Ancient History Mark (AHM) Clarifications
H Team,
In the Vertica Knowldge base , In link "https://community.dev.hpe.com/t5/Vertica-Knowledge-Base/Understanding-Vertica-Epochs/ta-p/233749", I got explanation of AHM as
"A large epoch map can increase the catalog size. The ancient history mark is the epoch prior to which historical data can be purged from physical storage.You cannot run any historical queries prior to the AHM. By default, Vertica advances the AHM at an interval of 5 minutes to be equal with LGE.".
I need clarification for the below set of items.
1) A large epoch map can increase the catalog size. What is epoch map, is there any catolog table for this ?
2) Explanation says that "You cannot run any historical queries prior to the AHM", what historical queries exactly means, is it only the retriving the DELETE data or some thing else ?
3) As per definition it says AHM will be moved to LGE at an interval 5 mins, if so the maximum retention of deleted data in Vertica is just 5 mins ?
4) If the above point is valid then what is the significae of the below parameters in relationship with AHM,
HistoryRetentionTime,HistoryRetentionEpochs and PurgeMergeoutPercent. Kindly elobarate
Regards,
Narayan.
Comments
Hi
See replay to your post
1) A large epoch map can increase the catalog size. What is epoch map, is there any catolog table for this ?
<Eli> Yes , there is such catalog table -> epochs table , you can run select * from epochs and you will see epoch map to timestamp info .
2)Explanation says that "You cannot run any historical queries prior to the AHM", what historical queries exactly means, is it only the retriving the DELETE data or some thing else ?
<Eli > Historical querys provide you the ability to access data that was already being deleted see more info her
https://my.vertica.com/docs/7.1.x/HTML/Content/Authoring/AnalyzingData/Queries/HistoricalSnapshotQueries.htm
3) As per definition it says AHM will be moved to LGE at an interval 5 mins, if so the maximum retention of deleted data in Vertica is just 5 mins ?
<Eli> Now that you know what is historical querys , you can understand that each 5min your ability to access historical data is shrink
I hope it help
Thanks
Hi,
Thanks for your nice explnation, now I am clear with the 3 clarifications, will it be possible for you share some light on 4 th question as well.
4) If the above point is valid then what is the significae of the below parameters in relationship with AHM,
HistoryRetentionTime,HistoryRetentionEpochs and PurgeMergeoutPercent. Kindly elobarate
My understanding is that after AHM is synced up with LGE in 5 mins time , then Mergeout process will purge the deleted records having EPOCH value less than AHM. Correct me if I am wrong.
If my above understanding is correct. In case if I set PurgeMergeoutPercent value as 25%, Will Mergeout not purge the Deleted data based on AHM and it will wait till it reaches 25% for purging.