1.1.6 RAID Overview
Data redundancy and speed are important considerations for any network attached storage system.
Most NAS systems use multiple disks to store data, meaning you should decide what type of RAID to
use before installing FreeNAS™. This section provides an overview of RAID types to assist you in
deciding which type best suits your requirements.
RAID 0: uses data striping to store data across multiple disks. It provides zero fault tolerance, meaning
if one disk fails, all of the data on all of the disks is lost. The more disks in the RAID 0, the more likely
the chance of a failure.
RAID 1: all data is mirrored onto two disks, creating a redundant copy should one disk fail. If the disks
are on separate controllers, this form of RAID is also called duplexing.
RAID 5: requires a minimum of 3 disks and can tolerate the loss of one disk without losing data. Disk
reads are fast but write speed can be reduced by as much as 50%. If a disk fails, it is marked as
degraded but the system will continue to operate until the drive is replaced and the RAID is rebuilt.
However, should another disk fail before the RAID is rebuilt, all data will be lost. If your FreeNAS™
system will be used for steady writes, RAID 5 is a poor choice due to the slow write speed.
RAID 6: requires a minimum of 4 disks and can tolerate the loss of 2 disks without losing data.
Benefits from having many disks as performance, fault tolerance, and cost efficiency are all improved
relatively with more disks. The larger the failed drive, the longer it takes to rebuild the array. Reads are
very fast but writes are slower than a RAID 5.
RAID 10: requires a minimum of 4 disks and number of disks is always even as this type of RAID
mirrors striped sets. Offers faster writes than RAID 5. Can tolerate multiple disk loss without losing
data, as long as both disks in a mirror are not lost.
RAID 60: requires a minimum of 8 disks. Combines RAID 0 striping with the distributed double parity
of RAID 6 by striping 2 4-disk RAID 6 arrays. RAID 60 rebuild times are half that of RAID 6.
RAIDZ1: ZFS software solution that is equivalent to RAID5. Its advantage over RAID 5 is that it
avoids the write-hole and doesn't require any special hardware, meaning it can be used on commodity
disks. If your FreeNAS™ system will be used for steady writes, RAIDZ is a poor choice due to the
slow write speed. Requires a minimum of 3 disks though 5 disks is recommended (over 3, 4, or 6
disks). It should be noted that you cannot add additional drives to expand the size of a RAIDZ1 after
you have created it. The only way to increase the size of a RAIDZ1 is to replace each drive with a
larger drive one by one while allowing time for restriping between each drive swap out. However, you
can combine two existing RAIDZ1's to increase the size of a ZFS volume (pool).
RAIDZ2: double-parity ZFS software solution that is similar to RAID-6. Its advantage over RAID 5 is
that it avoids the write-hole and doesn't require any special hardware, meaning it can be used on
FreeNAS™ 8.0.1 Guide Page 11 of 157commodity disks. Requires a minimum of 3 disks. RAIDZ2 allows you to lose 1 drive without any
degradation as it basically becomes a RAIDZ1 until you replace the failed drive and restripe. At this
time, RAIDZ2 on FreeBSD is slower than RAIDZ1.
RAIDZ3: triple-parity ZFS software solution. The current ZFS version used by FreeNAS™ does not
support this.
