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From Matthew Foley <ma...@yahoo-inc.com>
Subject Re: [jira] [Commented] (HDFS-1780) reduce need to rewrite fsimage on statrtup
Date Thu, 24 Mar 2011 16:43:47 GMT
>> Writing image is a (small) fraction of the other start up components. You can find
the startup timeline 
>> numbers in other jiras. What is the high level problem you are solving?

With other improvements underway, large cluster startup time is down to about 30 minutes.
 Of this, 5 minutes is writing the new FSImage files, even after the improvements of HDFS-1071.
 So this has become a significant, if not huge, part of the startup time.

>> For those who have been running hadoop for 4+ years, Namenode being able to write
back updated 
>> fsimage back saved us during upgrades. Please don't remove this completely, make
it optional.

Completely agree that backup copies of this info are vital.  However:

(1) Since the Edits files are also replicated, it is reasonable to think that having a matched
set of FSImage & Edits is sufficient for this protection; it is not vital to have them
compacted into an updated FSImage.  I believe the proposal is not to eliminate redundant backups,
the proposal is simply to not view the compacting operation as something vital to do at startup
time.

(2) Since most of us running production clusters use Checkpoint Namenodes to do the compacting
operation (combining the FSImage + Edits => new FSImage, and writing out redundant copies
of the new FSImage) in background, in order to keep the size of the Edits logs under control,
it is even less important to do a compaction operation during startup.  In fact, it seems
to me that only sites that do NOT use any sort of Checkpoint Namenode actually have any need
to do compaction from the Primary Namenode, at startup or otherwise.

So I think Daryn's suggestion is worthwhile.  Doing the compaction at startup should still
remain an option, for sites not using Checkpoint Namenodes.
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