Storage Systems: Storage Systems: Advanced TopicsAdvanced Topics

Learning Objectives:• To understand major characteristics of SSD• To understand Logical Volume Management –

its motivations and its facilities• To understand how Storage Area Networks

extend these features to multiple systems• To relate LVM and SAN to a most modern file

system implementation

COMP25212 1Feb 2013

Solid State DisksSolid State Disks

• Today’s technology: Flash Memory– Floating gate Field Effect Transistor

COMP25212

• Store {0, 1} on “Floating” gate

- no electrical connectionFeb 2013

NAND Flash OperationNAND Flash Operation

• No connection to “Floating” gate– Quantum tunneling– Block erase, bit writing– Memory wear (10K writes?)– Multi Level Cell e.g. {0, ¼, ½, ¾} 2 bits/cell– Read disturb

COMP25212Feb 2013

Flash ControllerFlash Controller

• Implement:– Error Correcting Codes– (Bad) Block Remapping– Write levelling– Read tracking and rewriting

• But note:– 10 year data retention(?)– Wear-out with write cycles– Performance falls as file system fills– Performance falls with device wearout

Feb 2013 COMP25212

Hard Disks and SSDHard Disks and SSD

Feb 2013 COMP25212

(2013 figures) Hard Disk SSD

Capacity/price 4Tb/£140 250Gb/£125

Streaming reads 205 Mb/s 530 Mb/s

Streaming writes 205 Mb/s 240 Mb/s

Random 4Kb read 15.5 mS 11 uS

Random 4Kb write 6.4 mS 23 uS

Power 6 - 8 W 0.3 - 4.2 W

(eg) Hitachi 7k4000 Samsung SSD 840

Storage VirtualizationStorage Virtualization

File System Code– assumes 1 file system– to 1 disk drive (partition)

Stripe/Mirror/RAID changes this

Storage Virtualization: break filesystem/drive relationship

COMP25212 6

OSOS

Device DriversDevice Drivers

File SystemFile System

Feb 2013

Logical Volume Logical Volume ManagementManagement

• Virtual mapping between file system code and physical device

• Analogous (but not identical!) to virtual memory addressing

• “Volume Group” of all drives in a pool• Storage space in “Volume Group” divided

into “Physical Extents” – usually all same size

• “Logical Volume” is set of “Physical Extents”

COMP25212 7Feb 2013

Logical Volume Logical Volume ManagementManagement

• Mirror/Stripe/RAID provided within LVM layer

• Resize filesystem

• “Snapshot” a live filesystem

COMP25212 8

OSOS

Device DriversDevice Drivers

File SystemFile System

LVM layerLVM layer

Volume Group

Feb 2013

e.g. Linux Servere.g. Linux Server

• / - mostly read – want fast seeks

• swap – read/write – want high bandwidth

• /opt – infrequent access

• /var – huge, infrequent access

• Mirror /

• Stripe swap

• spare space to /opt and /var

COMP25212 9Feb 2013

LVM Example:LVM Example:

• Now we can share resources more flexibly and more efficiently – but only on one server

COMP25212 10Feb 2013

Storage Area Storage Area NetworkingNetworking

• Implement LVM features in separate storage controller

• Connect multiple servers to storage controller• via SCSI, or FibreChannel, or Infiniband, or

…(n.b. SAN over Ethernet is conventionally called Networked Attached Storage (NAS)

• Now can share disk resources across multiple servers

• Rapid migration of disk images

COMP25212 11Feb 2013

Storage Area Network Storage Area Network ControllerController

COMP25212 12

8 “Blade Servers”

SA

N

Con

trol

ler

SA

N

Con

trol

ler

Feb 2013

SAN Key FeaturesSAN Key Features

• Functionality– Key element of “system virtualization”– Migrating virtual machines– “De-duping” – share common subsets of

file systems (think Virtual Machine images!)

• Management:– Manage storage separately from server

physical resources– Maximize flexibility of storage provisioning

COMP25212 13Feb 2013

ZFS – Volume Aware ZFS – Volume Aware File SystemFile System

Marketing claims:

• Lost a file?

• Run out of space?

• Difficult disk upgrade?

• Want to grow/shrink?

• Data Corruption?

COMP25212 14

Pools of storage

Feb 2013

ZFS techniquesZFS techniques

• Lost a file?– Copy-on-write – simple rollback/recovery

• Run out of space/Difficult disk upgrade?– Add new storage to live systems– Self-checking, self-healing

• Want to grow/shrink?

• Data Corruption?– end-to-end sumchecking

COMP25212 15Feb 2013

ZFS ImplementationZFS Implementation

• ZFS Combines File Service and Logical Volume Management

• Is this a good idea?

Discuss

COMP25212 16Feb 2013