#OPENZFS VS MACZFS UPDATE#
If the data passes this integrity check, the system can then update the faulty copy with known-good data so that redundancy can be restored. If redundancy exists, ZFS will fetch a copy of the data (or recreate it via a RAID recovery mechanism), and recalculate the checksum-ideally resulting in the reproduction of the originally expected value. If the storage pool consists of a single disk, it is possible to provide such redundancy by specifying “copies=2” (or “copies=3”), which means that data will be stored twice (thrice) on the disk, effectively halving (or, for “copies=3”, reducing to one third) the storage capacity of the disk. If the values do not match, then ZFS can heal the data if the storage pool has redundancy via ZFS mirroring or RAID. If the checksums match, the data are passed up the programming stack to the process that asked for it. When a block is accessed, regardless of whether it is data or meta-data, its checksum is calculated and compared with the stored checksum value of what it “should” be. ZFS stores the checksum of each block in its parent block pointer so the entire pool self-validates. In-flight data corruption or phantom reads/writes (the data written/read checksums correctly but is actually wrong) are undetectable by most filesystems as they store the checksum with the data.
This checksumming continues all the way up the file system’s data hierarchy to the root node, which is also checksummed, thus creating a Merkle tree. Next, the block pointer is checksummed, with the value being saved at its pointer. Each block of data is checksummed and the checksum value is then saved in the pointer to that block-rather than at the actual block itself. ZFS data integrity įor ZFS, data integrity is achieved by using a ( Fletcher-based) checksum or a ( SHA-256) hash throughout the file system tree. While it is also faster than UFS, it can be seen as the successor to UFS.
Initial research indicates that ZFS protects data better than earlier efforts.
#OPENZFS VS MACZFS DRIVER#
That is, it is designed to protect the user’s data on disk against silent data corruption caused by bit rot, current spikes, bugs in disk firmware, phantom writes (the write is dropped on the floor), misdirected reads/writes (the disk accesses the wrong block), DMA parity errors between the array and server memory or from the driver (since the checksum validates data inside the array), driver errors (data winds up in the wrong buffer inside the kernel), accidental overwrites (such as swapping to a live file system), etc.ĭata integrity is a high priority in ZFS because recent research shows that none of the currently widespread file systems-such as UFS, Ext, XFS, JFS, or NTFS-nor hardware RAID provide sufficient protection against such problems (hardware RAID has some issues with data integrity). One major feature that distinguishes ZFS from other file systems is that ZFS is designed with a focus on data integrity. See also: Hard disk error rates and handling and Silent data corruption
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