As you have already discovered, or perhaps are about to discover, maintaining the high-availability database is a tricky prospect. There are hundreds of possible sources of downtime hiding in the crooks and crannies of your enterprise. Many outage situations cascade into each other, where an attempt to resolve a problem creates another outage and quickly spirals further and further out of control. But almost all downtime scenarios can be prevented with careful planning or swift corrective action.
Throughout this book, we discuss numerous technologies that prevent or drastically reduce different database outages. But what are these downtime scenarios? Which possible outages can you protect yourself against? In order to illustrate the various types of situations that threaten the availability of your database, we will illustrate various database problems that can be prevented using the technologies outlined in this book. We have typically provided a worst-case scenario, but we wanted to place the high-availability technologies in Chapters 2 through 11 in a real-world context prior to exploring the configuration and administration of these technologies.
After each downtime scenario, a text box will describe which piece of technology could be employed to prevent the outage, or to recover from it quickly. If you are not interested in reading through the situations, you can skim through this chapter and just look for the boxes. The boxes provide a road map to the available technologies that are discussed in the rest of this book.
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{mospagebreak title=Horatio’s Woodscrews}
For the situations in this chapter, and for the workshops and examples throughout the book, we will use the database from the company Horatio’s Woodscrews, Inc. This is a fictitious company that sells…well…woodscrews. This company has a primary database that holds millions of records concerning woodscrews, woodscrew inventory, and woodscrew orders. The following is the code to create the application owner in the oracle database, along with the three primary tables: Woodscrew, woodscrew_inventory, and woodscrew_orders. These tables will be used throughout the book for all examples, so you might want to make note of this information. After building the tables, a few rows are added so we can manipulate them for labs, like this:
create tablespace ws_app_data datafile
‘/u01/product/oracle/oradata/orcl/ws_app_data01.dbf’ size 100m;
create tablespace ws_app_idx datafile
‘/u01/product/oracle/oradata/orcl/ws_app_idx01.dbf’ size 100m;create user ws_app identified by ws_app
default tablespace ws_app_data
temporary tablespace temp;grant connect, resource to ws_app;
connect ws_app/ws_app;create table woodscrew (
scr_id number not null,
manufactr_id varchar2(20) not null,
scr_type varchar2(20),
thread_cnt number,
length number,
head_config varchar2(20),
constraint pk_woodscrew primary key (scr_id, manufactr_id)
using index tablespace ws_app_idx);create index woodscrew_identity
on woodscrew(scr_type, thread_cnt, length, head_config)
tablespace ws_app_idx;create table woodscrew_inventory (
scr_id number not null,
manufactr_id varchar2(20) not null,
warehouse_id number not null,
region varchar2(20),
count number,
lot_price number);create table woodscrew_orders (
ord_id number not null,
ord_date date,
cust_id number not null,
scr_id number not null,
ord_cnt number,
warehouse_id number not null,
region varchar2(20),
constraint pk_wdscr_orders primary key (ord_id, ord_date)
using index tablespace ws_app_idx);—- Now, add rows to the tables.
insert into woodscrew values (
1000, ‘Tommy Hardware’, ‘Finish’, 30, 1.5, ‘Phillips’);
insert into woodscrew values (
1000, ‘Balaji Parts, Inc.’, ‘Finish’, 30, 1.5, ‘Phillips’);
insert into woodscrew values (
1001, ‘Tommy Hardware’, ‘Finish’, 30, 1, ‘Phillips’);
insert into woodscrew values (
1001, ‘Balaji Parts, Inc.’, ‘Finish’, 30, 1, ‘Phillips’);
insert into woodscrew values (
1002, ‘Tommy Hardware’, ‘Finish’, 20, 1.5, ‘Phillips’);
insert into woodscrew values (
1002, ‘Balaji Parts, Inc.’, ‘Finish’, 20, 1.5, ‘Phillips’);
insert into woodscrew values (
1003, ‘Tommy Hardware’, ‘Finish’, 20, 1, ‘Phillips’);
insert into woodscrew values (
1003, ‘Balaji Parts, Inc.’, ‘Finish’, 20, 1, ‘Phillips’);
insert into woodscrew values (
1004, ‘Tommy Hardware’, ‘Finish’, 30, 2, ‘Phillips’);
insert into woodscrew values (
1004, ‘Balaji Parts, Inc.’, ‘Finish’, 30, 2, ‘Phillips’);
insert into woodscrew values (
1005, ‘Tommy Hardware’, ‘Finish’, 20, 2, ‘Phillips’);
insert into woodscrew values (
1005, ‘Balaji Parts, Inc.’, ‘Finish’, 20, 2, ‘Phillips’);insert into woodscrew_inventory values (
1000, ‘Tommy Hardware’, 200, ‘NORTHEAST’, 3000000, .01);
insert into woodscrew_inventory values (
1000, ‘Tommy Hardware’, 350, ‘SOUTHWEST’, 1000000, .01);
insert into woodscrew_inventory values (
1000, ‘Balaji Parts, Inc.’, 450, ‘NORTHWEST’, 1500000, .015);
insert into woodscrew_inventory values (
1005, ‘Balaji Parts, Inc.’, 450, ‘NORTHWEST’, 1700000, .017);insert into woodscrew_orders values (
20202, ‘2003-09-22 00:02:02’, 2001, 1000, 20000, 64114, ‘NORTHEAST’);
insert into woodscrew_orders values (
20203, ‘2003-09-22 00:02:04’, 2001, 1001, 10000, 64114, ‘NORTHEAST’);
insert into woodscrew_orders values (
20204, ‘2003-09-22 00:02:06’, 2002, 1002, 10000, 64114, ‘NORTHWEST’);
insert into woodscrew_orders values (
20205, ‘2003-09-22 00:02:08’, 2002, 1003, 30000, 64114, ‘NORTHWEST’);
insert into woodscrew_orders values (
20206, ‘2003-10-04 00:02:12’, 2002, 1004, 10000, 80903, ‘SOUTHWEST’);
insert into woodscrew_orders values (
20207, ‘2003-10-04 00:02:14’, 2001, 1003, 20000, 80903, ‘SOUTHWEST’);
insert into woodscrew_orders values (
20208, ‘2003-10-04 00:02:16’, 2002, 1002, 30000, 64114, ‘SOUTHWEST’);
insert into woodscrew_orders values (
20209, ‘2003-10-04 00:02:18’, 2003, 1001, 40000, 90210, ‘NORTHWEST’);
insert into woodscrew_orders values (
20210, ‘2003-11-04 00:02:20’, 2005, 1000, 10000, 83401, ‘SOUTHEAST’);
insert into woodscrew_orders values (
20211, ‘2003-11-04 00:02:22’, 2002, 1005, 10000, 83401, ‘SOUTHEAST’);
insert into woodscrew_orders values (
20212, ‘2003-11-04 00:02:24’, 2001, 1004, 10000, 64114, ‘NORTHEAST’);
insert into woodscrew_orders values (
20213, ‘2003-11-04 00:02:26’, 2003, 1003, 10000, 64114, ‘NORTHEAST’);
insert into woodscrew_orders values (
20214, ‘2003-12-04 00:02:28’, 2002, 1001, 20000, 64114, ‘SOUTHEAST’);
insert into woodscrew_orders values (
20215, ‘2003-12-04 00:02:30’, 2001, 1000, 10000, 80903, ‘NORTHWEST’);
insert into woodscrew_orders values (
20216, ‘2003-12-04 00:02:32’, 2005, 1001, 50000, 80903, ‘SOUTHWEST’);
insert into woodscrew_orders values (
20217, ‘2003-12-04 00:02:34’, 2003, 1003, 70000, 90210, ‘SOUTHWEST’);
commit;
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{mospagebreak title=User-Defined Availability}
Sometimes database availability is not defined by the database administrator. Even if the DBA can connect to the database and select against tables, users may not be having the same experience from their application.
Take the example of Horatio’s Woodscrews. Due to massive transaction processing that occurs over the course of the day against the woodscrew_orders table, thousands of new rows could be added over the course of a day. When the CEO goes to check on his reports which show how many woodscrews are being sold, and to which customers, the report is creating an ad hoc SQL query against the entire table. Because of the amount of data, and the simplistic data design, these queries might not return in what the CEO envisions as a reasonable amount of time. The DBA of the company is inundated with uncomfortable calls from impatient executives wondering why the database is “down.”
The CEO has a button on his desktop application, see, that says nothing more than woodscrew_orders_by_customer. He has no interest in hearing about how long-running queries against massive amounts of data might take a little longer than his expectations. He just wants the report to be available.
Test and Development Availability
Just as with CEOs demanding faster reports, sometimes availability is defined more by a human resource issue than actual data availability. At Horatio’s Woodscrews, they run all their production systems on Solaris, but are investigating Linux as an alternative. The first step of the investigation is to set up the test and development shop with Linux servers, and then move a copy of production data to the new Linux servers for application testing.
The Linux servers arrived late in the week, and were configured with the Oracle software image over the weekend. On Monday, the development lead needs to get the data moved from the production Woodscrew database to the Linux servers. So the development lead contacts the senior DBA for the database and asks that they transition the data to the new Linux servers.
The dilemma for the DBA is that getting a read-consistent view of the Woodscrew tables is nearly impossible for long enough to get an export to complete. The only time there is a large enough window for such an operation would be the following weekend. The backups cannot be used, because they are backups of Solaris datafiles. But if they wait for the weekend, the development team will sit idle all week on a project that has imminent deadlines. How can they get the data moved across platforms in an expedient way?
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{mospagebreak title=Cyclical Database Resource Requirements}
User needs often cycle over the course of time, based on business needs, such that some users will not find the database to be inaccessible until there is a massive amount of activity in a certain area, and then suddenly there are not enough resources.
Accounts Receivable, at Horatio’s Woodscrews, has this problem. As the end of each month comes to a close, they have to close out the previous month’s accounts and run massive reports on the status of all opening and closing accounts. This month-end processing sends the database processing needs of the AR department through the roof—but only for a week or so, before it settles back into a more routine usage pattern.
At the same time, Human Resources typically finds its peak at the lead-up to the end of the month, as it processes employee hours, salaries, and payments. They will need the most resources, then, at a different time than the AR department.
The reporting of these two groups always affects the Order Entry group, as they have a relatively steady database resource usage pattern over the entire course of the month. However, as the summer approaches, the hardware stores begin to increase the stock of woodscrews, and so orders will steadily increase as the summer approaches, and then steadily decrease as weather cools.
What Reports Were Those, Exactly?
The problem with attempting to allocate the correct resources to the correct applications at the correct time of month or year is that often it is nearly impossible to get trended data on usage over time.
The DBA at Horatio’s Woodscrews is faced with this dilemma. The different application groups have been complaining about the performance of their reports at different times, but the DBA has not been able to get accurate data on which problem reports or application batch processes are truly in need of more resources. He’s been reading up on his performance tuning techniques, but the month is coming to an end again, and he can already hear his pager beginning to beep.
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{mospagebreak title=Out of Space in the Woodscrew Tablespace}
Availability can be significantly hampered by routine database management tasks. Take something as straightforward as datafile management. Where the files reside, how big they are, and on what disks, can lead to significant availability problems.
Horatio’s Woodscrew database recently ran out of disk space while autoextending a datafile in the WS_APP_DATA tablespace. They had turned autoextend on so that they would not have to be faced with disk management. But business has been booming, and the orders and customer tables are growing dramatically. In addition, new reports are being generated by upper management looking to mine the data for more edge in the marketplace. In a word, the single datafile on a single disk volume has begun to fall short of their needs. They recently had disk problems, and the system administrator is reporting excessive I/O on WS_APP_DATA datafiles. A disk burnout is imminent.
However, reorganizing the datafiles across new disk volumes not only means an explicit investigation into which tables, exactly, are getting hit the hardest, but also the outage that would be required while the tablespace is reorganized across new datafiles on new volumes. Such a massive undertaking could take days.
Downtime for Hardware Fixes
Database availability can be taken out of the hands of the DBA when an outage is related to a hardware problem on the database server. At Horatio’s Woodscrews, they have had users complaining about losing their connection to the database intermittently. The DBA could find nothing wrong with their setup, but the system administrator ran some diagnostics that pointed to a flaky network interface card. The solution is to replace the network card as soon as possible. Doing so means taking the database down, shutting down the entire server, and replacing the card. With month-end reports running around the clock in order to complete on time, shutting down the database will mean that the reports will have to wait until after the database comes back up.
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{mospagebreak title=Restarting Long-Running Transactions}
In the event of some kind of instance outage, there will inevitably be transactions that will stop, and then have to be restarted manually. This can lead to significant shortcomings in time-sensitive reporting and processing.
At Horatio’s Woodscrews, the developers that were sitting idle due to the Linux servers having no data decided to try and get a little work done by connecting to the production database to review some of the database structure. However, one of the developers had installed a beta version of the ODBC drivers. When this ODBC driver connected to the production database, it caused a heap corruption that resulted in an ORA-600. The ORA-600 caused the database to crash.
Just like that, all the long-running reports that had been generated by the Accounts Receivable teams began to receive the error ORA-3113 “End-of-File on Communication.” The DBA’s pager started to beep, and beep, and beep. He was able to restart the database, but all of the long-running transactions had to be restarted.
Slow Crash Recovery
Even after the DBA restarted the database, it seemed to be hung at the startup command. There were no errors, but when he checked the alert log, he noted that the last line indicated the database was performing instance recovery (also known as crash recovery). This recovery must be done to recover any changes in the redo logs that have not been committed to the datafiles. Depending on the size of the redo logs, and the frequency of checkpoints, this can take a considerable amount of time. When there is a lot of pressure to make a database available as quickly as possible, waiting on a poorly tuned crash recovery session can be excruciating.
Dealing with Block Corruption (ORA 1578)
At Horatio’s Woodscrews, the system administrators finally installed the new network card. With the system down, they took the opportunity to install new drivers for their storage area network (SAN) device. Then they contacted the DBA to let him know he could start the database.
The DBA starts the database, and begins to run through the system checks to make sure all the applications will start up correctly. Then he initiates all the reports that failed because of the shutdown. Everything seems to be going smoothly, until he notices that one of the reports has an error:
ORA-1578: ORACLE data block corrupted (file # 121, block # 68)
As the DBA began to investigate the problem, the system administrators called him back: the new SAN driver is causing problems, and he should shut down the database immediately. But the damage has already been done—database corruption.
But there is only one corrupt block, in the WS_APP_DATA01.DBF file. Just one block out of thousands of perfectly good blocks. With datablock corruption, the solution to the problem is to restore the entire datafile from backup, and then apply archivelogs to recover the file. Because the datafile contains parts of the Woodscrew, woodscrew_orders, and woodscrew_inventory tables, these objects will be offline until the file can be restored and recovered. The brief outage for the hardware fix now has been extended to include the restore of a file from backup, and then the application of the archivelogs to that file. The tablespace will not be available until recovery completes.
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{mospagebreak title=Waiting for the File to Restore from Tape}
For our poor DBA at Horatio’s Woodscrews, it only gets worse. He needs to restore the file WS_APP_DATA01.DBF from backup and then perform media recovery using archivelogs. He contacts the backup administrator and asks that the file be restored from tape.
After some shuffling around, the backup administrator finds the last backup and begins the tape restore. Because of the size of the file, this will take a few hours to get the file queued over the network from the tape jukebox to the database server. The DBA asks if there are any backups of the file on disk, knowing that there is plenty of unused space on the new SAN that could be used for disk backup of important files. The backup administrator just runs the tape servers, though, and knows nothing about the SAN. That’s a different department.
RAC and the Single Point of Failure
The DBA for Horatio’s Woodscrews is no fool. He knows that he needs to transition the data to a RAC cluster. As part of the preparation, he has configured the Sales Department’s smaller database on a RAC cluster with two nodes. He set it up quickly, with minimal configuration, and it’s been running smoothly for a few weeks.
However, the network configuration issues that affected the production database had also affected the Sales system. One of the nodes in the cluster was not available on the network. The DBA wasn’t worried, however, because the other node was still available.
But one of the files was asking for media recovery, so the DBA issued the recovery command and it asked for an archivelog that was required for recovery. The DBA pressed ENTER to accept the default archivelog. But the media recovery session failed. He looked a little closer, and noted that the requested archivelog was on the unavailable node. Suddenly, his RAC cluster was not operational as he waited for the other node to become available so he could perform recovery.
Rewinding the Database
Perhaps the most difficult outage situations are those tricky logical errors introduced by the users themselves—when a user updates the wrong table, or updates the wrong values. These types of errors are tough to overcome because they are not perceived by the database as errors, but just another transaction. Typically, user errors do not occur in a vacuum; an erroneous update can occur alongside hundreds of correct updates. Pretty soon, the bad data is buried by thousands of additional updates. How can you fish just one of those transactions out? Can you “rewind” the database back to a previous point in time?
At Horatio’s Woodscrews, the problem was reported by the Accounting team. As they went through their month-end processing, they began to realize that the data seemed incorrect. All the roll-up values seemed to be impossibly high, even during a good month. They could not figure out what had happened, but the data had been incorrectly entered at some point in the morning. Now, in the afternoon, they came to the DBA and asked that he “start over” all of their tables as they had looked in the morning.
The options for restoring just a few tables are limited, without rolling the entire database back to a previous point-in-time. Oracle has provided for a tablespace point-in-time recovery (TSPITR), where just a single tablespace is restored to a previous point. But that is labor-intensive, and the smallest unit that can be restored is the entire tablespace. The Accounting group does not want to redo all their work for the day, just the work in a few tables.
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{mospagebreak title=The Dropped Table}
Like an incorrect update to the table, an inadvertently dropped table can be catastrophic. Unlike an inadvertent DML statement (insert, update, or delete), a drop cannot be explored and fixed manually. Once dropped, the table must be restored from a backup of some sort.
The DBA at Horatio’s Woodscrews did this one to himself: he was trying to clean up unused objects in the production database. There were some leftover tables that had been used for testing purposes in a now-abandoned user’s schema. The DBA was reviewing the objects and then dropping them to free up the extents. However, for the Woodscrew table, he put the wrong username in the DROP statement by accident, and he knew it immediately: ws_app.woodscrew had just been dropped.
The Truncated Table
Another deadly user error can be the use of TRUNCATE to remove rows of an incorrect table. With no undo generated, a truncate is permanent in a way that even a Flashback Transaction or LogMiner operation cannot assist with. Once truncated, the data is missing, and nothing can be done except to restore from a backup and then cancel recovery prior to the truncate.
The DBA was still trying to figure out how to restore the Woodscrew table when he was interrupted by a page. Someone in the Sales group, impatient because their cluster was down, had logged into the production database with a borrowed password. The salesperson had been trying to delete rows from a small sales table, but it was taking too long so he used TRUNCATE. But the production table he truncated had more than just rows for his region, and suddenly the woodscrew_inventory table was empty:
select count(*) from WS_APP.WOODSCREW_INVENTORY;
no rows selected.
Connecting Online, Identical Databases
The DBA at Horatio’s Woodscrews finally found the time to move the production data from the Solaris production database to the Linux servers run by the Test and Development team. The development lead was very excited about the performance they were getting from the Linux boxes, and the application code ported with very little trouble.
In a few weeks, however, the Solaris box was simply overworked. New orders were coming in faster than ever, and the new warehouses were coming online in four new distribution areas. With all the new data, the database began to bog down. The Chief Information Officer approached the DBA and asked a simple question: how can we leverage the Linux servers for our production system? Can we start connecting the Order Entry group to the Linux database, for instance, and just keep the internal groups running against the Solaris system?
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{mospagebreak title=Complete and Total Disaster}
Disaster struck Horatio’s Woodscrews on a particularly wet spring day. Rain had been falling steadily for weeks in the city. The DBA was sleeping soundly, dreaming of data protection systems, when his pager began to beep. He called the number groggily, looking at the clock. It was the middle of the night.
A water main had busted near the Woodscrew building, and the basement had flooded. The basement held all of the database systems—the production Solaris box, the Sales RAC cluster, the test and dev servers, everything. The utility company was marking the area as a disaster area, and not letting anyone near the building.
All the servers were lost. The DBA had to think fast. Did they move the tape backups off-site, as was proposed a few months back? He could not remember, so he called the backup administrator. Did they have those old Solaris boxes they’d retired last year someplace? The DBA dialed the system administrator.
A conference call was put together in the middle of the night. Where were the archived tape backups? Were they moved to a different building? When was the last time the tapes were moved to the other location? How much data would be lost in the flood?
The questions flew around to the system administrator—Where could they find new systems to install Oracle? Would there be enough disk space without the SAN device? Should they contact the vendors to get emergency hardware shipped in?
Then the questions came to the DBA: Could we rebuild the data once the drives were salvaged from the flood? Could we hobble along on one of the older servers at the other office? How fast could he get Oracle installed, patched, and ready to start the file restore from the archive tapes?
And, finally, the ultimate question: Who’s going to call the CEO and tell him what happened?
Where to Go from Here
The downtime scenarios that we have tried to illustrate in this chapter are but the tip of the iceberg. We wanted to show you that there are common, everyday situations that can be fixed using new functionality in Oracle Database 10g. Often, this is functionality that you already have at your fingertips and can leverage right now, with just a few steps that we outline in the following chapters.
We also wanted to show you some of the more disruptive events that can immobilize a database: user errors, such as a truncated table, or a hardware failure. These types of events happen less frequently, but the outage can be extremely long without careful planning that accounts for the possibility. From RAC to Flashback, careful planning and forethought will guide you through these disruptions.
Even less predictable, and less frequent, is the complete loss of a system due to natural disaster or site network outage. In situations where the entire database is lost and cannot be quickly restored, there are important questions that begin to circulate, and some of them have to do with who’s at fault for the lack of a disaster plan. Oracle Data Guard offers one of the most advanced disaster-proofing solutions on the market.
So, now that you’ve explored just a few of the situations that might cause an outage, its time to explore the technologies that can prevent (or significantly reduce) those disruptive downtimes.
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