Post on 01-Jan-2016
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Fault tolerance and disaster recovery
Unit objectives
• Discuss disk configuration
• Discuss Windows-based replication and NDS/eDirectory partitions and replicas
• Discuss backup and UPS
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
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.
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.
RAID 0• 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.
Disk 0
Disk 1
Controller
256K Data
128K Data
128K Data
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.
Disk 0
Disk 1
Controller
256K Data
256K Data
256K Data
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.
Software mirroring (RAID1)
Disk 0
Disk 1
Controller
Disk 0
Disk 1
Controller
RAID 10 or RAID 01
• RAID 10 is mirroring (1) then striping (0)
• RAID 01 is striping (0) then mirroring (1)
Disk 0
Disk 1
256K Data
Disk 2
Disk 3
128K Data
RAID 1+0
128K Data
1. Write Single Stripe2. Mirror3. Write Single Stripe4. Mirror
Disk 0
Disk 1
Disk 2
Disk 3
128K Data
RAID 0+1
128K Data
1. Write Single Stripe2. Write Single Stripe3. Mirror
256K Data
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!!
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.
Disk 0
Disk 1
Controller
256K Data
16K Data
16K Data
Disk 116K Parity
RAID 5 Data Calculation• EOR to calc parity and EOR to re-calc data
Stripe 1 goes to Disk 1 -> 10101010 (170)Stripe 2 goes to Disk 2 -> 10111101 (189)Parity Stripe to Disk 3 is 10101010 EOR
10111101------------00010111 (23)
00010111 is written to disk 3 .
Recovery (Disk 2 has failed)…Take data from Disk 1 10101010 EORTake parity from Disk 3 00010111
------------Data on Disk 2 is 10111101
Disk striping with parity• An implementation of RAID Level 5
• Normally used on larger networks where data integrity is a critical concern
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”
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
Replication• Available in Windows NT networks
• Helps to copy data automatically from a source system (exporter) to a destination system (importer)
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
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–
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).
NDS / eDirectory partitions• Involves division of NDS/e-Directory database
• Provides two primary benefits:– Fault tolerance– Performance Increase
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.
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
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.
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.
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.
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.
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
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
Removable media• An alternative to tapes
• Includes:– Removable hard drives– Floptical media– Rewriteable CD-ROMs and DVDs
• Provide a convenient way to archive data
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
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
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
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
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