Direct Attached Storage (DAS)
Module 3.1
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage (DAS)
After completing this module you will be able to:
 Discuss the benefits of DAS based storage strategy
 Describe the elements of DAS
 Describe the connectivity options for DAS
 Discuss DAS management considerations
 Identify the best environments for DAS solutions
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 2
What is DAS?
Internal
© 2006 EMC Corporation. All rights reserved.
External Direct
Connect
Direct Attached Storage - 3
DAS Benefits
 Ideal for local data provisioning
 Quick deployment for small environments
 Simple to deploy in simple configurations
 Reliability
 Low capital expense
 Low complexity
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 4
Physical Elements of DAS
Connectivity
Storage
CPU

Motherboard

Internal

Hard disk(s)

Clustered group of processors

External

CD-ROM drive

Processor cards

Optical drive

Complete system

Removable media

Tape devices/tape library

RAID/intelligent array(s)

Portable media drives
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 5
DAS Connectivity
Block-Level Access Protocols:
 ATA (IDE) and SATA
– Primarily for internal bus
 SCSI
– Parallel (primarily for internal bus)
– Serial (external bus)
 Buss and Tag
– Primarily for external mainframe
– Precursor to ESCON and FICON
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 6
DAS Connectivity: Internal
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 7
Internal DAS Connectivity Examples
Parallel Connectivity Cables
50-wire
SCSI-2 cable
© 2006 EMC Corporation. All rights reserved.
80-wire
IDE
cable
Serial Connectivity Cable
34-wire
floppy
cable
Serial ATA cable
Direct Attached Storage - 8
DAS Connectivity: External
External DAS
HBA
Host
© 2006 EMC Corporation. All rights reserved.
Storage Device
Example of an external
connectivity cable
Direct Attached Storage - 9
DAS Management: Internal
 Host provides:
– Disk partitioning (Volume management)
– File system layout
– Data addressing
 Direct Attached Storage managed individually through the
server and the OS
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 10
DAS Management: External
 Array based management
 Availability – multi-path I/O
 Lower TCO for managing data and storage Infrastructure
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 11
DAS Performance Considerations
Factors to be considered for DAS performance:
• Hard disks
• Memory cache
• Virtual memory (paging)
• Storage controllers
• Protocol supported (e.g. SCSI, FireWire, USB, etc.)
• RAID level
• Bus
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 12
Internal DAS Application Example
Hard Drive
40 Pin Ribbon Cable
Motherboard
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 13
External DAS Application Example
ESCON HBA
© 2006 EMC Corporation. All rights reserved.
Cable for external
DAS connectivity
Direct Attached Storage - 14
DAS Challenges
 Hosts must be directly connected
 Data availability
 Data slowdowns possible
– CPU congestion, caching, multi-pathing
 Scalability is limited
– Number of connectivity ports to hosts
– Number of addressable disks
– Distance limitations
 Downtime required for maintenance
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 15
Module Summary
Key points covered in this module:
 DAS can be:
– An integrated part of the host computer
– Directly connected to a single server
 DAS is made up of a CPU, connectivity, and
storage devices
– There are several options within each of these categories
 DAS connectivity uses block-level access
protocols
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 16
 Check Your Knowledge
 What are the physical elements of DAS?
 Give an example of when DAS is a good solution.
 Describe internal DAS connectivity.
 Describe external DAS connectivity.
 What are some areas that you need to consider as part of
DAS management?
© 2006 EMC Corporation. All rights reserved.
Direct Attached Storage - 17