZ for details.

The following resources can also help you determine the RAID configuration best suited to your storage needs:

- Getting the Most out of ZFS Pools
- A Closer Look at ZFS, Vdevs and Performance

Warning

NO RAID SOLUTION PROVIDES A REPLACEMENT FOR A RELIABLE BACKUP STRATEGY. BAD STUFF CAN STILL HAPPEN AND YOU WILL BE GLAD THAT YOU BACKED UP YOUR DATA WHEN IT DOES. See Periodic Snapshot Tasks and Replication Tasks if you would like to use replicated ZFS snapshots as part of your backup strategy.

While ZFS provides many benefits, there are some caveats to be aware of:

- At 90% capacity, ZFS switches from performance- to space-based optimization, which has massive performance implications. For maximum write performance and to prevent problems with drive replacement, add more capacity before a pool reaches 80%. If you are using iSCSI, it is recommended to not let the pool go over 50% capacity to prevent fragmentation issues.
- When considering the number of disks to use per vdev, consider the size of the disks and the amount of time required for resilvering, which is the process of rebuilding the vdev. The larger the size of the vdev, the longer the resilvering time. When replacing a disk in a RAIDZ*, it is possible that another disk will fail before the resilvering process completes. If the number of failed disks exceeds the number allowed per vdev for the type of RAIDZ, the data in the pool will be lost. For this reason, RAIDZ1 is not recommended for drives over 1 TB in size.
- It is recommended to use drives of equal sizes when creating a vdev. While ZFS can create a vdev using disks of differing sizes, its capacity will be limited by the size of the smallest disk.

If you are new to ZFS, the Wikipedia entry on ZFS provides an excellent starting point to learn more about its features. These resources are also useful to bookmark and refer to as needed:

- FreeBSD ZFS Tuning Guide
- ZFS Administration Guide
- Becoming a ZFS Ninja (video)
- Slideshow explaining VDev, zpool, ZIL and L2ARC and other newbie mistakes!
- A Crash Course on ZFS
- ZFS: The Last Word in File Systems - Part 1 (video)