Class: VirtFS::BlockIO

Inherits:
Object
  • Object
show all
Defined in:
lib/virtfs/block_io.rb

Overview

Block IO Adapter, used internally by VirtFS to perform read and write operations via an underlying ‘block-like’ interface.

The class requires a handle to an object defining #raw_read and #raw_write methods facilitating raw block access.

This class behaves similarily to a tradional disk device, providing a ‘seek’ position at which read/write operations occur after which the pos is updated

Constant Summary collapse

MIN_BLOCKS_TO_CACHE = 
64

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(io_obj) ⇒ BlockIO

BlockIO initializer

Parameters:

  • io_obj (RawIO) —

    instance of class defining #raw_read and #raw_write



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# File 'lib/virtfs/block_io.rb', line 21

def initialize(io_obj)
  @io_obj          = io_obj
  @block_size      = @io_obj.block_size   # Size of block in bytes
  @size            = @io_obj.size         # Size of file in bytes
  @size_in_blocks  = @size / @block_size  # Size of file in blocks
  @start_byte_addr = 0
  @end_byte_addr   = @size - 1
  @lba_end         = @size_in_blocks - 1
  @seek_pos        = 0
  @cache_range     = Range.new(-1, -1)
  @offset          = 0
end

Instance Attribute Details

#offset ⇒ Object

Optional absolute block offset, always applied



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# File 'lib/virtfs/block_io.rb', line 15

def offset
  @offset
end

Instance Method Details

#close ⇒ Object

Close block device, after this no additional read/writes can occur



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# File 'lib/virtfs/block_io.rb', line 103

def close
  @io_obj.close
end

#read(len) ⇒ Array<Byte>?

Read len bytes from the block device and update seek_pos

Parameters:

  • len (Integer) —

    number of bytes to read

Returns:

  • (Array<Byte>, nil) —

    array of up to len bytes read from block device, or nil if no bytes can be read



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# File 'lib/virtfs/block_io.rb', line 39

def read(len)
  return nil if @seek_pos >= @end_byte_addr
  len = @end_byte_addr - @seek_pos if (@seek_pos + len) > @end_byte_addr

  start_sector, start_offset = @seek_pos.divmod(@block_size)
  end_sector = (@seek_pos + len - 1) / @block_size
  num_sector = end_sector - start_sector + 1

  rbuf = bread_cached(start_sector, num_sector)
  @seek_pos += len

  rbuf[start_offset, len]
end

#seek(amt, whence = IO::SEEK_SET) ⇒ Object

Advance seek pointer via the specified mechanism

Parameters:

  • amt (Integer) —

    absolute amount which to advance seek pointer

  • whence (IO::SEEK_SET, IO::SEEK_CUR, IO::SEEK_END) (defaults to: IO::SEEK_SET) —

    mechanism which to update seek pos

See Also:

  • IO::SEEK_SET
  • IO::SEEK_CUR
  • IO::SEEK_END


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# File 'lib/virtfs/block_io.rb', line 85

def seek(amt, whence = IO::SEEK_SET)
  case whence
  when IO::SEEK_CUR
    @seek_pos += amt
  when IO::SEEK_END
    @seek_pos = @end_byte_addr + amt
  when IO::SEEK_SET
    @seek_pos = amt + @start_byte_addr
  end
  @seek_pos
end

#size ⇒ Integer

Returns size of block device.

Returns:

  • (Integer) —

    size of block device



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# File 'lib/virtfs/block_io.rb', line 98

def size
  @size
end

#write(buf, len) ⇒ Integer?

Write buffer of specified length to block device and update seek_pos

Parameters:

  • buf (Array<Byte>) —

    bytes to write to disk device

Returns:

  • (Integer, nil) —

    number of bytes written to device, or nil if none can be written



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# File 'lib/virtfs/block_io.rb', line 58

def write(buf, len)
  return nil if @seek_pos >= @end_byte_addr
  len = @end_byte_addr - @seek_pos if (@seek_pos + len) > @end_byte_addr

  start_sector, start_offset = @seek_pos.divmod(@block_size)
  end_sector = (@seek_pos + len - 1) / @block_size
  num_sector = end_sector - start_sector + 1

  rbuf = bread(start_sector, num_sector)
  rbuf[start_offset, len] = buf[0, len]

  bwrite(start_sector, num_sector, rbuf)
  @seek_pos += len

  len
end