[SERVER-18885] Readahead startup warning is inconsistent with the documentation Created: 09/Jun/15  Updated: 11/Oct/18  Resolved: 11/Oct/18

Status: Closed
Project: Core Server
Component/s: MMAPv1
Affects Version/s: None
Fix Version/s: None

Type: Bug Priority: Major - P3
Reporter: Joanna Cheng Assignee: Gregory Wlodarek
Resolution: Done Votes: 0
Labels: None
Remaining Estimate: Not Specified
Time Spent: Not Specified
Original Estimate: Not Specified

Issue Links:
Related
related to SERVER-7335 Warn at startup on machines with sub-... Closed
Operating System: ALL
Sprint: Storage NYC 2018-10-22
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 Description   

2015-06-04T10:20:40.494-0400 [initandlisten] ** WARNING: Readahead for DBPATH is set to READAHEAD_VALUE
2015-06-04T10:20:40.494-0400 [initandlisten] **          We suggest setting it to 256KB (512 sectors) or less
2015-06-04T10:20:40.494-0400 [initandlisten] **          http://dochub.mongodb.org/core/readahead

http://dochub.mongodb.org/core/readahead links to http://docs.mongodb.org/manual/administration/production-notes/#readahead, where we recommend:

Ensure that readahead settings for the block devices that store the database files are appropriate. For random access use patterns, set low readahead values. A readahead of 32 (16kb) often works well.
For a standard block device, you can run sudo blockdev --report to get the readahead settings and sudo blockdev --setra <value> <device> to change the readahead settings. Refer to your specific operating system manual for more information.

These 2 suggestions are inconsistent. We should just refer to the docs and keep our recommendation in one place.



 Comments   
Comment by Gregory Wlodarek [ 11/Oct/18 ]

Marking this as gone away because if others need to use a non-standard read-ahead value (32kB), that's fine, we don't need to warn them.

We don't recommend read-ahead settings for WiredTiger since WiredTiger does its own selection of read sizes, typically determined by its pagesize, which wasn't the case for MMAP, we relied on implicit paging in by the OS.

In regards to the inconsistency with the documentation, the documentation states that 32kB is a value that often works well, and MongoDB recommends a value of 256kB orĀ less. 256kB being the reason to let at least one read-ahead of one large document, since 256kB is the chunk-size of GridFS.

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