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What is network attached storage (NAS)?
What is a Network Attached Storage (NAS)?
NAS is the abbreviation of Network Attached Storage, which represents an HDD based storage box can be connected with an RJ-45 network cable. It could be accessed by all the devices located on the same IP network range, some advanced version could be also connected by a outside device (i.e. another IP network range or WWW), and would be identified as an accessible storage disk through “Network Neighborhood” (Microsoft Windows) / Apple Macintosh OS or Linux Samba, with user/group privilege setting function equipped, it could even prevent important data/files from being accessed by unauthorized people.
Using a NAS, you don’t have to keep your original data-center PC server operating all day long anymore, a smaller size, more stable, less power-consumption, less heat, less cost, easier to access device would make your life more convenient!
What is private storage network? This is very united to the term "cloud computing"
Why Cloud?
* A private hard disk across the Internet
* Your files from any computer or cell phone
* Share and collaborate
* World-class backup and security
1. Private cloud computing can consist of on-demand applications, or storage, or both -- it doesn't matter.
2. It can be based on in the Internet, or on an Intranet -- it doesn't matter.
3. It is easily scalable. Extra storage nodes can often simply be plugged into the system.
4. The underlying technology consists essentially of commodity elements, including servers, storage arrays and data controllers.
5. You can buy or rent cloud storage -- or both. Cloud storage is not just about sending your files off into the Internet. Renting cloud storage (public cloud) or buying cloud storage (private cloud inside your firewall) is a choice, not unlike buying or renting a car.
6. Cloud storage is all about bulk storage, not Tier 1 transactional applications. It is usually not about primary Tier 1, mission critical, transactional storage. It is about data that needs to be accessed only occasionally, and possibly never.
7. You can start with small cloud and scale up as needed. You don't have to be at Amazon scale to realize the major benefits of cloud storage. Moore's Law is your friend; you get great savings when you build your storage cloud with a few servers or hundreds. You can easily start up a useful storage cloud with as few as three to five servers.
Service providers can build out different kinds of clouds with hundreds, rather than thousands of nodes, and thus provide very different services.
Service providers could host a separate cloud for an enterprise customer in a co-location center, thus providing economy as well as security.
8. Storage clouds can be tuned for specific uses. When people think of Amazon S3, they are looking at just one type of cloud. By buying different servers (tuning CPU, memory and hard drives), you can change the performance characteristics of your cloud. You can have clouds tuned for archival needs and be very cost-effective, or you can tune clouds for streaming media performance. The latter, by its very nature, will be more expensive to run and maintain.
Service providers can now build different types of clouds and offer different kinds of services.
9. Clouds are designed to be self-managing. By its very design, a cloud cannot work if it is heavily in need of IT supervision. Automation in storage tiering, provisioning, deduplication and in other factors is a key element.
What is storage area networks (SAN)?
What is a storage area network (SAN)?
Storage is increasingly recognised as a resource in its own right, not merely as a peripheral device. Where in the past direct-attached storage (DAS) was physically connected to the specific server that used it, organisations now want all their servers to be able to share the same storage resources. This is done by creating a second network that connects the servers to shared storage at the device level. Called a storage area network, or SAN, this is separate from the local area network (LAN) and usually exists within a datacentre.
Using this topology, servers no longer need to route data requests through other servers, but have direct access to many different storage devices. There can be operational advantages too, with several servers sharing a single Raid subsystem, for example. But perhaps more importantly, SANs can minimise wastage through over-provisioning. With DAS, each server will normally be allocated extra storage capacity for possible growth, but in a SAN, excess capacity can be pooled.
The key to this is networking technology that can provide the connectivity and bandwidth necessary to compete with DAS. Today, this usually means Fibre Channel, but SANs could also be based on Escon or Ficon technology, and in the future on Gigabit Ethernet.
What is strip size?
Strip size
Strip size multiplied by the number of disks in the array equals the stripe size. Strip size can be any power of 2, from 4 KB to 128 MB.
When defining a striped logical volume, at least two physical drives are required. The size of the logical volume in partitions must be an integral multiple of the number of disk drives used. See Tuning logical volume striping for a detailed discussion.
Strip size is the most important factor affecting RAID 0 performance. However, you should also be aware that a two HD array only provides about a 17% improvement over a single HD on reads. So don't expect some earth shattering performance gain going to RAID 0 when loading games. Strip size depends on the size of the blocks of data you use the most and with which you want to exploit RAID 0. Let's say your avg block size is 4k while playing your favorite games. Divide that number in half to exploit RAID 0. Thus, 4k / 2 = 2k strip size. And you can set NTFS cluster size to 2k via Disk Management in WinXP.
What is the meaning of database must be mounted exclusively?
A database is started in EXCLUSIVE mode by default. Therefore, the
ORA-01102 error is misleading and may have occurred due to one of the
following reasons:
- there is still an "sgadef<sid>.dbf" file in the "ORACLE_HOME/dbs"
directory
- the processes for Oracle (pmon, smon, lgwr and dbwr) still exist
- shared memory segments and semaphores still exist even though the
database has been shutdown
- there is a "ORACLE_HOME/dbs/lk<sid>" file
The "lk<sid>" and "sgadef<sid>.dbf" files are used for locking shared memory.
It seems that even though no memory is allocated, Oracle thinks memory is
still locked. By removing the "sgadef" and "lk" files you remove any knowledge
oracle has of shared memory that is in use. Now the database can start.
POSSIBLE SOLUTION:
Verify that the database was shutdown cleanly by doing the following:
1. Verify that there is not a "sgadef<sid>.dbf" file in the directory
"ORACLE_HOME/dbs".
% ls $ORACLE_HOME/dbs/sgadef<sid
>.dbf
If this file does exist, remove it.
% rm $ORACLE_HOME/dbs/sgadef<sid
>.dbf
2. Verify that there are no background processes owned by "oracle"
% ps -ef | grep ora_ | grep $ORACLE_SID
If background processes exist, remove them by using the Unix
command "kill". For example:
% kill -9 <Process_ID_Number>
3. Verify that no shared memory segments and semaphores that are owned
by "oracle" still exist
% ipcs -b
If there are shared memory segments and semaphores owned by "oracle",
remove the shared memory segments
% ipcrm -m <Shared_Memory_ID_Number>
and remove the semaphores
% ipcrm -s <Semaphore_ID_Number>
NOTE: The example shown above assumes that you only have one
database on this machine. If you have more than one
database, you will need to shutdown all other databases
before proceeding with Step 4.
4. Verify that the "$ORACLE_HOME/dbs/lk<sid>" file does not exist
5. Startup the instance
What is the meaning of database must be mounted as RESTRICTED?
You can start an instance and database in a variety of ways:
* start the instance without mounting a database
* start the instance and mount the database, but leave it closed
* start the instance, and mount and open the database in:
o unrestricted mode (accessible to all users)
o restricted mode (accessible to database administrators only)
About 8i Parallel Server Concepts:
Database Mount Lock (DM)
The mount lock shows whether or not any instance has mounted a particular database. This lock is only used with OPS. It is the only multi-instance lock used by OPS in exclusive mode, where it prevents another instance from mounting the database in shared mode.
In OPS single shared mode, this lock is held in shared mode. Another instance can successfully mount the same database in shared mode. In OPS exclusive mode, however, another instance will not able to get the lock.
