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siva3155 · 5 years ago

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300+ TOP Memory Management Interview Questions and Answers

Memory Management Interview Questions for freshers experienced :-

1. What is the significance of having storage clause? We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updations etc. 2. What is the functionality of SYSTEM table space? To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage. 3. How does Space allocation table place within a block? Each block contains entries as follows Fixed block header Variable block header Row Header,row date (multiple rows may exists) PCTEREE (% of free space for row updation in future). 4. What is the role of PCTFREE parameter is storage clause? This is used to reserve certain amount of space in a block for expansion of rows. 5. What is the OPTIMAL parameter? To avoid the space wastage we use OPTIMAL parameter. 6. What is a shared pool? The Shared Pool environment contains both fixed and variable structures. The Fixed structures remain relatively the same size, whereas the variable structures grow and shrink based on user and program requirements. Used To Store Most Recently Executed SQL Statements Most Recently used Data definitions It Consists of two Key performance - related memory structures Library Cache & Data Dictionary Cache Shared Pool is sized by SHARED_POOL_SIZE 7. What is mean by Program Global Area (PGA)? It is area in memory that is used by a Single Oracle User Process. 8. What is a data segment? Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored. 9. What are the factors causing the reparsing of SQL statements in SGA? Due to insufficient Shared SQL pool size. Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE. LOGICAL & PHYSICAL ARCHITECTURE OF DATABASE. 10. What is Database Buffers? Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.

Memory Management Interview Questions 11. What is dictionary cache? Dictionary cache is information about the databse objects stored in a data dictionary table. 12. Which parameter in Storage clause will reduce number of rows per block? PCTFREE parameter Row size also reduces no of rows per block. 13. What is meant by free extent? A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free. 14. How will you force database to use particular rollback segment? For perticular transaction Alter system set rollback segment 'name'; For database, we can set in pfile. Rollback_segment='name' . 15. How can we organize the tablespaces in Oracle database to have maximum performance? Store data in tablespaces to avoid disk contension.system tablespace-recursive callsuserdata-users objectsindex tablespace-for indexesrollback segmnets-undo tablespace or manual rollback segmentsplace application specific data in respective tablespaces.Place all these tablespaces in saperate disks.Try to implement raid-0 (striping) for better performance. 16. How will you swap objects into a different table space for an existing database? Export the user Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql. Drop necessary objects. Run the script newfile.sql after altering the tablespaces. Import from the backup for the necessary objects. 17. What is redo log buffer? Changes made to entries are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first brought to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size. 18. What is meant by recursive hints? Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of Data Dictionary Cache. 19. How can we plan storage for very large tables? Limit the number of extents in the table Separate Table from its indexes. Allocate Sufficient temporary storage. 20. How will you estimate the space required by a non-clustered tables? Calculate the total header size Calculate the available dataspace per data block Calculate the combined column lengths of the average row Calculate the total average row size. Calculate the average number rows that can fit in a block Calculate the number of blocks and bytes required for the table. After arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table. 21. It is possible to use raw devices as data files and what are the advantages over file system files? Yes. The advantages over file system files. I/O will be improved because Oracle is bye-passing the kernnel which writing into disk. Disk Corruption will be very less. 22. What is a Control file? The Control File is a small binary file necessary for the database to start and operate successfully. Each Control file is associated with only one Oracle database. Before a database is opened, the control file is read to determine if the database is in a valid state to USE. The Control file is not accessible, the database does not function properly. 23. How will you monitor rollback segment status? By using dictionaray view's called v$rollstat,dba_rollback_segs. 24. How will you monitor the space allocation? This can be monitored in DB_data_files. 25. Why query fails sometimes? Due to syntax errors. 26. How the space utilization takes place within rollback segments? By correctly fixing optimal size. 27. How will you create multiple rollback segments in a database? create rollback segment roll1tablespace roll1. 28. What is a rollback segment entry? When ever changes happend to the database previous change will be there in the rollback segment. 29. What is hit ratio? Hit Ratio is the ratio of shared SQL and PL/SQL items found in the Library Cache versus physical storage.It can also be defined in a mathematical expression as 1 - ((physical reads) / (db block gets + consistent reads)). 30. What are disadvantages of having raw devices? We should depend on export/import utility for backup/recovery (fully reliable) The tar command cannot be used for physical file backup, instead we can use dd command which is less flexible and has limited recoveries. StumbleUpon Digg Delicious Twitter FaceBook LinkedIn Google Yahoo MySpace Tell Your Friend 31. What is use of rollback segments in Oracle database? When a user updated a particular table (for example 100 rows) the old value will be retained in the roll back segments(Oracle 8) and now it is Undo segment (oracle 9i). If the user issue a rollback command the old value will be taken from the rollback segment(that too if undo_retention parameter set properly in the parameter file). 32. What is advantage of having disk shadowing / mirroring? Shadow set of disks save as a backup in the event of disk failure. In most Operating System if any disk failure occurs it automatically switchover to place of failed disk. Improved performance because most OS support volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks. 33. How redo logs can be achieved? LGWR process wirtes all change vectors from theredo log buffer to online redo log file sequentially. 34. What is redo log file mirroring? Multiplexing Redo log file called Mirroing. ( Keeping multiple copies in different disks) 35. How to implement the multiple control files for an existing database? Edit init.ora file set control_files parameter with multiple location shutdown immediate copy control file to multiple locations & confirm from init.ora contol_files parameter start the database. run this query for changes confirmation - select name from v$controlfile; 36. What is SGA? How it is different from Ver 6.0 and Ver 7.0? The System Global Area in a Oracle database is the area in memory to facilitates the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database. The structure is Database buffers, Dictionary cache, Redo Log Buffer and Shared SQL pool (ver 7.0 only) area. 37. What is a Shared SQL pool? The data dictionary cache is stored in an area in SGA called the Shared SQL Pool. This will allow sharing of parsed SQL statements among concurrent users. 38. What is mean by Program Global Area (PGA)? It is area in memory that is used by a Single Oracle User Process. 39. List the factors that can affect the accuracy of the estimations? The space used transaction entries and a deleted record does not become free immediately after completion due to delayed cleanout. Trailing nulls and length bytes are not stored. Inserts of, updates to and deletes of rows as well as columns larger than a single datablock, can cause fragmentation and chained row pieces. 40. What are the different kind of export backups? Full back - Complete database Incremental - Only affected tables from last incremental date/full backup date. Cumulative backup - Only affected table from the last cumulative date/full backup date. 41. What is cold backup? What are the elements of it? Cold backup is taking backup of all physical files after normal shutdown of database. We need to take. All Data files. All Control files. All on-line redo log files. The init.ora file (Optional) 42. What is a logical backup? Logical backup involves reading a set of database records and writing them into a file. Export utility is used for taking backup and Import utility is used to recover from backup. 43. What is hot backup and how it can be taken? Taking backup of archive log files when database is open. For this the ARCHIVELOG mode should be enabled. The following files need to be backed up. All data files. All Archive log, redo log files. All control files. 44. What is the use of FILE option in EXP command? To give the export file name. 45. What is the use of GRANT option in EXP command? A flag to indicate whether grants on databse objects will be exported or not. Value is 'Y' or 'N'. 46. What is the use of INDEXES option in EXP command? A flag to indicate whether indexes on tables will be exported. 47. What is the use of ROWS option in EXP command? Flag to indicate whether table rows should be exported. If 'N' only DDL statements for the database objects will be created. 48. What is the use of PARFILE option in EXP command? Name of the parameter file to be passed for export. 49. What is the use of ANALYSE ( Ver 7) option in EXP command? A flag to indicate whether statistical information about the exported objects should be written to export dump file. 50. What is the use of FULL option in EXP command? A flag to indicate whether full databse export should be performed. 51. What is the use of OWNER option in EXP command? List of table accounts should be exported. 52. What is the use of TABLES option in EXP command? List of tables should be exported. 53. What is the use of RECORD LENGTH option in EXP command? Record length in bytes. 54. What is the use of INCTYPE option in EXP command? Type export should be performed COMPLETE,CUMULATIVE,INCREMENTAL 55. What is the use of RECORD option in EXP command? For Incremental exports, the flag indirects whether a record will be stores data dictionary tables recording the export. 56. What is the use of ROWS option in IMP command? A flag to indicate whether rows should be imported. If this is set to 'N' then only DDL for database objects will be executed. 57. What is the use of INDEXES option in IMP command? A flag to indicate whether import should import index on tables or not. 58. What is the use of GRANT option in IMP command? A flag to indicate whether grants on database objects will be imported. 59. What is the use of SHOW option in IMP command? A flag to indicate whether file content should be displayed or not. 60. What is the use of FILE option in IMP command? The name of the file from which import should be performed. 61. What is use of LOG (Ver 7) option in EXP command? The name of the file which log of the export will be written. Memory Management Questions and Answers Pdf Download Read the full article

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longevitytechnologies-blog · 7 years ago

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7 Ways to Keep DNS Safe

The Domain Name System (DNS) has long been a favored target for threat actors looking to disrupt victims. Whether criminals are looking to use DNS to misdirect traffic in order to steal information, gain access, or launch attacks that deny access to a victim's resources, it is a critical link that can become a huge vulnerability. DNS vulnerability was put under the spotlight in the Mirai attack on the DynDNS service in 2016. In that case, attacking a single DNS source had an impact on scores of major organizations. And that's one of the great attractions DNS has as a target: Disrupting DNS can have an outsize impact on the organization (or organizations) hit by an attack. Other qualities make DNS a favorite tool for hackers. Because the information returned is considerably larger than the query, and DNS is a service that nearly every firewall will allow to pass, DNS servers make useful amplification tools in DDoS attacks. That usefulness means DNS servers and services need to be protected in two different dimensions. First, DNS must be protected so that it continues to resolve names consistently and correctly for the organization it serves. Next, it must be protected so that it can't be used as a weapon against other organizations and individuals. Many of the steps to protect one will protect the other, but some defensive mechanisms focus on one aspect or the other. Many of the protective steps on this list can be taken without rushing out to buy new networking hardware. The question for many organizations will be how to prioritize defensive steps and ensure that all the steps taken work in harmony to protect an organization's network, applications, and users.

DNSSEC

To a great extent, protecting DNS today begins with DNSSEC. The DNS Security Extensions handle one set of tasks, but it's an extremely important set in the overall scheme of things. DNSSEC is all about making sure that the server (or service) you want to talk to is the one you're actually talking to. DNSSEC uses a DNSSEC-validating DNS resolver to check DNS signatures and ensure that the resolution information has not been changed and the responding server is the correct server. It's important to note that the signatures in DNSSEC aren't used for any sort of encryption — they're only responsible for validating the identity of the servers involved. It's also important to note that DNSSEC can protect more than Web pages. Any service that uses a DNS-based address, from email to instant messaging, can benefit from the server authentication provided by DNSSEC.

Distribute DNS

DNS is, by its very nature, a distributed architecture. From the root servers, to authoritative servers, to the local DNS servers that organizations might host to speed queries, DNS was designed to spread its workload to a huge number of servers. That's a good thing, but it can be even better if an organization optimizes the distribution of its resolving servers. Distribution in service to security can mean having multiple DNS servers or services available in a redundant configuration. As an example, if your organization manages its own DNS server, a contract with a DNS service such as Oracle Dyn, Edgecast, or Cloudflare can be a significant contributor to system resilience in the case of a DDoS attack against the local server or an identified case of a server being used as the reflector in an attack on another. Building the switchover into the playbook for attack remediation (and practicing the procedures in the playbook on a regular basis) will be instrumental in minimizing the effects of a DNS-targeting DDoS attack on the organization.

SIEM

Security information and event management (SIEM) systems are not specific to DNS, but they can be critical for ensuring the continued operation of the DNS server in the case of an attack. As with any other network service being monitored, the key to effective use of the SIEM is linking the DNS server's information source to the SIEM. While there are elements of the linking that will be specific to the particular SIEM in question, other aspects are common to many systems. In addition, staff will need to decide which events are appropriate for which types of alerts and automated responses. It's obvious that SIEMs are not the only security devices that are available to protect DNS. In particular, a next-generation firewall (NGFW) may be the central component in a security matrix that includes DNS protection. But for most organizations, including DNS information in the SIEM's input will be an important early step in protecting their DNS.

Response Policy Zones (Blacklist)

One of the most common ways that DNS is used for an attack is when a DNS server is fed incorrect or counterfeit data. This article has already presented DNSSEC as one method for fighting incorrect server information. But another method, the DNS Response Policy Zone (RPZ), can be particularly useful when either bad or preferred servers are known. The first use of the RPZ is to block access to known bad destinations via a black listQ@. These could be recognized command-and-control (C&C) servers for botnets and malware or websites that are known to be hotbeds of malware distribution. A number of different organizations maintain lists of these known servers — organizations that include Dissect Cyber, Infoblox, Spamhaus, and SURBL. Depending on the nature of the bad site, users entering the link can be dropped, redirected to the "real" site for which the masquerade is underway, or directed to an information page explaining that the chosen site is bad and can't be visited from the organization's equipment and network. These responses demonstrate another set of uses from a different RPZ capability: the whitelist.

Response Policy Zones (Whitelist)

Just as RPZs can prevent users from visiting known bad sites, they can ensure that careless or clumsy users go to known good sites that can educate and protect them from malware writers and themselves. RPZ was developed as part of the Berkeley Internet Name Domain (BIND) system that is part of virtually every Linux distribution and is the default DNS server for Linux. When blocking sites, as in the blacklist, RPZ is often described as being a DNS firewall. When it doesn't simply block but redirects the user to a synthesized, known good or informational site, it is part of a "walled garden" of safe destinations. BIND is open-source and vendor-neutral. A number of different companies and organizations provide the reputation information on which RPZ responses are built; it is possible to subscribe to multiple reputation lists for more complete or comprehensive coverage.

Rate Limit

DNS becomes a weapon when a DNS server is flooded with requests from a spoofed address; when data is returned, it overwhelms the innocent victim. The weaponization can be reduced when the rate of responses to queries from a single address is limited. In BIND, rate limiting is relatively straightforward to implement using a standard command. With this, an administrator can specify how many queries from a single address per second will receive a response. In other DNS systems, such as the Windows DNS server, the process is similar, though the options for fine-tuning the limits might vary so that queries from known good addresses or subnets are handled differently than those from "the wild." Rate limiting, set up properly, can restrict the ability of hackers to use the DNS server as a weapon while allowing it to serve the DNS needs of the organization's legitimate users. Issues like IPv6, with its larger headers, make setting rate limits more complex but also make using rate limiting more critical to maintain good Internet neighbor cred in the online neighborhood. DNS remains a critical protocol for Internet activity of all types. Unfortunately, those activities will continue to include malicious actors for the foreseeable future. Using available services and technology will allow organizations to be safer in their DNS use while maintaining the performance and functionality their users demand. via 7 Ways to Keep DNS Safe Read the full article

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