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Exam AWS Certified Database - Specialty topic 1 question 48 discussion

Exam question from Amazon's AWS Certified Database - Specialty
Question #: 48
Topic #: 1
[All AWS Certified Database - Specialty Questions]

A company is looking to migrate a 1 TB Oracle database from on-premises to an Amazon Aurora PostgreSQL DB cluster. The company's Database Specialist discovered that the Oracle database is storing 100 GB of large binary objects (LOBs) across multiple tables. The Oracle database has a maximum LOB size of
500 MB with an average LOB size of 350 MB. The Database Specialist has chosen AWS DMS to migrate the data with the largest replication instances.
How should the Database Specialist optimize the database migration using AWS DMS?

  • A. Create a single task using full LOB mode with a LOB chunk size of 500 MB to migrate the data and LOBs together
  • B. Create two tasks: task1 with LOB tables using full LOB mode with a LOB chunk size of 500 MB and task2 without LOBs
  • C. Create two tasks: task1 with LOB tables using limited LOB mode with a maximum LOB size of 500 MB and task 2 without LOBs
  • D. Create a single task using limited LOB mode with a maximum LOB size of 500 MB to migrate data and LOBs together
Show Suggested Answer Hide Answer
Suggested Answer: B 🗳️
by BillyMadison at July 22, 2020, 5:54 p.m.
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vicks316
Highly Voted 3 years, 7 months ago
C sounds correct since as per link https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html#CHAP_BestPractices.LOBS, "AWS DMS migrates LOB data in two phases: 1. AWS DMS creates a new row in the target table and populates the row with all data except the associated LOB value. 2.AWS DMS updates the row in the target table with the LOB data." This means that we would need two tasks, one per phase and use limited LOB mode for best performance.
upvoted 17 times
khchan123
2 years, 11 months ago
I think it is C. The explanation from vicks316 is irrelevant. From this link -> https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Tasks.LOBSupport.html Using the Max LOB size (K) option with a value greater than 63KB impacts the performance of a full load configured to run in limited LOB mode. During a full load, DMS allocates memory by multiplying the Max LOB size (k) value by the Commit rate, and the product is multiplied by the number of LOB columns. When DMS can’t pre-allocate that memory, DMS starts consuming SWAP memory, and that impacts performance of a full load. So, if you experience performance issues when using limited LOB mode, consider decreasing the commit rate until you achieve an acceptable level of performance. You can also consider using inline LOB mode for supported endpoints once you understand your LOB distribution for the table.
upvoted 4 times
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JobinAkaJoe
3 years, 6 months ago
This doesn't mean we need two tasks. It just explains how LOBs are copied to target in any scenario. First all columns except LOBs then the LOB.
upvoted 2 times
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Shunpin
Highly Voted 3 years, 4 months ago
Selected Answer: B
Limited LOB maxsize is 100MB. This question is about LOB average 350MB. Full LOB is better option
upvoted 14 times
Sathish_dbs
1 year, 7 months ago
the link says it not a hard limit 100MB, it is recommended.
upvoted 1 times
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Shunpin
3 years, 4 months ago
https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Tasks.LOBSupport.html
upvoted 4 times
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MultiAZ
Most Recent 1 year, 3 months ago
Selected Answer: C
I believe the right answer is C As for B, 500 chunk size is way too big and will lead to many failures.
upvoted 1 times
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rrshah83
1 year, 4 months ago
Selected Answer: C
Limited LOB with max size of 500MB makes migration fast.
upvoted 1 times
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jitesh_k
1 year, 5 months ago
Full LOB mode has no information about chunk size. Full LOB mode – In full LOB mode AWS DMS migrates all LOBs from source to target regardless of size. In this configuration, AWS DMS has no information about the maximum size of LOBs to expect. Thus, LOBs are migrated one at a time, piece by piece. Full LOB mode can be quite slow. https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Tasks.LOBSupport.html Either C or D - but C seems more optimal.
upvoted 1 times
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Sathish_dbs
1 year, 7 months ago
the question is confusing, but the lesson here full lob -> incur performance issue, limited lob - smooth, hopefully the same question doesn't repeat, if so then let it come with some additional clarity on performance, either way 500MB is huge and we expect performance issue, this is seems to be meaningless question
upvoted 1 times
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roymunson
1 year, 7 months ago
Selected Answer: B
It can not be C because of "Limited LOB mode – In limited LOB mode, you set a maximum LOB size for DMS to accept. That enables DMS to pre-allocate memory and load the LOB data in bulk. LOBs that exceed the maximum LOB size are truncated, and a warning is issued to the log file. In limited LOB mode, you can gain significant performance over full LOB mode. We recommend that you use limited LOB mode whenever possible. The maximum recommended value is 102400 KB (100 MB)." https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Tasks.LOBSupport.html
upvoted 2 times
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cuibap
1 year, 9 months ago
To optimize for large size LOB, we should use multiple tasks => eliminate A & D. B: we can't configure such large LOB chunk size of 500 MB. Please note that "chunk size" is different from "max LOB size". So I select C but can anyone explain what benefit of task 2 without LOBs?
upvoted 1 times
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rn30
1 year, 10 months ago
B looks to be right Task1 with LOB Tables: Use the full LOB mode in AWS DMS to ensure efficient streaming of LOB data during replication. Set the LOB chunk size to match the maximum LOB size in Oracle (500 MB). This ensures optimal data transfer and minimizes network overhead. Task2 without LOBs: Migrate the remaining tables that do not contain LOBs separately in a task without LOB-specific configurations.
upvoted 2 times
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saikarthikeya777
1 year, 10 months ago
Selected Answer: C
full lob is quite slow ! to optimize we need to use limited lob even if 500mb is not recommended size . Maybe i am super confused
upvoted 2 times
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backbencher2022
2 years, 1 month ago
B is correct. Option C could have been correct however, please note the word "recommended size" for Limited LOB mode which is 100MB. Agreed that 100 MB is not the max limit for limited LOB however, anything above 100MB in limited LOB is not recommended which makes full LOB with 2 different tasks a better option.
upvoted 2 times
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anantarb
2 years, 3 months ago
Selected Answer: B
The Max recommended LOB size is 100MB in limited LOB mode. So can't be C. B is correct. https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Tasks.LOBSupport.html
upvoted 3 times
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teo2157
2 years, 3 months ago
Selected Answer: C
https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html#CHAP_BestPractices.LOBS. Limited LOB mode migrates all LOB values up to a user-specified size limit (default is 32 KB). LOB values larger than the size limit must be manually migrated. Limited LOB mode, the default for all migration tasks, typically provides the best performance. However, ensure that the Max LOB size parameter setting is correct. Set this parameter to the largest LOB size for all your tables.
upvoted 1 times
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parle101
2 years, 3 months ago
Selected Answer: C
C is the answer - https://aws.amazon.com/premiumsupport/knowledge-center/dms-improve-speed-lob-data/
upvoted 1 times
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khun
2 years, 4 months ago
Selected Answer: C
C. is the answer. Faster = 2 task Limited Lob and No Lob Full LOB mode – Migrate complete LOBs regardless of size. AWS DMS migrates LOBs piecewise in chunks controlled by the Max LOB Size parameter. This mode is slower than using limited LOB mode. Limited LOB mode – Truncate LOBs to the value specified by the Max LOB Size parameter. This mode is faster than using full LOB mode.
upvoted 2 times
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shuo82
2 years, 4 months ago
Selected Answer: C
C is correct, 100MB is the maximum recommended value, not a limitation. B is wrong, because 500MB is too huge for chunk setting, connection error would be encountered. When a task is configured to use full LOB mode, AWS DMS retrieves LOBs in pieces. The LOB chunk size (K) option determines the size of each piece. When setting this option, pay particular attention to the maximum packet size allowed by your network configuration. If the LOB chunk size exceeds your maximum allowed packet size, you might see disconnect errors. The recommended value for LobChunkSize is 64 kilobytes. Increasing the value for LobChunkSize above 64 kilobytes can cause task failures.
upvoted 2 times
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rags1482
2 years, 6 months ago
https://docs.aws.amazon.com/dms/latest/userguide/CHAP_BestPractices.html#CHAP_BestPractices.LOBS Answer : C
upvoted 1 times
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