device_allocator.go

/*
Copyright 2016 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package aws

import "fmt"

// ExistingDevices is a map of assigned devices. Presence of a key with a device
// name in the map means that the device is allocated. Value is irrelevant and
// can be used for anything that DeviceAllocator user wants.
// Only the relevant part of device name should be in the map, e.g. "ba" for
// "/dev/xvdba".
type ExistingDevices map[mountDevice]awsVolumeID

// On AWS, we should assign new (not yet used) device names to attached volumes.
// If we reuse a previously used name, we may get the volume "attaching" forever,
// see https://aws.amazon.com/premiumsupport/knowledge-center/ebs-stuck-attaching/.
// DeviceAllocator finds available device name, taking into account already
// assigned device names from ExistingDevices map. It tries to find the next
// device name to the previously assigned one (from previous DeviceAllocator
// call), so all available device names are used eventually and it minimizes
// device name reuse.
// All these allocations are in-memory, nothing is written to / read from
// /dev directory.
type DeviceAllocator interface {
	// GetNext returns a free device name or error when there is no free device
	// name. Only the device suffix is returned, e.g. "ba" for "/dev/xvdba".
	// It's up to the called to add appropriate "/dev/sd" or "/dev/xvd" prefix.
	GetNext(existingDevices ExistingDevices) (mountDevice, error)
}

type deviceAllocator struct {
	firstDevice        mountDevice
	lastAssignedDevice mountDevice
	length             int
}

// NewDeviceAllocator creates new DeviceAlllocator that allocates device names
// of given length ("aaa" for length 3) and with given first device, so all
// devices before the first device are left to the operating system.
// With length 2 and firstDevice "ba", it will allocate device names
// ba, bb, ..., bz, ca, ... cz, ..., da, ... zz, so a..z and aa..az can be used
// by the operating system.
func NewDeviceAllocator(length int, firstDevice mountDevice) DeviceAllocator {
	lastDevice := make([]byte, length)
	for i := 0; i < length; i++ {
		lastDevice[i] = 'z'
	}
	return &deviceAllocator{
		firstDevice:        firstDevice,
		lastAssignedDevice: mountDevice(lastDevice),
		length:             length,
	}
}

func (d *deviceAllocator) GetNext(existingDevices ExistingDevices) (mountDevice, error) {
	candidate := d.lastAssignedDevice

	for {
		candidate = d.nextDevice(candidate)
		if _, found := existingDevices[candidate]; !found {
			d.lastAssignedDevice = candidate
			return candidate, nil
		}
		if candidate == d.lastAssignedDevice {
			return "", fmt.Errorf("no devices are available")
		}
	}
}

func (d *deviceAllocator) nextDevice(device mountDevice) mountDevice {
	dev := []byte(device)
	for i := d.length - 1; i >= 0; i-- {
		if dev[i] != 'z' {
			dev[i]++
			return mountDevice(dev)
		}
		dev[i] = 'a'
	}
	// all parts of device were 'z', jump to the first device
	return d.firstDevice
}