Non-standard RAID levels

[18] The ZFS filesystem provides RAID-Z, a data/parity distribution scheme similar to RAID 5, but using dynamic stripe width: every block is its own RAID stripe, regardless of blocksize, resulting in every RAID-Z write being a full-stripe write.

This, when combined with the copy-on-write transactional semantics of ZFS, eliminates the write hole error.

RAID-Z does not require any special hardware, such as NVRAM for reliability, or write buffering for performance.

This would be impossible if the filesystem and the RAID array were separate products, whereas it becomes feasible when there is an integrated view of the logical and physical structure of the data.

Going through the metadata means that ZFS can validate every block against its 256-bit checksum as it goes, whereas traditional RAID products usually cannot do this.

[22] Windows Home Server Drive Extender is a specialized case of JBOD RAID 1 implemented at the file system level.

[25][26] BeyondRAID is not a true RAID extension, but consolidates up to 12 SATA hard drives into one pool of storage.

[28] Unraid is a proprietary Linux-based operating system optimized for media file storage.

Disadvantages include closed-source code, high price[citation needed], slower write performance than a single disk[citation needed] and bottlenecks when multiple drives are written concurrently.

However, Unraid allows support of a cache pool which can dramatically speed up the write performance.

Cache pool data can be temporarily protected using Btrfs RAID 1 until Unraid moves it to the array based on a schedule set within the software.

[citation needed] Advantages include lower power consumption than standard RAID levels, the ability to use multiple hard drives with differing sizes to their full capacity and in the event of multiple concurrent hard drive failures (exceeding the redundancy), only losing the data stored on the failed hard drives compared to standard RAID levels which offer striping in which case all of the data on the array is lost when more hard drives fail than the redundancy can handle.

[31] Some filesystems, such as Btrfs,[32] and ZFS/OpenZFS (with per-dataset copies=1|2|3 property),[33] support creating multiple copies of the same data on a single drive or disks pool, protecting from individual bad sectors, but not from large numbers of bad sectors or complete drive failure.

The decrease in declustered rebuild impact and client overhead can be a factor of three to four times less than a conventional RAID.

File system performance becomes less dependent upon the speed of any single rebuilding storage array.

[34] Dynamic disk pooling (DDP), also known as D-RAID, maintains performance even when up to 2 drives fail simultaneously.