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From Jiannan Wang <jian...@yahoo-inc.com>
Subject Another Scan-And-Compare GC Implementation
Date Mon, 18 Mar 2013 10:14:21 GMT
Hi there,
   I'm preparing the patch for new 64-bits zookeeper-based ledger manager with radix tree
(BOOKKEEPER-553), but I get a problem.

   A LedgerManager implementation needs to complete a method called "getLedgerRanges", which
scans all ledger ids in order from metadata storage to do local Scan-And-Compare GC. However,
for a radix tree ledger manager implementation, scan all ledgers in order (in BFS way) may
require large memory usage.

   After digging the Scan-And-Compare GC, I find it's not necessary to make any order requirement.
What Scan-And-Compare GC approach wants to know is the ledger id that exists in local bookie
server but not in metadata storage. So we can do it in another way:
   1. Get a snapshot of local ledger id list "L".
   2. Get all the ledger id from metadata storage and remove it from list "L" (here we do
not require the metadata storage return ledger id range in any order guarantee).
   3. After step 2 finish looping all ledger id in metadata storage, GC the remaining ledger
id in "L" list.
   By this, we don't require a "ORDER SCAN" now, LedgerManager#asyncProcessLedgers is enough
to do this job.

   Of cause, this implementation has one drawback: GC process can only take place after iterating
all ledger id in metadata storage. But I just don't think we need specific order guarantee
for Scan-And-Compare GC, there are already some other better improved GC approaches.

- Jiannan


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