package mirage-block-ramdisk

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Basic operation
Resizing support
Querying sparseness information

mirage-block-ramdisk
- Ramdisk

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An in-memory BLOCK device also known as a Ramdisk

Basic operation

include V1_LWT.BLOCK with type id = string

type page_aligned_buffer = Cstruct.t

The type for page-aligned memory buffers.

type error = [

| `Unknown of string
(*
an undiagnosed error
*)
| `Unimplemented
(*
operation not yet implemented in the code
*)
| `Is_read_only
(*
you cannot write to a read/only instance
*)
| `Disconnected
(*
the device has been previously disconnected
*)

]

The type for IO operation errors.

include V1.DEVICE
  with type error := error
  with type 'a io = 'a Lwt.t
  with type id = string

type 'a io = 'a Lwt.t

The type for potentially blocking I/O operation

type t

The type representing the internal state of the device

type id = string

This type is no longer used and will be removed once other * modules stop using it in their type signatures.

val disconnect : t -> unit io

Disconnect from the device. While this might take some time to complete, it can never result in an error.

type info = {

read_write : bool;
(*
True if we can write, false if read/only
*)
sector_size : int;
(*
Octets per sector
*)
size_sectors : int64;
(*
Total sectors per device
*)

}

Characteristics of the block device. Note some devices may be able to make themselves bigger over time.

val get_info : t -> info io

Query the characteristics of a specific block device

val read : 
  t ->
  int64 ->
  page_aligned_buffer list ->
  [ `Error of error | `Ok of unit ] io

read device sector_start buffers returns a blocking IO operation which attempts to fill buffers with data starting at sector_start. Each of buffers must be a whole number of sectors in length. The list of buffers can be of any length.

val write : 
  t ->
  int64 ->
  page_aligned_buffer list ->
  [ `Error of error | `Ok of unit ] io

write device sector_start buffers returns a blocking IO operation which attempts to write the data contained within buffers to t starting at sector_start. When the IO operation completes then all writes have been persisted.

Once submitted, it is not possible to cancel a request and there is no timeout.

The operation may fail with:

`Unimplemented: the operation has not been implemented, no data has been written.
`Is_read_only: the device is read-only, no data has been written.
`Disconnected: the device has been disconnected at application request, an unknown amount of data has been written.
`Unknown: some other permanent, fatal error (e.g. disk is on fire), where an unknown amount of data has been written.

Each of buffers must be a whole number of sectors in length. The list of buffers can be of any length.

The data will not be copied, so the supplied buffers must not be re-used until the IO operation completes.

val connect : name:string -> [ `Ok of t | `Error of error ] io

Connect to the named ramdisk.

val create : 
  name:string ->
  size_sectors:int64 ->
  sector_size:int ->
  [ `Ok of t | `Error of error ] io

Create an in-memory block device (a "ramdisk") with a given name, total size in sectors and sector size. Two calls to connect with the same name will return the same block device

val destroy : name:string -> unit

Destroy removes an in-memory block device. Subsequent calls to connect will create a fresh empty device.

Resizing support

val resize : t -> int64 -> [ `Ok of unit | `Error of error ] io

resize t new_size_sectors attempts to resize the connected device to have the given number of sectors. If successful, subsequent calls to get_info will reflect the new size.

Querying sparseness information

val seek_unmapped : t -> int64 -> [ `Ok of int64 | `Error of error ] io

seek_unmapped t start returns the offset of the next guaranteed zero-filled region (typically guaranteed because it is unmapped)

val seek_mapped : t -> int64 -> [ `Ok of int64 | `Error of error ] io

seek_mapped t start returns the offset of the next regoin of the device which may have data in it (typically this is the next mapped region)