Disk

A Disk wraps one virtio-blk disk attached to a VM. It exposes block-level operations and a fault-injection surface useful for exercising I/O error paths in the guest.

Constructing #

vm:attach_disk opts:

Disks are 512-byte sectors throughout. The read_sectors and write_sectors ops express offsets and counts in sectors.

Methods #

disk:size() #

Returns the disk's size in bytes. When a backing image is attached, returns the live stat() size of the image file (so a test that resized the underlying file with truncate / fallocate reads honest output). Without an image, returns the modeled size from attach_disk.

disk:read_sectors(offset, n) #

Read n sectors starting at sector offset. Returns the bytes as a Lua string. Errors if:

• The disk is detached (disk:read_sectors: disk is detached).
• No backing image (disk:read_sectors: no backing image — disk:with_image required).
• An eio_read fault is active (disk:read_sectors: EIO (fault_inject)).
• The host-side read fails (passes the underlying error through).

Honours active faults:

• eio_read — short-circuits to EIO before any I/O.
• slow — sleeps 50 ms per call. After the sleep, re-checks for eio_read so a concurrent fault injection during the sleep still takes effect. After the actual I/O, checks one more time for the same reason.

disk:write_sectors(offset, data) #

Write data (a Lua string) starting at sector offset. The data does not have to be a multiple of 512 bytes; it is written verbatim from the offset. Returns nothing.

Errors and fault handling mirror read_sectors:

• Detached → error.
• No backing image → error.
• eio_write active before the I/O → EIO.
• slow → 50 ms sleep, with eio_write re-checks after the sleep and after the I/O.

disk:fault_inject(mode) #

Activate a fault. Valid modes: "eio_read", "eio_write", "slow". Unknown modes error with the valid list (disk:fault_inject: unknown mode \X` (valid: eio_read, eio_write, slow)`).

Multiple modes can be active simultaneously. slow and eio_* compose naturally — a slow + eio_read combination delays the I/O before erroring.

disk:clear_faults() #

Clear every active fault. Subsequent reads / writes succeed normally.

disk:active_faults() #

Returns a Lua array of the currently-active fault mode names. Useful for tests that need to assert the harness state.

disk:detach() #

Mark the disk as detached and issue a best-effort QMP device_del against the parent VM. Subsequent read_sectors / write_sectors error. The QMP call is best-effort: a disk that was never QMP-added (host-bookkeeping-only attach) silently surfaces "Device not found" but the local detached flag is still set.

disk:is_detached() / disk:id() #

Accessors. :id() returns the disk identifier as given to attach_disk.

Example: full fault-injection pattern #

test("guest sees EIO and recovers after clear", function(t)
    local img = "/tmp/test-image"
    -- Pre-create the backing file.
    local f = io.open(img, "w"); f:write(string.rep("\0", 512 * 1024)); f:close()

    local vm = provium:vm("v", "peios"):boot()
    local disk = vm:attach_disk({id = "vda", size = 512 * 1024, image = img})

    -- Confirm baseline read works.
    local body = disk:read_sectors(0, 1)
    t:assert_eq(#body, 512)

    -- Inject EIO.
    disk:fault_inject("eio_read")
    local ok, err = pcall(function() disk:read_sectors(0, 1) end)
    t:assert(not ok)
    t:assert(tostring(err):find("EIO"))

    -- Clear and confirm reads recover.
    disk:clear_faults()
    body = disk:read_sectors(0, 1)
    t:assert_eq(#body, 512)
end)

See also #

• VM — disks come from vm:attach_disk / vm:disk.
• Disks and fault injection — patterns for exercising I/O error paths.