Summary:
Two things:
1. Use hash-based index for data column family
2. Use Get() instead of Iterator Seek() when DB is opened read-only
Test Plan: added read-only test in unit test
Reviewers: yinwang
Reviewed By: yinwang
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D22323
Summary:
I will move compression related options in a separate diff since this
diff is already pretty lengthy.
I guess I will also need to change JNI accordingly :(
Test Plan: make all check
Reviewers: yhchiang, igor, sdong
Reviewed By: igor
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D21915
Summary: We need to start compression at level 1, while OptimizeForLevelComapaction() only sets up rocksdb to start compressing at level 2. I also adjusted some other things.
Test Plan: compiles
Reviewers: yinwang
Reviewed By: yinwang
Differential Revision: https://reviews.facebook.net/D22203
Summary:
Add WriteBatchWithIndex so that a user can query data out of a WriteBatch, to support MongoDB's read-its-own-write.
WriteBatchWithIndex uses a skiplist to store the binary index. The index stores the offset of the entry in the write batch. When searching for a key, the key for the entry is read by read the entry from the write batch from the offset.
Define a new iterator class for querying data out of WriteBatchWithIndex. A user can create an iterator of the write batch for one column family, seek to a key and keep calling Next() to see next entries.
I will add more unit tests if people are OK about this API.
Test Plan:
make all check
Add unit tests.
Reviewers: yhchiang, igor, MarkCallaghan, ljin
Reviewed By: ljin
Subscribers: dhruba, leveldb, xjin
Differential Revision: https://reviews.facebook.net/D21381
* Script for building the unity.cc file via Makefile
* Unity executable Makefile target for testing builds
* Source code changes to fix compilation of unity build
Summary:
Since we have enough memory to hold all primary keys loaded from spatial index, it is better if we first load all of them (store them in unordered_set for deduplication) and then query on primary key column family one by one.
We need to dedup all IDs, so we'll end up storing all of them in memory even with the current approach.
Test Plan: ./spatial_db_test is happy
Reviewers: yinwang
Reviewed By: yinwang
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D20949
Summary: This will help debugging
Test Plan: ran, observed output
Reviewers: yinwang
Reviewed By: yinwang
Differential Revision: https://reviews.facebook.net/D20937
Summary: I changed SpatialDB API so that we only specify list of indexes when we create the database. That way, whoever is querying the DB doesn't need to know the full list of indexes and their options.
Test Plan: spatial_db_test
Reviewers: yinwang
Reviewed By: yinwang
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D20571
Summary:
This diff is adding spatial index support to RocksDB.
When creating the DB user specifies a list of spatial indexes. Spatial indexes can cover different areas and have different resolution (i.e. number of tiles). This is useful for supporting different zoom levels.
Each element inserted into SpatialDB has:
* a bounding box, which determines how will the element be indexed
* string blob, which will usually be WKB representation of the polygon (http://en.wikipedia.org/wiki/Well-known_text)
* feature set, which is a map of key-value pairs, where value can be int, double, bool, null or a string. FeatureSet will be a set of tags associated with geo elements (for example, 'road': 'highway' and similar)
* a list of indexes to insert the element in. For example, small river element will be inserted in index for high zoom level, while country border will be inserted in all indexes (including the index for low zoom level).
Each query is executed on single spatial index. Query guarantees that it will return all elements intersecting the specified bounding box, but it might also return some extra non-intersecting elements.
Test Plan: Added bunch of unit tests in spatial_db_test
Reviewers: dhruba, yinwang
Reviewed By: yinwang
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D20361
Summary:
All public headers need to be under `include/rocksdb` directory. Otherwise, clients include our header files like this:
#include <rocksdb/db.h>
#include <utilities/backupable_db.h> // still our public header!
Also, internally, we include:
#include "utilities/backupable/backupable_db.h" // internal header
#include "utilities/backupable_db.h" // public header
which is confusing.
This way, when we install rocksdb as a system library, we can just copy `include/rocksdb` directory to system's header files. We can't really copy `utilities` directory to system's header files.
Test Plan: compiles
Reviewers: dhruba, ljin, yhchiang, sdong
Reviewed By: sdong
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D20409
Summary:
This is a rough sketch of our new document API. Would like to get some thoughts and comments about the high-level architecture and API.
I didn't optimize for performance at all. Leaving some low-hanging fruit so that we can be happy when we fix them! :)
Currently, bunch of features are not supported at all. Indexes can be only specified when creating database. There is no query planner whatsoever. This will all be added in due time.
Test Plan: Added a simple unit test
Reviewers: haobo, yhchiang, dhruba, sdong, ljin
Reviewed By: ljin
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D18747
Summary:
A generic rate limiter that can be shared by threads and rocksdb
instances. Will use this to smooth out write traffic generated by
compaction and flush. This will help us get better p99 behavior on flash
storage.
Test Plan:
unit test output
==== Test RateLimiterTest.Rate
request size [1 - 1023], limit 10 KB/sec, actual rate: 10.374969 KB/sec, elapsed 2002265
request size [1 - 2047], limit 20 KB/sec, actual rate: 20.771242 KB/sec, elapsed 2002139
request size [1 - 4095], limit 40 KB/sec, actual rate: 41.285299 KB/sec, elapsed 2202424
request size [1 - 8191], limit 80 KB/sec, actual rate: 81.371605 KB/sec, elapsed 2402558
request size [1 - 16383], limit 160 KB/sec, actual rate: 162.541268 KB/sec, elapsed 3303500
Reviewers: yhchiang, igor, sdong
Reviewed By: sdong
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D19359
Summary:
After evaluating options for JSON storage, I decided to implement our own. The reason is that we'll be able to optimize it better and we get to reduce unnecessary dependencies (which is what we'd get with folly).
I also plan to write a serializer/deserializer for JSONDocument with our own binary format similar to BSON. That way we'll store binary JSON format in RocksDB instead of the plain-text JSON. This means less storage and faster deserialization.
There are still some inefficiencies left here. I plan to optimize them after we develop a functioning DocumentDB. That way we can move and iterate faster.
Test Plan: added a unit test
Reviewers: dhruba, haobo, sdong, ljin, yhchiang
Reviewed By: haobo
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D18831
Summary: added a new option to BackupEngine: if share_files_with_checksum is set to true, sst files are stored in shared_checksum/ and are identified by the triple (file name, checksum, file size) instead of just the file name. This option is targeted at distributed databases that want to backup their primary replica.
Test Plan: unit tests and tested backup and restore on a distributed rocksdb
Reviewers: igor
Reviewed By: igor
Differential Revision: https://reviews.facebook.net/D18393
Summary:
Our valgrind tests are failing because ttl_test is kind of flakey. This diff should fix valgrind issue and make ttl_test less flakey and much faster.
Instead of relying on Env::Default() for getting current time, I expose `Env*` to all TTL functions that are interested in time. That way, I can insert a custom test Env which is then used to provide exactly the times we need. That way, we don't need to sleep anymore -- we control the time.
Test Plan: ttl_test in normal and valgrind run
Reviewers: dhruba, haobo, sdong, yhchiang
Reviewed By: haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D18399
Summary:
This enables user to add a TTL column family to normal DB.
Next step should be to expand StackableDB and create StackableColumnFamily, such that users can for example add geo-spatial column families to normal DB.
Test Plan: added a test
Reviewers: dhruba, haobo, ljin
Reviewed By: haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D18201
Summary: Read-only BackupEngine can connect to the same backup directory that is already running BackupEngine. That enables some interesting use-cases (i.e. restoring replica from primary's backup directory)
Test Plan: added a unit test
Reviewers: dhruba, haobo, ljin
Reviewed By: ljin
CC: leveldb
Differential Revision: https://reviews.facebook.net/D18297
Summary:
This will enable people using TTL DB to do so with multiple column families. They can also specify different TTLs for each one.
TODO: Implement CreateColumnFamily() in TTL world.
Test Plan: Added a very simple sanity test.
Reviewers: dhruba, haobo, ljin, sdong, yhchiang
Reviewed By: haobo
CC: leveldb, alberts
Differential Revision: https://reviews.facebook.net/D17859
Summary:
Introducing RocksDBLite! Removes all the non-essential features and reduces the binary size. This effort should help our adoption on mobile.
Binary size when compiling for IOS (`TARGET_OS=IOS m static_lib`) is down to 9MB from 15MB (without stripping)
Test Plan: compiles :)
Reviewers: dhruba, haobo, ljin, sdong, yhchiang
Reviewed By: yhchiang
CC: leveldb
Differential Revision: https://reviews.facebook.net/D17835
Summary: Compiling for iOS has by default turned on -Wmissing-prototypes, which causes rocksdb to fail compiling. This diff turns on -Wmissing-prototypes in our compile options and cleans up all functions with missing prototypes.
Test Plan: compiles
Reviewers: dhruba, haobo, ljin, sdong
Reviewed By: ljin
CC: leveldb
Differential Revision: https://reviews.facebook.net/D17649
Summary: Fix some signed and unsigned comparisons to make some other build script happy.
Test Plan: Build and run those changed tests
Reviewers: ljin, igor, haobo
Reviewed By: igor
CC: yhchiang, dhruba, kailiu, leveldb
Differential Revision: https://reviews.facebook.net/D17463
Summary: The previous change D15087 changed existing compaction filter, which makes the commonly used class not backward compatible. Revert the older interface. Use a new interface for V2 instead.
Test Plan: make all check
Reviewers: haobo, yhchiang, igor
CC: danguo, dhruba, ljin, igor, leveldb
Differential Revision: https://reviews.facebook.net/D17223
Summary:
This patch stores gps locations in rocksdb.
Each object is uniquely identified by an id. Each object has
a gps (latitude, longitude) associated with it. The geodb
supports looking up an object either by its gps location
or by its id. There is a method to retrieve all objects
within a circular radius centered at a specified gps location.
Test Plan: Simple unit-test attached.
Reviewers: leveldb, haobo
Reviewed By: haobo
CC: leveldb, tecbot, haobo
Differential Revision: https://reviews.facebook.net/D15567
Summary:
This diff adds a new CompactionFilterV2 API that roll up the
decisions of kv pairs during compactions. These kv pairs must share the
same key prefix. They are buffered inside the db.
typedef std::vector<Slice> SliceVector;
virtual std::vector<bool> Filter(int level,
const SliceVector& keys,
const SliceVector& existing_values,
std::vector<std::string>* new_values,
std::vector<bool>* values_changed
) const = 0;
Application can override the Filter() function to operate
on the buffered kv pairs. More details in the inline documentation.
Test Plan:
make check. Added unit tests to make sure Keep, Delete,
Change all works.
Reviewers: haobo
CCs: leveldb
Differential Revision: https://reviews.facebook.net/D15087
Summary:
* PartialMerge api now takes a list of operands instead of two operands.
* Add min_pertial_merge_operands to Options, indicating the minimum
number of operands to trigger partial merge.
* This diff is based on Schalk's previous diff (D14601), but it also
includes necessary changes such as updating the pure C api for
partial merge.
Test Plan:
* make check all
* develop tests for cases where partial merge takes more than two
operands.
TODOs (from Schalk):
* Add test with min_partial_merge_operands > 2.
* Perform benchmarks to measure the performance improvements (can probably
use results of task #2837810.)
* Add description of problem to doc/index.html.
* Change wiki pages to reflect the interface changes.
Reviewers: haobo, igor, vamsi
Reviewed By: haobo
CC: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D16815
Summary: Might be useful if client doesn't want to effect running system during backup too much.
Test Plan: added a test case
Reviewers: dhruba, haobo, ljin
Reviewed By: haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D17091
Summary:
Added an option to BackupableDB implementation that allows users to persist in-memory databases. When the restore happens with keep_log_files = true, it will
*) Not delete existing log files in wal_dir
*) Move log files from archive directory to wal_dir, so that DB can replay them if necessary
Test Plan: Added an unit test
Reviewers: dhruba, ljin
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D16941
Summary:
I had this diff for a while to test column families implementation. Last night, I ran it sucessfully for 10 hours with the command:
time ./db_stress --threads=30 --ops_per_thread=200000000 --max_key=5000 --column_families=20 --clear_column_family_one_in=3000000 --verify_before_write=1 --reopen=50 --max_background_compactions=10 --max_background_flushes=10 --db=/tmp/db_stress
It is ready to be committed :)
Test Plan: Ran it for 10 hours
Reviewers: dhruba, haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D16797
Summary: We should dump options in backupable DB log, just like with to with RocksDB. This will aid debugging.
Test Plan: checked the log
Reviewers: ljin
Reviewed By: ljin
CC: leveldb
Differential Revision: https://reviews.facebook.net/D16719
Summary:
(1) Report corruption if backup meta file has tailing data that was not read. This should fix: https://github.com/facebook/rocksdb/issues/81 (also, @sdong reported similar issue)
(2) Don't use OS buffer when copying file to backup directory. We don't need the file in cache since we won't be reading it twice
(3) Don't delete newer backups when somebody tries to backup the diverged DB (restore from older backup, add new data, try to backup). Rather, just fail the new backup.
Test Plan: backupable_db_test
Reviewers: ljin, dhruba, sdong
Reviewed By: ljin
CC: leveldb, sdong
Differential Revision: https://reviews.facebook.net/D16287
Summary: Even if user flushes the memtables before getting live files, we still can't guarantee that new data didn't come in (to already-flushed memtables). If we want backups to provide consistent view of the database, we still need to get WAL files.
Test Plan: backupable_db_test
Reviewers: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D16299
Summary:
The change to the public behavior:
* When opening a DB or creating new column family client gets a ColumnFamilyHandle.
* As long as column family handle is alive, client can do whatever he wants with it, even drop it
* Dropped column family can still be read from (using the column family handle)
* Added a new call CloseColumnFamily(). Client has to close all column families that he has opened before deleting the DB
* As soon as column family is closed, any calls to DB using that column family handle will fail (also any outstanding calls)
Internally:
* Ref-counting ColumnFamilyData
* New thread-safety for ColumnFamilySet
* Dropped column families are now completely dropped and their memory cleaned-up
Test Plan: added some tests to column_family_test
Reviewers: dhruba, haobo, kailiu, sdong
CC: leveldb
Differential Revision: https://reviews.facebook.net/D16101
Summary: Clean up IOErrors so that it only indicates errors talking to device.
Test Plan: make all check
Reviewers: igor, haobo, dhruba, emayanke
Reviewed By: igor
CC: leveldb
Differential Revision: https://reviews.facebook.net/D15831
Summary:
There are three SanitizeOption-s now : one for DBOptions, one for ColumnFamilyOptions and one for Options (which just calls the other two)
I have also reshuffled some options -- table_cache options and info_log should live in DBOptions, for example.
Test Plan: make check doesn't complain
Reviewers: dhruba, haobo, kailiu, sdong
CC: leveldb
Differential Revision: https://reviews.facebook.net/D15873
Summary:
if it happens (randomly) to corrupt shared file in the test, then the
checksum will be inconsistent between meta files from different backup.
BackupEngine will then detect this issue and fail. But in reality, this
does not happen since the checksum is checked on every backup. So here,
only corrupt checksum of private file to let BackupEngine to construct
properly (but fail during restore).
Test Plan: run test with valgrind
Reviewers: igor
Reviewed By: igor
CC: leveldb
Differential Revision: https://reviews.facebook.net/D15531
Summary: I think it looks nicer. In RocksDB we have both styles, but I think that static method is the more common version.
Test Plan: backupable_db_test
Reviewers: ljin, benj, swk
Reviewed By: ljin
CC: leveldb
Differential Revision: https://reviews.facebook.net/D15519
Summary:
Lots of clients have problems with using StackableDB interface. It's nice to have BackupableDB as a layer on top of DB, but not necessary.
This diff exports BackupEngine, which can be used to create backups without forcing clients to use StackableDB interface.
Test Plan: backupable_db_test
Reviewers: dhruba, ljin, swk
Reviewed By: ljin
CC: leveldb, benj
Differential Revision: https://reviews.facebook.net/D15477
Summary: Keep checksum of each backuped file in meta file. When it restores these files, compute their checksum on the fly and compare against what is in the meta file. Fail the restore process if checksum mismatch.
Test Plan: unit test
Reviewers: haobo, igor, sdong, kailiu
Reviewed By: igor
CC: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D15381
Summary: By removing some includes form options.h and reply on forward declaration, we can more easily reason the dependencies.
Test Plan: make all check
Reviewers: kailiu, haobo, igor, dhruba
Reviewed By: kailiu
CC: leveldb
Differential Revision: https://reviews.facebook.net/D15411
Summary:
This diff introduces some features that were requested by two internal customers:
* Ability for backups not to share table files, because we can't guarantee that equal filename means equal content accross replicas
* Ability for two threads to call EnableFileDeletions() and DisableFileDeletions()
* Ability to stop backup from another thread and not slow down the DB close
* Copy the files to the temporary folder first and then atomically rename
Test Plan: Added some tests to backupable_db_test
Reviewers: dhruba, sanketh, muthu, sdong, haobo
Reviewed By: haobo
CC: leveldb, sanketh, muthu
Differential Revision: https://reviews.facebook.net/D14769
Summary:
We don't want two threads to clash if they concurrently call DisableFileDeletions() and EnableFileDeletions(). I'm adding a counter that will enable file deletions only after all DisableFileDeletions() calls have been negated with EnableFileDeletions().
However, we also don't want to break the old behavior, so I added a parameter force to EnableFileDeletions(). If force is true, we will still enable file deletions after every call to EnableFileDeletions(), which is what is happening now.
Test Plan: make check
Reviewers: dhruba, haobo, sanketh
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14781
Summary:
<This diff is for Column Family branch>
Sharing some of the work I've done so far. This diff compiles and passes the tests.
The biggest change is in options.h - I broke down Options into two parts - DBOptions and ColumnFamilyOptions. DBOptions is DB-specific (env, create_if_missing, block_cache, etc.) and ColumnFamilyOptions is column family-specific (all compaction options, compresion options, etc.). Note that this does not break backwards compatibility at all.
Further, I created DBWithColumnFamily which inherits DB interface and adds new functions with column family support. Clients can transparently switch to DBWithColumnFamily and it will not break their backwards compatibility.
There are few methods worth checking out: ListColumnFamilies(), MultiNewIterator(), MultiGet() and GetSnapshot(). [GetSnapshot() returns the snapshot across all column families for now - I think that's what we agreed on]
Finally, I made small changes to WriteBatch so we are able to atomically insert data across column families.
Please provide feedback.
Test Plan: make check works, the code is backward compatible
Reviewers: dhruba, haobo, sdong, kailiu, emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14445
Summary:
I realized that manifest will get deleted by PurgeObsoleteFiles in DBImpl, but it is sill cleaner to delete
files before we restore the backup
Test Plan: backupable_db_test
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14619
Summary: We now delete backups with newer sequence number, so the clients don't have to handle confusing situations when they restore from backup.
Test Plan: added a unit test
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14547
Summary: Valgrind complained about BackupableDB. This fixes valgrind errors. Also, I cleaned up some code.
Test Plan: valgrind does not complain anymore
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14529
Summary:
In this diff I present you BackupableDB v1. You can easily use it to backup your DB and it will do incremental snapshots for you.
Let's first describe how you would use BackupableDB. It's inheriting StackableDB interface so you can easily construct it with your DB object -- it will add a method RollTheSnapshot() to the DB object. When you call RollTheSnapshot(), current snapshot of the DB will be stored in the backup dir. To restore, you can just call RestoreDBFromBackup() on a BackupableDB (which is a static method) and it will restore all files from the backup dir. In the next version, it will even support automatic backuping every X minutes.
There are multiple things you can configure:
1. backup_env and db_env can be different, which is awesome because then you can easily backup to HDFS or wherever you feel like.
2. sync - if true, it *guarantees* backup consistency on machine reboot
3. number of snapshots to keep - this will keep last N snapshots around if you want, for some reason, be able to restore from an earlier snapshot. All the backuping is done in incremental fashion - if we already have 00010.sst, we will not copy it again. *IMPORTANT* -- This is based on assumption that 00010.sst never changes - two files named 00010.sst from the same DB will always be exactly the same. Is this true? I always copy manifest, current and log files.
4. You can decide if you want to flush the memtables before you backup, or you're fine with backing up the log files -- either way, you get a complete and consistent view of the database at a time of backup.
5. More things you can find in BackupableDBOptions
Here is the directory structure I use:
backup_dir/CURRENT_SNAPSHOT - just 4 bytes holding the latest snapshot
0, 1, 2, ... - files containing serialized version of each snapshot - containing a list of files
files/*.sst - sst files shared between snapshots - if one snapshot references 00010.sst and another one needs to backup it from the DB, it will just reference the same file
files/ 0/, 1/, 2/, ... - snapshot directories containing private snapshot files - current, manifest and log files
All the files are ref counted and deleted immediatelly when they get out of scope.
Some other stuff in this diff:
1. Added GetEnv() method to the DB. Discussed with @haobo and we agreed that it seems right thing to do.
2. Fixed StackableDB interface. The way it was set up before, I was not able to implement BackupableDB.
Test Plan:
I have a unittest, but please don't look at this yet. I just hacked it up to help me with debugging. I will write a lot of good tests and update the diff.
Also, `make asan_check`
Reviewers: dhruba, haobo, emayanke
Reviewed By: dhruba
CC: leveldb, haobo
Differential Revision: https://reviews.facebook.net/D14295
Summary: Now DBWithTTL takes DB* and can behave more like StackableDB. This saves us a lot of duplicate work by defining interfaces
Test Plan: ttl_test with ASAN - OK
Reviewers: emayanke
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14481
Summary: As title
Test Plan: make clean and make
Reviewers: igor
Reviewed By: igor
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14469
Summary: We need access to options for BackupableDB
Test Plan: make check
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb, reconnect.grayhat
Differential Revision: https://reviews.facebook.net/D14331
Summary: This is part of https://reviews.facebook.net/D14295 -- smaller diff that is easier to review
Test Plan: make asan_check
Reviewers: dhruba, haobo, emayanke
Reviewed By: emayanke
CC: leveldb, kailiu, reconnect.grayhat
Differential Revision: https://reviews.facebook.net/D14301
Summary:
Sure, let me put 8 bytes in that int32_t.
Brought to you by ASAN!
Test Plan: ttl_test
Reviewers: dhruba, haobo, kailiu, emayanke
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14193
Summary: The work to make sure mac os compiles rocksdb is not completed yet. But at least we can start cleaning some warnings captured only by g++ from mac os..
Test Plan: ran make in mac os
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D14049
Summary:
strict essentially means that we MUST find the startsequence. Thus we should return if starteSequence is not found in the first file in case strict is set. This will take care of ending the iterator in case of permanent gaps due to corruptions in the log files
Also created NextImpl function that will have internal variable to distinguish whether Next is being called from StartSequence or by application.
Set NotFoudn::gaps status to give an indication of gaps happeneing.
Polished the inline documentation at various places
Test Plan:
* db_repl_stress test
* db_test relating to transaction log iterator
* fbcode/wormhole/rocksdb/rocks_log_iterator
* sigma production machine sigmafio032.prn1
Reviewers: dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D13689
