RAID

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 RAIDto 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 (stripe): provides optimal performance and allows you to add disks as needed. Provides zero redundancy, 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 (mirror): provides redundancy as data is copied (mirrored) to two or more drives. Provides good read performance but may have slower write performance, depending upon how the mirrors are setup and the number of ZILs and L2ARCs.

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. This type of RAID can survive the failure of any one drive. If you lose a second drive from the same mirrored set, you will lose the array. However, if you lose a second drive from a different mirrored set, the array will continue to operate in a degraded state. RAID 10 significantly outperforms RAIDZ2, especially on writes.

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.

RAIDZ2: double-parity ZFS software solution that is similar to RAID-6. It avoids the write-hole and doesn't require any special hardware, meaning it can be used on commodity 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. RAIDZ3 offers three parity drives and can operate in degraded mode if up to three drives fail with no restrictions on which drives can fail.