1. Fault tolerance and disaster recovery
Unit objectives
Discuss disk configuration
Discuss Windows-based replication and NDS/eDirectory partitions and replicas
Discuss backup and UPS

2. Topic A Topic A: System fault tolerance Topic B: Replication
Topic C: Backup and UPS

3. Disaster planning
When creating a disaster plan, some key points to be considered are:
Plan for the worst
Implement physical data security
Protect your critical systems

4. RAID
A set of specifications describing hard disk fault tolerance configurations
The specification:
RAID Level 0
RAID Level 1
RAID Level 2
RAID Level 3
RAID Level 4
RAID Level 5
There is also a RAID 10 (as in RAID “one plus zero”), a RAID 01, and several other “composite” levels.

5. RAID
Both the book and the power point are weak on RAID; the power point also presents the material out of order – so I’ve included 10 slides on RAID from other power points, with editing:
Most popular levels are RAID 0, 1, 5 and 10.
RAID = “Redundant Array of Inexpensive or Independent Disks.”
More disks give more heads give faster transfer rates.
Sometimes the “read” is faster and sometimes the “write” is faster; sometimes both are faster.

6. RAID 0 128K Data Controller Disk 0 Disk 1 256K Data 128K Data
Disk striping (no parity): (data is written across disks in a stripe).
Stripe size is a multiple of 2, size depends on the RAID level in use, RAID 1 and 0 have a high stripe size, usually 128K whereas RAID 5 has a low stripe size usually 16K.
RAID 0 offers no redundancy (no fault tolerance).
The 2 128K stripes of data are written in parallel.
Note that because you have the same amount of data, but written across 2 or more disks, you have more locations from which to read it. This results in faster disk reads – you have 2 heads reading at once.
“Disk writes” are also faster; you have 2 places to which to write!
Disadvantage: There is no fault tolerance, because there is no parity.
Controller
128K Data
Disk 0
Disk 1
256K Data
128K Data

7. RAID 1 (Mirroring)
Minimum of 2 disks, literally one disk is a complete mirror of the other.
One fails the other takes over
When reading can read both disks (two copies of the data) – very fast read and write access.
Um, no – reads are faster – you have 2 identical disks from which to read, but you have to write everything twice! This isn’t as slow as it seems, especially when using 2 controllers – duplexing – but it isn’t fast either.
Controller
256K Data
Disk 0
256K Data
Disk 1
256K Data

8. RAID 1 (Duplexing)
Controller Mirroring (2 controllers) each with a disk
Just like mirroring but with two controllers instead of one, so if one controller goes down, you still have one disk that is fine.
Controller
Disk 0
Software mirroring (RAID1)
Disk 1
Controller
Disk 0
Disk 1

9. RAID 10 or RAID 01 RAID 10 is mirroring (1) then striping (0)
RAID 01 is striping (0) then mirroring (1)
RAID 0+1
RAID 1+0
128K Data
128K Data
128K Data
128K Data
256K Data
256K Data
Disk 0
Disk 2
Disk 0
Disk 2
Disk 1
Disk 3
Disk 1
Disk 3
Write Single Stripe
Mirror
Write Single Stripe
Mirror

10. RAID 10 or RAID 01
Minimum of 4 disks because data needs mirroring and striping.
Massive difference when comes to fault tolerance so be careful!
RAID 10 allows for more fault tolerance – any disk can go so long as it has a mirror.
RAID 01 has poor fault tolerance – loose 1 disk in both mirrors and array fails.
Make absolutely sure you are getting what you think you are getting – there is a difference!!

11. RAID 5 Minimum of 3 disks required.
Uses parity to recalculate data in case of disk failure
an EOR formula = “exclusive or” 1+0 or 0+1 = 1, 0+0 or 1+1 = 0
Critical failure occurs on failure of 2 disks.
Performance degradation on single disk failure.
Uses a smaller stripe size to aid parity calculation.
Controller
16K Data
Disk 0
16K Data
Disk 1
256K Data
16K Parity
Disk 1

12. RAID 5 Data Calculation EOR to calc parity and EOR to re-calc data
Stripe 1 goes to Disk 1 -> (170) Stripe 2 goes to Disk 2 -> (189)
Parity Stripe to Disk 3 is EOR
(23)
is written to disk 3 .
Recovery (Disk 2 has failed)…
Take data from Disk EOR
Take parity from Disk
Data on Disk 2 is

13. Disk mirroring (back to Thomson book)

14. Activity A – page 20-5
Discussing RAID

15. Disk duplexing

16. Discussing disk duplexing
Activity A – page 20-7
Discussing disk duplexing

17. Disk striping with parity
An implementation of RAID Level 5
Normally used on larger networks where data integrity is a critical concern

18. Discussing disk striping with parity
Activity A – page 20-9
Discussing disk striping with parity

19. Volume sets Combines space from up to 32 drives
Cannot contain the system or the boot partition
If one disk area is destroyed, the entire set fails
This is the simpler version of a “striped set”

20. Disk striping Also combines space from up to 32 drives
Each segment must be the same size

21. Managing disk configuration
Activity A – page 20-11
Managing disk configuration

22. Topic B Topic A: System fault tolerance Topic B: Replication
Topic C: Backup and UPS

23. Replication
Offers additional data redundancy on Windows-based networks
Can specify certain data to be copied from one system to another
Common uses include
replication of login scripts to all domain servers
replication of mandatory user profiles
replication of frequently used files across multiples servers to balance the server load

24. Replication Available in Windows NT networks
Helps to copy data automatically from a source system (exporter) to a destination system (importer)

25. Key points about replication
Runs as a background service
After any changes, files must be closed before they can be replicated
Can specify to replicate files immediately after a change in the subdirectory tree
Individual subdirectories might be locked
An exporter can send files to importers
An importer can receive files from one or more exporters
The import directory might be locked
A Windows NT Server might act as both an exporter and an importer

26. Active Directory
Fault tolerance of directory services information is built into the directory model
Every domain controller holds a copy of Active Directory
So, by this syllogism, fault tolerance is “assured”
All domain controllers contain Active Directory
All Active Directory provides fault tolerance
Therefore, all domain controllers provide fault tolerance 

27. File Replication Service
In Windows 2000/Server 2003, the File Replication Service (FRS) replaces the LAN Manager Replication system used in Windows NT
Used to replicate system policies as well as login scripts.
Allows for file replication for domain-based Distributed File System (DFS).

28. Discussing replication
Activity B page 20-13
Discussing replication

29. NDS / eDirectory partitions
Involves division of NDS/e-Directory database
Provides two primary benefits:
Fault tolerance
Performance Increase

30. NDS/e-Directory Partitions & Replicas
Used to store information about all of the objects known to the network
A partition is a logical division of the eDirectory database. A directory partition forms a distinct unit of data in the tree that stores directory information.
Partitions can be created at container level objects, like Organization, Organizational Unit or any objects marked as a container.
An eDirectory has one [ROOT] partition which contains all the objects by default.
Partitions are set up as parent-child objects.

31. NDS/eDirectory directory partitions

32. Managing NDS/eDirectory partitions
Activity B-2
Managing NDS/eDirectory partitions

33. NDS/eDirectory replicas
A replica is a copy or an instance of a user-defined partition that is distributed to a server
Each partition has at least one replica
Examples of Types:
Master replica
Read/write replica
Read-only replica
Subordinate reference

34. NDS/eDirectory Replica Types
There are six types of replicas:
1. Master replica: There can be only one Master replica for a partition. The Master is a read-writeable replica that, most importantly, controls the partition operations and the obituary process.
This type of replica also performs the following operations:
Managing objects (add, remove, move)
Authenticating objects
Managing attributes (add, remove)
By default the first server in the tree holds the Master replica of the [ROOT] partition.

35. NDS/eDirectory Replica Types
2. Read-Write replica: This replica type allows modification to objects and will automatically propagate them to the other replicas based on the timestamps.
You can designate a Read-Write replica as a Master replica.
3. Read-Only replica: This replica type is only readable.
It does not perform any write operations
It will forward all writing requests to a Read-Write replica.
The replica can be designated as a Master replica.

36. NDS/eDirectory Replica Types
4. Filtered Read-Write Replica: This replica contains only a special set of classes and attributes specified by the filter.
The replica can be written and the changes will be synchronized to the other replicas.
5. Filtered Read-Only Replica: The same rules applies to this replica type as the ones to the Filtered Read-Write Replica, but the replica is only readable
Therefore all writing requests are forwarded to a writeable replica.

37. NDS/eDirectory Replica Types
6. Subordinate reference replica: System-generated replicas that don't contain all the objects, attributes and values like a master or a read/write replica.
Therefore, they don't provide fault tolerance.
They are internal pointers generated to contain enough information for eDirectory to resolve object names across partition boundaries.
You cannot create a Subordinate references replica; eDirectory will create it when the server holds a replica of the parent partition, but not one of the child partitions.
It holds no partition data, only information about the "real" replica-holder servers.
So it cannot be designated as a Master without adding a Read-Write or Read-Only replica.

38. Discussing NDS/eDirectory replicas
Activity B Page 20-17
Discussing NDS/eDirectory replicas

39. Replica ring
Made up by the servers that hold replicas for that partition
Documentation of the replica ring might consist of a replica table containing:
A list of servers
A list of partitions
The type of replica stored on each server

40. Discussing replica rings
Activity B-4
Discussing replica rings

41. Fault tolerance Topic A: System fault tolerance Topic B: Replication
Topic C: Backup and UPS

42. Backup and UPS
Backup of the data is necessary to protect the data in the event of loss of power
Three types of backup schemes:
Full backup
Incremental backup
Differential backup

43. Discussing backup schemes
Activity C-1
Discussing backup schemes

44. Removable media An alternative to tapes Includes:
Removable hard drives
Floptical media
Rewriteable CD-ROMs and DVDs
Provide a convenient way to archive data

45. Discussing removable media
Activity C-2
Discussing removable media

46. Backup storage Storing backups in your office isn’t a good idea
Always keep backups in a secure, access-controlled location
Also have backups stored at offsite locations

47. Disaster recovery site options
Cold sites
Warm sites
Hot sites

48. Cold sites
Cold site
Usually a single room in which your data center can be recreated in case of a disaster
Can be on site or off site
Doesn’t actually hold any equipment
Coming back on line after a disaster can take quite a bit of time
Least expensive backup site solution

49. Warm sites Warm site Can be either on site or off site
Contains a fair amount of equipment to create a semi-duplicate of your current data center
Can be live in much less time than a cold site
Is more expensive to create and maintain than a cold site

50. Hot sites
Hot site
Is a complete duplication of your current data center
Is typically off site
Can be up and running in a matter of hours
Very expensive to create and maintain

51. Discussing backup storage
Activity C-3
Discussing backup storage

52. Uninterruptible power supply
Makes sure that the server is powered down, thereby protecting network data
Keeps your system from going down unexpectedly due to line power loss
Uses a battery to supply power to the system

53. Activity C-4
Discussing UPS

54. Unit summary Discussed disk configuration
Discussed Windows replication and NDS/eDirectory partitions and replication
Discussed backup and UPS