Thanks Ivan, >From the bookkeeper tutorial( http://zookeeper.apache.org/doc/r3.4.1/bookkeeperOverview.html) It says `A server maintains an in-memory data structure (with periodic snapshots for example) and logs changes to that structure before it applies the change.', but why what I see by opening the ledger file and journal file, both of them are same, i.e., entry data. (for example, if I use hedwig to pub a topic, both the ledger and journal file only have the topic/contents, which is what I called entry data, no change information, e.g., pub, is logged.) why there is no such `change' information that can be used to recover failure.
Maybe my understanding is wrong, plz correct me. Thanks a lot. Best, Jialin On Mon, Jul 21, 2014 at 5:31 AM, Ivan Kelly <[email protected]> wrote: > We have considered something like this in the past. However, it would > mean that reads will affect the latency or writes, as they will move > the disk head. > > It's also the case that the interleaved entrylog performs really badly > on reads. Work has been done recently to improve this, by buffering > entries and sorting them by ledger id before flushing to the > entrylog. This means that reads for a specific ledger will be > sequential as opposed to jumping all over the place as it has to do > now. If we used the journal for this, then we wouldn't be able to do > this processing, as the point of the journal is to ensure that the > entry is on persistent storage before replying to the client. If we > buffered enough to get benefit from sorting, write latency would be > enormous. > > -Ivan > > On Sat, Jul 19, 2014 at 01:55:16PM -0700, Jaln wrote: > > Thank you so much, Rakesh, > > Without consideration of performance, can we just maintain one file. For > > example journal file, and the index for each entry. > > > > Best, > > Jaln > > > > > > On Fri, Jul 18, 2014 at 11:23 PM, Rakesh Radhakrishnan < > > [email protected]> wrote: > > > > > Hi Jaln, > > > > > > >>>>>>for the data in the journal file(*.txn) and the entry log > > > file(*.log), are > > > >>>>>>they similar? > > > >>>>>>for example, when I add an entry, this opeartion and the entry > data > > > will be > > > >>>>>>logged in the journal file, > > > >>>>>>and the entry data will be logged in the entry log file (*.log), > > > right? > > > > > > As I mentioned earlier, when an entry is added Bookie server will add > only > > > this entry to the journal file and will send a response back to the > > > client after the successful flush to the disk. Later during > checkpointing > > > time, server will read the journal entries and add it to the entry > logger > > > files. Also, it will generate index files corresponding to each > ledgers for > > > the faster access. This old journal file will be garbage collected, > because > > > all these entries are mapped it to the entry logger. > > > > > > >>>>>what's the purpose of the two files? > > > AFAIK, adding to entry log and generating index is a costly I/O > operation > > > and will affect the performance. Thats the reason, first will only add > > > transactions to journal file and send a response quickly. Later will > add it > > > to the entrylog file & index files offline. > > > > > > Total bookie stored data = entry logger data + journal data(most recent > > > data) > > > > > > *For example:* I'm calling write operation as transaction. Assume > client > > > has performed 20 transactions. All these exists only in the journal > file. > > > Say, now checkpointing triggered. It will add these 20 transactions to > the > > > entry logger file and generate indexes. Again assume user performed 10 > more > > > transactions. Now we have total 30 transactions. > > > > > > Bookie data(30 transactions) = 20 + 10. > > > > > > Regards, > > > Rakesh > > > > > > > > > > > > On Sat, Jul 19, 2014 at 9:52 AM, Jaln <[email protected]> wrote: > > > > > > > Thanks Rakesh, > > > > for the data in the journal file(*.txn) and the entry log > file(*.log), > > > are > > > > they similar? > > > > for example, when I add an entry, this opeartion and the entry data > will > > > be > > > > logged in the journal file, > > > > and the entry data will be logged in the entry log file (*.log), > right? > > > > what's the purpose of the two files? > > > > > > > > Thanks, > > > > Jaln > > > > > > > > On Fri, Jul 18, 2014 at 8:16 PM, Rakesh Radhakrishnan < > > > > [email protected]> wrote: > > > > > > > > > Hi Jaln, > > > > > > > > > > No, both are different. I hope you are asking about 'entry log' > files > > > and > > > > > 'journal' files > > > > > > > > > > *Journal : *When client performs a write operation (such as adding > an > > > > entry > > > > > etc), it is first recorded in the journal file. Journal will be > flushed > > > > and > > > > > synced after every write operation before a success code is > returned to > > > > the > > > > > client. This ensures that no operation is lost due to machine > failure. > > > > > > > > > > *Entry Log : *It is not updated for every write operation, bookie > > > server > > > > > will do it lazily. Because writing out the ledger involves - update > > > > ledger > > > > > index files to faster look up and add entry to the logger file. > This > > > will > > > > > be a costly operation and will affect the performance. > > > > > > > > > > In Bookie, there is a dedicated thread to play journal > transactions and > > > > add > > > > > it to the logger lazily, this is called as checkpointing operation. > > > This > > > > > will be performed periodically, now the data will be persisted to > > > ledger > > > > > index files and entry logger. By default the 'flushInterval' is 100 > > > > > milliseconds. Probably you can configure a bigger value to see the > > > > > difference. > > > > > > > > > > *"SyncThread"* is a background thread which help checkpointing. > After a > > > > > ledger storage is checkpointed, the journal files added before > > > checkpoint > > > > > will be garbage collected. > > > > > > > > > > Cheers, > > > > > Rakesh > > > > > > > > > > > > > > > On Sat, Jul 19, 2014 at 1:41 AM, Jaln <[email protected]> > wrote: > > > > > > > > > > > Hi, > > > > > > is the ledger file and journal file same? > > > > > > I run the bookkeeper and generate the bookie, > > > > > > inside the bookie, I found the journal file and ledger file are > > > almost > > > > > > same. > > > > > > > > > > > > Best, > > > > > > Jialin > > > > > > > > > > > > > > > > > > > -- Genius only means hard-working all one's life
