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From Meghdoot bhattacharya <>
Subject Re: Mesos replicated log dual writes
Date Tue, 05 Jun 2018 16:30:12 GMT
Thx Joseph. Writing back the entire blob is costly instead of writing back only the 2 bytes.
In this case this record is just a chunk of large snapshot that is getting written. So it

Should I open a jira to track and resolve this or something like this is already open?


> On Jun 5, 2018, at 9:10 AM, Joseph Wu <> wrote:
> The two-byte difference is most likely coming from the "learned" field:
> The first time an entry is recorded, the entry is "unlearned", basically meaning that
the entry has not been written to a quorum of masters (yet).  Once a quorum of masters indicate
to each other that they have written an entry, they then flip the bit to "learned".
> This process is mandatory for Paxos to work, but I can see how overwriting an entry could
be expensive.  There's no particular restriction on why we must overwrite the entire entry,
except that we currently use the position number (i.e. 6506690 in your example) as the key
in the leveldb implementation.  
>> On Tue, Jun 5, 2018 at 12:30 AM, meghdoot bhattacharya <>
>> Recently investigation of logs of aurora around snapshot creations in replicated
log come up with this
>> I0601 21:57:27.322444   144 log.cpp:577] Attempting to append 524304 bytes to the
logI0601 21:57:27.322501   144 coordinator.cpp:348] Coordinator attempting to write APPEND
action at position 6506690I0601 21:57:27.323122   139 replica.cpp:539] Replica received write
request for position 6506690 from __req_res__(3685)@x.x.x.x:yyyyI0601 21:57:27.443131   139
leveldb.cpp:341] Persisting action (524331 bytes) to leveldb took 119.847993msI0601 21:57:27.443208
  139 replica.cpp:710] Persisted action APPEND at position 6506690I0601 21:57:27.443437  
139 replica.cpp:693] Replica received learned notice for position 6506690 from @
21:57:27.478754   139 leveldb.cpp:341] Persisting action (524333 bytes) to leveldb took 35.278735msI0601
21:57:27.478818   139 replica.cpp:710] Persisted action APPEND at position 6506690
>> It seems there are dual writes to leveldb for the same record (with 2 byte diff).
Becomes expensive if that is the case for large records. Hoping to get some insights.
>> Thx

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