hesela.devZFS gang block
A ZFS gang block represents one block as smaller data fragments reached through a separately checksummed header.
OpenZFS 2.3.4 can fall back to gang allocation when it cannot obtain the requested block size, including under severe free-space fragmentation. A member can itself be a gang block, forming a tree. This does not create free space: allocation can still fail. [1]
In this version, a gang header holds up to three member block pointers and an embedded checksum trailer. Three is the direct-member limit, not a limit on all leaves of a nested tree. [2]
The parent block pointer retains the data checksum, not the header checksum, keeping the data signature used for deduplication independent of the allocation layout. [1] The header instead uses embedded SHA-256 with a verifier derived from the identity block pointer's vdev, offset and birth transaction group. Verification substitutes that expected tuple before recomputing the hash. A header cannot be validated correctly by hashing its stored bytes unchanged; this context binding is not a secret-key authentication scheme. [3]
Before freeing or claiming members, OpenZFS assembles the entire header tree. This avoids freeing one part before discovering that another header cannot be read. On a header-assembly error, the issue stage discards its in-memory traversal tree without dispatching the member operation; that cleanup is not a partial on-disk free. Successful assembly does not certify that payload reads or writes will succeed. Header replication is separately controlled; splitting payload into members is not itself redundancy. [1]
Relations
Advantages
- Allows a larger logical operation to use smaller allocations when an allocation of the requested size is unavailable. [1]
Limitations
- Requires additional metadata traversal; no throughput or latency improvement is implied. [1]
- A valid header does not establish that every referenced data fragment is readable and correct. [1, 3]
Citations
- OpenZFS 2.3.4: zio.c, gang-tree assembly, allocation and failure paths
- OpenZFS 2.3.4: zio.h, gang-header layout and member-count formula
- OpenZFS 2.3.4: zio_checksum.c, checksum table, gang verifier, compute and verification
Sources & caveats
- Implementation review is pinned to OpenZFS zfs-2.3.4, commit 34f96a15c73eab27dd6ad17bb5f1263bf26e37d7. It is not a claim about every OpenZFS version.
- No native gang-block allocation, corruption injection, recovery or performance experiment was performed. Header validity, payload integrity and redundancy are separate questions.