Jagadish Paranjape firstname.lastname@example.org
1. Server Virtualization:
Hardware resources are time
(Computer Science 555 - Research Paper)
multiplexed to give appearance of dedicated
Role of “Storage Virtualization” in
hardware resources to the Operating Systems
running within a ‘Virtual Machine’.
Virtualization allows sharing of hardware
1. Introduction: resources. Operating Systems running within
Server Virtualization is seen as enabling the virtual machines can host application
technology for ‘Cloud Computing’. Although servers or any other application like any
virtual machines play a key role in providing operating system running on the real hardware
computation as a service, ‘Storage can.
Virtualization’ and ‘Network Virtualization’ Virtual machines offer benefits like
technologies are critical in providing isolation, live migration to another hardware
‘Infrastructure as a Service’ - ‘IaaS’. This paper resource. All the instructions executed by
will focus on the storage related technologies applications running within a VM can be
and particularly ‘Storage Virtualization’ and its monitored and execution of potentially
importance as a complementary technology to malicious instructions can be avoided. Since
server virtualization in realizing IaaS. virtualized Operating Systems run at lower
At the beginning a brief overview of privilege levels than the Virtual Machine
how virtualization technologies are used in a Monitors that control them, running a Virtual
‘modern data center’ is provided. Then ‘Storage Machine is secure than running an OS directly
Virtualization’ is discussed in detail. Then some over the hardware.
techniques like Façade and VectorDot for Thus by hosting multiple VMs on the
maintaining storage performance are discussed. same server hardware, the capacity of the
Paper concludes with discussion on emerging hardware ban be better utilized reducing the
ideas in IaaS. total power consumption, isolation provided by
virtual machines adds security to the
applications running in different VMs, live
2. Virtualization Technologies: migration of VMs enables load balancing
Modern data centers with server between the hardware servers. 
virtualization, storage virtualization and
network virtualization are at the core of 2. Network Virtualization:
‘Infrastructure as a Services’ (Iaas). Data centers Virtual networks are built using overlay
are composed of layers of resources such as networks that can be used for creating virtual
servers, switches and shared storage.  server farms within a data center..Virtual
Servers can host multiple Virtual Servers machines on the virtual network are made
using virtualization technologies like Xen, invisible to the internet. Traffic thresholds can
VMWare. Network switches connect servers be configured for the virtual machines in order
with storage devices. Data center storage can be to restrict network bandwidth usage by that
in the form of a SAN – Storage Area Network, VM. Virtual networks also provide additional
NAS – Network Attached Storage or a security to the network traffic passing through
combination of both. them as it is protected from other virtual
networks in the same data center. . Thus
virtual networks can be used for creating 1. Characteristics of Storage
multiple virtual farms belonging to different Virtualization:
customers sharing other data center resources 3.1.1 Device Transparency:
like physical servers and storage.
Violin is one such system that allows Physical storage devices are transparent
network virtualization. In Violin, A virtual to the applications accessing the storage pool.
Local Area Network VLAN consists of virtual Thus multiple storage devices with different
machines as virtual hosts. Virtual machines performance, capacity and other storage
belonging to the same VLAN may reside on attributes can be part of the same storage pool .
different physical servers. Virtual hosts are .
connected to a ‘virtual switch’. Another ‘Virtual 3.1.2 Location Transparency:
Machine’ acts as a ‘Virtual Router’ with multiple
The physical location where the data
virtualized network interface cards (VNIC) and
actually resides is transparent to the
connects multiple VLANs. .
applications. The virtualization software
manages the mapping between the apparent
3. Storage Virtualization: data location and the actual physical location
Lumb et al. point out that the concept where the data gets stored..
of storage virtualization existed since IBM MVS
2. Types of Storage virtualization:
mainframe operating system . Storage
3.2.1 File Level Virtualization:
virtualization is an abstraction that gives a
unified view of underlying heterogeneous This is more traditional way of storage
physical storage devices (a storage pool ).The virtualization. This is implemented by using
virtualization layer divides the physical storage Distributed File Systems with a root DFS server
into logical units and provides indirection maintaining the information necessary for
(mapping) between the logical storage location resolving the file names to the ‘appropriate
in to physical storage location. physical storage locations’.  Using File Level
SNIA – Storage Networking Industry Virtualization, multiple file systems are made to
Association  defines Storage Virtualization appear as a single file system..
as follows: NAS – Network Attached Storage
operates at the granularity of a File. NAS device
1. “The act of abstracting, hiding or isolating
is connected to a LAN and all the client requests
the internal functions of a storage (sub)system
for files are handled by the NAS appliance. The
or service from applications, host computers,
client applications are unaware of the location
general network resources, for the purpose of
and device type of the physical storage.
enabling application and network-independent
management of storage or data”. .
3.2.2 Block Level Virtualization:
2. “The application of virtualization to storage
services or devices for the purpose of This is implemented at a lower level
aggregation functions or devices, hiding than a file. At the core of it is the abstraction of
complexity, or adding new capabilities to lower a ‘virtual disk’  . Clients of a Virtual disk
level storage resources”. . (File systems, databases etc) get a view of
Virtual Disk as a raw byte storage space
composed of “blocks” just like a regular physical
Jagadish Paranjape email@example.com
disk.[Petal:9] controller hardware (Eg: IBM SVC – SAN Volume
It is the job of the storage virtualization Controller ) or as a software in the switches
layer to abstract shared physical storage into used for connecting servers with the physical
‘Virtual Disks’ and resolve ‘virtual disk’ access storage. (Eg: EMC-Invista ). (Ameek et al.
requests into ‘physical storage’ access requests. Discuss their datacenter testbed ‘Harmony’ and
The virtual address can be in the form <Virtual storage virtualization setup in  in greater
Disk Identifier, Offset> and it needs to be detail).
mapped to appropriate physical address which 3.3.2 Out of Band Indirection:
can be <Server location, Physical Disk Identifier,
Offset>.[Petal:9] In case of the out of band indirection
the virtualization software / hardware
SAN – Storage Area Networks operate at
the granularity of disk blocks and hence are well responsible for indirection is not within the data
path. The virtual servers make an explicit
suited for block level virtualization. SAN can be
used as backing store for NAS and then SAN indirection request to the out of band
server/device for resolving the virtual address
aware clients can access the storage at block
level where as the legacy clients can continue to of the storage and then request the physical
device for their storage needs. [8-465]. In this
access files using the NAS interface. .
For providing Storage as a service, SAN approach special software needs to be installed
at each server for communicating with out of
based block level virtualization looks more
promising as it gives appearance of a raw disk band-device.
In SoftUDC server based storage
space by abstracting storage into Virtual Disks.
Virtual Disk capacities can be configured based virtualization is used. The storage virtualization
layer (Virtual Volume Manager) is implemented
on the application needs. Also at block level
virtualization other services like storage as part of ‘gatekeeper’ layer that sits along Xen-
VMM-Virtual Machine Monitor  of every
performance, availability can be built in .
Since NAS operates at granularity of a File, physical server hosting multiple virtual servers.
Gatekeeper monitors all I/O and network traffic
achieving flexibility similar to that of block
based virtualization could be difficult. and is responsible for access control and I/O
security. Virtual Volume Manager is responsible
3. Types of Indirection: for mapping of the VSDs-Virtual Storage Devices
Depending on the location where the with the actual physical storage devices in the
indirection software layer (which converts storage-pool..
virtual disk accesses requests to the physical
storage access requests) is installed, it is 3.3.3 Indirection and scaling:
classified as ‘in-band’ indirection or ‘out of
For scaling in band indirection based
systems, additional hardware that performs
3.3.1 In band indirection:
indirection is required. The number of servers
When the indirection takes place within that can access the storage pool using single in-
the data path between the nodes hosting the band indirection appliance gets restricted by the
virtual servers and the physical storage device, it bandwidth of the indirection device. (In above
is called as in-band indirection.  example multiple IBM SVC appliances will be
In data center environment this level of required).. Scaling of the systems using out
indirection can be provided by a special of band indirection is easier as the bandwidth of
the data path is not constrained by the number 5. Benefits of Storage
of servers connected to the out of band Virtualization:
indirection appliance. Out of band virtualization Ease of Management: Storage virtualization
suffers from other problems. It requires a driver simplifies the storage management task. The
module to be installed on each host OS and thus administrator only configures the ‘Virtual Disks’
if an operating systems does not have and the actual storage management is handled
compatible driver module, that operating by the virtualization software.
system cannot be configured to access that Improved Storage Utilization: Since multiple
virtualized storage.  Virtual Disks can share the same physical
storage device, there is better utilization of the
4. Multi-dimentional Storage
Virtualization: Flexible storage allocation: Storage can be
Huang et al. discuss need of a multi dynamically allocated / unallocated based on
dimensional storage virtualization and have application demands. Eg: For running certain
proposed ‘Stonehenge’ system. Generally task if a snapshot of a database is required,
storage virtualization considers dimension of additional storage can be dynamically allocated
capacity  in which additional physical storage and after the task is completed, the additional
can be added to the system without having to resources can be released.
re-configure all the system. ’Stonehenge’ Performance optimization: Using Techniques
virtualizes capacity as well as efficiency and like “stripping” the virtual disk can be
other physical disk attributes like bandwidth, configured to perform at a higher bandwidth
latency and availability. than an individual physical disk can. .
‘Stonehenge’ partitions physical storage Inbuilt Reliability: Replication factor can be
into multiple ‘virtual disks’ and allows assigning configured as part of attributes of the virtual
of disk attributes like - Availability: Expressed in storage.
terms of degree of replication, Bandwidth: Non-disruptive addition or removal of physical
expressed in terms of number of disk access storage: Additional physical storage can be
requests per unit time, Capacity: Total amount added to the storage pool or unused disks can
of user data that can be put into the virtual disk, be removed from the pool transparently to the
Delay: Worst case end to end delay that a disk applications using the pool. .
access request, Elasticity: The delay and
bandwidth characteristics of the disk expressed 6. Load Balancing and storage
in terms of probabilities.. performance guarantees:
’Stonehenge‘ maps the attributes that Data Centers with capacities of
user assigns to the virtual disk to a set of hundreds of terabytes and aggregate transfer
physical storage servers. If capacity rates of hundreds of gigabytes/second are
requirements of a virtual disk cannot be fulfilled possible with fast switched fabrics and large disk
by a single physical server, then the virtual disk arrays.  Such Data Centers are capable of
is assigned to multiple physical storage servers. serving storage needs of multiple organizations.
Similarly, if single physical server cannot fulfill Typically a storage service provider (SSP-)
the bandwidth requirements of a virtual disk, owns a data center and offers storage as a
then the virtual disk is ‘striped’ and partially service to multiple customers.
assigned to multiple physical servers.  The SSP provides certain performance
Jagadish Paranjape firstname.lastname@example.org
guarantees- “SLO: Service Level Objective” to availability..
the customer as part of “SLA: Service Level Ameek Et al. present ‘VectorDot’ an
Agreement”. SLO specifies capacity, availability algorithm for selecting the server where VMs
and performance requirements that the storage should be moved from an overloaded server.
should meet.  This can be seen as ‘Cloud The algorithm analyses the current state of the
Storage Service’ as part of IaaS. Load balancing resources in the data center. The state of the
is critical for meeting the SLAs. data center comprises of status (current usage
3.6.1 Real time scheduling based approach in value, capacity, threshold value) of each of the
Façade : Lumb et al. define server node, switch node, storage node, storage
“performance isolation” as ‘the performance virtualization appliance, network connection
experienced by a workload from a given topology, VMs and VDisks. .A node
customer should not suffer because of the (server/storage/switch) is considered to be
variations on the workloads from other overloaded when current usage value at the
customers’. Such isolation can be provided by node exceeds the threshold value. Finding a
over provisioning of the resources to the node where the overloaded node needs to be
customers, but results in increased costs. Other moved is difficult to determine. Even if a server
solution could be allocating separate physical node is less loaded , if the switch to which it is
storage resources to different customers, but connected is overloaded then that server
this solution is inflexible and adding new cannot be selected as a destination. The
physical storage becomes difficult and needs VectorDot algorithm  categorizes above
reconfiguration. constraints as ‘multidimensional’ and
‘Façade’  is a virtualization layer ‘hierarchical’ and tries to determine which items
between the hosts and the storage devices and need to be moved and to which server.
is designed for achieving ‘performance isolation’ In SoftUDC approach : When a virtual
- for meeting the SLO. Façade achieves this by machine needs to be migrated from one node
real time scheduling and controlling the storage to another node (may be as a part of load
device queue. balancing), the Virtual Volume Manager
It is based on two assumptions:  migrates the VSD – Virtual Storage Device along
a. ’Reducing the length of the device queue with it.Since VSD only acts as storage access
reduces the latency at the device’. point, there is no physical movement of data.
b. ’Increasing the device queue may increase . So only the virtual to physical storage
the device throughput’. mapping related information contained in the
VSDs is moved along with the Virtual Server
3.6.2 Virtual Server and Virtual Storage when it migrates from one node to another
migration based approaches: node.
A server can get overloaded due to
excessive demand generated for a shared 4. Current trends in ‘Cloud Storage’:
resource Eg: Memory, CPU, network switches Live Migration of VMs within same data
and storage - disks. Virtual Machines hosting centre (same LAN) is already used as a way of
the applications and corresponding virtual disks load balancing. Research is being conducted in
accessed by them can be migrated from Live Migration of VMs between data centers (in
overloaded physical server to a less loaded Wide Area Networks). This type VM
physical server without impacting application migration may be required for moving the VMs
between multiple cloud computing vendors Façade: virtual storage devices with
without disrupting the application availability. In performance guarantees.
intra data center migration only the migration of Conference On File And Storage
the ‘Virtual Disks’ which act as ‘access points’ Technologies 2003
 to storage is sufficient but in inter data Proceedings of the 2nd USENIX
center migration, actual contents of backing Conference on File and Storage
storage needs to be moved as well for avoiding Technologies.
performance degradation that will arise due to http://portal.acm.org/citation.cfm?
I/O over the network.  id=1090694.1090710
Another interesting idea is ‘MetaCDN’  SoftUDC: A Software-Based Data Center
 creating an affordable ‘Content Delivery for Utility Computing
Network’ by aggregating cloud storage services Mahesh Kallahalla,Mustafa Uysal,Ram
provided by multiple vendors such as Amazon’s Swaminathan,David E.Lowell,Mike
S3  and Nirvanix SDN.. Wray,Tom Christian,
Nigel Edwards, Chris I. Dalton, Frederic
Gittler. - HP Labs.
IEEE computer , Volume 37 Issue 11,
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 Paul Barham,Boris Dragovic,Keir
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Guillermo A. Alvarez
Jagadish Paranjape email@example.com
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Thekkath Conference on Mass Storage Systems and
Petal:Dtributed Virtual Disks Technologies (MSST)
Digital Equipment Corporation http://citeseerx.ist.psu.edu/viewdoc/su
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Storage Systems and Technologies 2002 University.
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Storage Systems http://www.vmware.com/virtualization
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Ogawa,Hidemoto Nakada,Satoshi Association.
Itoh,Satoshi Sekiguchi Frank Bunn – VERITAS software, Nick
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Over WAN. Technical Tutorial – Storage
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Heide , Ulrich Rückert , Kay Zahir Tari
Salzwedel , Mario Vodisek MetaCDN: Harnessing 'Storage Clouds'
V:Drive - Costs and Benefits of an Out- for high performance content delivery
of-Band Storage Virtualization System Journal of Network and Computer
Application, Volume 32 , Issue 5
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