`blk`#

github.com/go-fsctl/blk

Pure-Go generic Linux block-device ioctls: BLK* from linux/fs.h, BLKPG from linux/blkpg.h, and the zoned queries from linux/blkzoned.h. Size and geometry, discard and zero-out, the read-only flag, and partition-table maintenance — with no cgo and without shelling out to blockdev, hdparm or partx.

These operate on real devices

Discard, SecureDiscard and ZeroOut destroy data, and the partition calls edit a live kernel partition table. Point them at a loop device (see loop/) or a scratch disk — never at the machine you are working on.

Platforms#

The ioctls exist only on Linux. Everywhere else every operation returns ErrUnsupported rather than failing in some device-specific way:

_, err := blk.GetSize64(fd)
// darwin: blk: block-device ioctls are only supported on Linux

The ABI definitions and the ioctl-number derivation stay available on every platform, so tooling and tests that only need the numbers build anywhere:

blk.BLKGETSIZE64  // 0x80081272
blk.BLKDISCARD    // 0x1277

Install#

go get github.com/go-fsctl/blk

Every call takes an already-open file descriptor, so opening the device — and deciding whether to open it O_RDONLY — stays with the caller.

f, err := os.OpenFile("/dev/loop0", os.O_RDWR, 0)
if err != nil { return err }
defer f.Close()
fd := int(f.Fd())

Size and geometry#

bytes, err := blk.GetSize64(fd)          // BLKGETSIZE64, the one to use
sectors, err := blk.GetSize(fd)          // BLKGETSIZE, in 512-byte sectors

blk.GetBlockSize(fd)        // BLKBSZGET — the soft block size
blk.SetBlockSize(fd, 4096)  // BLKBSZSET
blk.GetSectorSize(fd)       // BLKSSZGET — logical sector size
blk.GetPhysBlockSize(fd)    // BLKPBSZGET — physical, which may be larger
blk.GetIOMin(fd)            // BLKIOMIN — minimum I/O the device prefers
blk.GetIOOpt(fd)            // BLKIOOPT — optimal I/O size
blk.GetAlignmentOffset(fd)  // BLKALIGNOFF

The logical and physical sizes differ on 512e drives — a 4096-byte physical sector presented as 512 — which is what GetAlignmentOffset exists to tell you about.

Discard, secure discard, zero-out#

blk.Discard(fd, start, length)        // BLKDISCARD
blk.SecureDiscard(fd, start, length)  // BLKSECDISCARD
blk.ZeroOut(fd, start, length)        // BLKZEROOUT
blk.GetDiscardZeroes(fd)              // BLKDISCARDZEROES

GetDiscardZeroes reports whether a discarded region reads back as zeroes. It is a property of the device, not a promise of the call: where it is false, a discard frees blocks without defining what a read returns, and ZeroOut is what actually writes zeroes.

The read-only flag, and flushing#

blk.SetReadOnly(fd, true)   // BLKROSET
ro, err := blk.GetReadOnly(fd) // BLKROGET
blk.FlushBuf(fd)            // BLKFLSBUF — drop the buffer cache for this device

Partitions#

blk.AddPartition(fd, blk.Partition{Number: 1, Start: 1 << 20, Length: 64 << 20})
blk.ResizePartition(fd, blk.Partition{Number: 1, Start: 1 << 20, Length: 128 << 20})
blk.DelPartition(fd, 1)
blk.RereadPartitionTable(fd)  // BLKRRPART

Partition is {Number int; Start, Length int64}, with Start and Length in bytes.

The BLKPG calls tell the kernel about one partition without re-reading the whole table, which RereadPartitionTable does and which fails with EBUSY when any partition is mounted. That is the reason BLKPG exists: it is the way to add or remove a partition on a disk that is in use.

Zoned devices#

blk.GetNumZones(fd)  // BLKGETNRZONES
blk.GetZoneSize(fd)  // BLKGETZONESZ, in 512-byte sectors

Licence#

BSD-3-Clause.

blk#