Summary:
Most code change in this diff is code cleanup/rewrite. The logic changes include:
(1) add universal compaction to db_crashtest2.py
(2) randomly set --test_batches_snapshots to be 0 or 1 in db_crashtest2.py. Old codes always use 1.
(3) use different tmp directory as db directory in different runs. I saw some intermittent errors in my local tests. Use of different tmp directory seems to be able to solve the issue.
Test Plan: Have run "make crashtest" for multiple times. Also run "make all check"
Reviewers: emayanke, dhruba, haobo
Reviewed By: emayanke
Differential Revision: https://reviews.facebook.net/D12369
Test Plan:
- make all check;
- make release;
- make stringappend_test; ./stringappend_test
Reviewers: haobo, emayanke
Reviewed By: haobo
CC: leveldb, kailiu
Differential Revision: https://reviews.facebook.net/D12381
Summary: Also expanded class LogFile to have startSequene and FileSize and exposed it publicly
Test Plan: make all check
Reviewers: dhruba, haobo
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D12087
Summary:
Updated db_bench and utilities/merge_operators.h to allow for dynamic benchmarking
of merge operators in db_bench. Added a new test (--benchmarks=mergerandom), which performs
a bunch of random Merge() operations over random keys. Also added a "--merge_operator=" flag
so that the tester can easily benchmark different merge operators. Currently supports
the PutOperator and UInt64Add operator. Support for stringappend or list append may come later.
Test Plan:
1. make db_bench
2. Test the PutOperator (simulating Put) as follows:
./db_bench --benchmarks=fillrandom,readrandom,updaterandom,readrandom,mergerandom,readrandom --merge_operator=put
--threads=2
3. Test the UInt64AddOperator (simulating numeric addition) similarly:
./db_bench --value_size=8 --benchmarks=fillrandom,readrandom,updaterandom,readrandom,mergerandom,readrandom
--merge_operator=uint64add --threads=2
Reviewers: haobo, dhruba, zshao, MarkCallaghan
Reviewed By: haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11535
Summary: I changed the db_stress configs, but forgot to update the scripts using the old configs.
Test Plan: 'make blackbox_crash_test' and 'make whitebox_crash_test' start running normally now (I haven't run them til the end, though).
Reviewers: vamsi
Reviewed By: vamsi
CC: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D12303
Summary:
Minor fix to current codes, including: coding style, output format,
comments. No major logic change. There are only 2 real changes, please see my inline comments.
Test Plan: make all check
Reviewers: haobo, dhruba, emayanke
Differential Revision: https://reviews.facebook.net/D12297
Summary: The ability to dump internal keys associated with certain user keys, directly from sst files, is very useful for diagnosis. Will incorporate it directly into ldb later.
Test Plan: run it
Reviewers: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D12075
Summary: rocksdb replicaiton will need this when writing value+TS from master to slave 'as is'
Test Plan: make
Reviewers: dhruba, vamsi, haobo
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11919
Summary: Removed KeyMayExistImpl because KeyMayExist demanded Get like semantics now. Removed no_io from memtable and imm because we need the proper value now and shouldn't just stop when we see Merge in memtable. Added checks to block_cache. Updated documentation and unit-test
Test Plan: make all check;db_stress for 1 hour
Reviewers: dhruba, haobo
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11853
Summary: filter_deletes options introduced in db_stress makes it drop Deletes on key if KeyMayExist(key) returns false on the key. code change was simple and tested so not wasting reviewer's time.
Test Plan: maek crash_test; python tools/db_crashtest[1|2].py
CC: dhruba, vamsi
Differential Revision: https://reviews.facebook.net/D11769
Summary:
Introduced KeyMayExist checking during writebatch-delete and removed from Outer Delete API because it uses writebatch-delete.
Added code to skip getting Table from disk if not already present in table_cache.
Some renaming of variables.
Introduced KeyMayExistImpl which allows checking since specified sequence number in GetImpl useful to check partially written writebatch.
Changed KeyMayExist to not be pure virtual and provided a default implementation.
Expanded unit-tests in db_test to check appropriately.
Ran db_stress for 1 hour with ./db_stress --max_key=100000 --ops_per_thread=10000000 --delpercent=50 --filter_deletes=1 --statistics=1.
Test Plan: db_stress;make check
Reviewers: dhruba, haobo
Reviewed By: dhruba
CC: leveldb, xjin
Differential Revision: https://reviews.facebook.net/D11745
Summary:
Wrote a new function in db_impl.c-CheckKeyMayExist that calls Get but with a new parameter turned on which makes Get return false only if bloom filters can guarantee that key is not in database. Delete calls this function and if the option- deletes_use_filter is turned on and CheckKeyMayExist returns false, the delete will be dropped saving:
1. Put of delete type
2. Space in the db,and
3. Compaction time
Test Plan:
make all check;
will run db_stress and db_bench and enhance unit-test once the basic design gets approved
Reviewers: dhruba, haobo, vamsi
Reviewed By: haobo
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11607
Summary: db_stress should alos print complete statistics like db_bench. Needed this when I wanted to measure number of delete-IOs dropped due to CheckKeyMayExist to be introduced to rocksdb codebase later- to make deltes in rocksdb faster
Test Plan: make db_stress;./db_stress --max_key=100 --ops_per_thread=1000 --statistics=1
Reviewers: sheki, dhruba, vamsi, haobo
Reviewed By: dhruba
Differential Revision: https://reviews.facebook.net/D11655
Summary:
There is a new option called hybrid_mode which, when switched on,
causes HBase style compactions. Files from L0 are
compacted back into L0. This meat of this compaction algorithm
is in PickCompactionHybrid().
All files reside in L0. That means all files have overlapping
keys. Each file has a time-bound, i.e. each file contains a
range of keys that were inserted around the same time. The
start-seqno and the end-seqno refers to the timeframe when
these keys were inserted. Files that have contiguous seqno
are compacted together into a larger file. All files are
ordered from most recent to the oldest.
The current compaction algorithm starts to look for
candidate files starting from the most recent file. It continues to
add more files to the same compaction run as long as the
sum of the files chosen till now is smaller than the next
candidate file size. This logic needs to be debated
and validated.
The above logic should reduce write amplification to a
large extent... will publish numbers shortly.
Test Plan: dbstress runs for 6 hours with no data corruption (tested so far).
Differential Revision: https://reviews.facebook.net/D11289
Summary:
There is a new option called hybrid_mode which, when switched on,
causes HBase style compactions. Files from L0 are
compacted back into L0. This meat of this compaction algorithm
is in PickCompactionHybrid().
All files reside in L0. That means all files have overlapping
keys. Each file has a time-bound, i.e. each file contains a
range of keys that were inserted around the same time. The
start-seqno and the end-seqno refers to the timeframe when
these keys were inserted. Files that have contiguous seqno
are compacted together into a larger file. All files are
ordered from most recent to the oldest.
The current compaction algorithm starts to look for
candidate files starting from the most recent file. It continues to
add more files to the same compaction run as long as the
sum of the files chosen till now is smaller than the next
candidate file size. This logic needs to be debated
and validated.
The above logic should reduce write amplification to a
large extent... will publish numbers shortly.
Test Plan: dbstress runs for 6 hours with no data corruption (tested so far).
Differential Revision: https://reviews.facebook.net/D11289
Summary: As title, now that db_stress supports --map_read properly
Test Plan: make crash_test
Reviewers: vamsi, emayanke, dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11391
Summary: as title, also removed an incorrect assertion
Test Plan: make check; db_stress --mmap_read=1; db_stress --mmap_read=0
Reviewers: dhruba, emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11367
Summary:
Rocksdb allos specifying the number of files in L0 that triggers
compactions. Expose this api as a command line parameter for
running db_stress.
Test Plan: Run test
Reviewers: sheki, emayanke
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11343
Summary:
This diff simplifies EnvOptions by treating it as POD, similar to Options.
- virtual functions are removed and member fields are accessed directly.
- StorageOptions is removed.
- Options.allow_readahead and Options.allow_readahead_compactions are deprecated.
- Unused global variables are removed: useOsBuffer, useFsReadAhead, useMmapRead, useMmapWrite
Test Plan: make check; db_stress
Reviewers: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11175
Summary: These extra options caught some bugs. Will be run via Jenkins now with the crash_test
Test Plan: ./make crashtest
Reviewers: dhruba, vamsi
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11151
Summary:
I think the check for "error" that I added had caused
false alarm. Fixed that.
Test Plan:
Revert Plan: OK
Task ID: #
Reviewers: emayanke, dhruba
Reviewed By: emayanke
Differential Revision: https://reviews.facebook.net/D11139
Summary: Make Statistics usable by client
Test Plan: make check; db_bench
Reviewers: dhruba
Reviewed By: dhruba
Differential Revision: https://reviews.facebook.net/D10899
Summary:
This is initial version. A few ways in which this could
be extended in the future are:
(a) Killing from more places in source code
(b) Hashing stack and using that hash in determining whether to crash.
This is to avoid crashing more often at source lines that are executed
more often.
(c) Raising exceptions or returning errors instead of killing
Test Plan:
This whole thing is for testing.
Here is part of output:
python2.7 tools/db_crashtest2.py -d 600
Running db_stress
db_stress retncode -15 output LevelDB version : 1.5
Number of threads : 32
Ops per thread : 10000000
Read percentage : 50
Write-buffer-size : 4194304
Delete percentage : 30
Max key : 1000
Ratio #ops/#keys : 320000
Num times DB reopens: 0
Batches/snapshots : 1
Purge redundant % : 50
Num keys per lock : 4
Compression : snappy
------------------------------------------------
No lock creation because test_batches_snapshots set
2013/04/26-17:55:17 Starting database operations
Created bg thread 0x7fc1f07ff700
... finished 60000 ops
Running db_stress
db_stress retncode -15 output LevelDB version : 1.5
Number of threads : 32
Ops per thread : 10000000
Read percentage : 50
Write-buffer-size : 4194304
Delete percentage : 30
Max key : 1000
Ratio #ops/#keys : 320000
Num times DB reopens: 0
Batches/snapshots : 1
Purge redundant % : 50
Num keys per lock : 4
Compression : snappy
------------------------------------------------
Created bg thread 0x7ff0137ff700
No lock creation because test_batches_snapshots set
2013/04/26-17:56:15 Starting database operations
... finished 90000 ops
Revert Plan: OK
Task ID: #2252691
Reviewers: dhruba, emayanke
Reviewed By: emayanke
CC: leveldb, haobo
Differential Revision: https://reviews.facebook.net/D10581
Summary: db can't reopen safely with disable_wal set!
Test Plan: make db_stress; run db_stress with disable_wal and reopens set and see error
Reviewers: dhruba, vamsi
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D10857
Summary: Will help while debugging if the generated value is truncated at proper length.
Test Plan: make db_stress;/db_stress --max_key=10000 --db=/tmp/mcr --threads=1 --ops_per_thread=10000
Reviewers: dhruba, vamsi
Reviewed By: vamsi
Differential Revision: https://reviews.facebook.net/D10845
Summary: ldb works with raw data from the database and needs to be aware of ttl-database to work with it meaningfully. '-ttl' option now tells it that. Also added onto the ldb_test.py test. This option may be specified alongwith put, get, scan or dump. There is no support to provide a ttl-value and it uses default forever because there is no use-case for this currently.
Test Plan: make ldb_test; python tools/ldb_test.py
Reviewers: dhruba, sheki, haobo, vamsi
Reviewed By: sheki
CC: leveldb
Differential Revision: https://reviews.facebook.net/D10797
Summary:
When opened with DBTimestamp::Open call, timestamps are prepended to and stripped from the value during subsequent Put and Get calls respectively. The Timestamp is used to discard values in Get and custom compaction filter which have exceeded their TTL which is specified during Open.
Have made a temporary change to Makefile to let us test with the temporary file TestTime.cc. Have also changed the private members of db_impl.h to protected to let them be inherited by the new class DBTimestamp
Test Plan: make db_timestamp; TestTime.cc(will not check it in) shows how to use the apis currently, but I will write unit-tests shortly
Reviewers: dhruba, vamsi, haobo, sheki, heyongqiang, vkrest
Reviewed By: vamsi
CC: zshao, xjin, vkrest, MarkCallaghan
Differential Revision: https://reviews.facebook.net/D10311
Summary:
- don't see a point exposing table.h to the public.
- fixed make clean to remove also *.d files.
Test Plan: make check; db_stress
Reviewers: dhruba, heyongqiang
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D10479
Summary: Primarily a refactor. Introduced LDBTool interface to which customers can plug in their options and this will create their own version of ldb tool.
Test Plan: made ldb tool and tried it.
Reviewers: dhruba, heyongqiang
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D10191
Summary: To know which options the crashtest was run with. Also changed print to sys.stdout.write which is more standard.
Test Plan: python tools/db_crashtest.py
Reviewers: vamsi, akushner, dhruba
Reviewed By: akushner
Differential Revision: https://reviews.facebook.net/D10119
Summary: make crash_test will now invoke the crash_test. Also some cleanup in the db_crashtest.py file
Test Plan: make crash_test
Reviewers: akushner, vamsi, sheki, dhruba
Reviewed By: vamsi
Differential Revision: https://reviews.facebook.net/D9987
Summary: The crash_test depends on db_stress to work with pre-existing dir
Test Plan: make db_stress; Run db_stress with 'destroy_db_initially=0'
Reviewers: vamsi, dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D10041
Summary: For sanity w.r.t. the way we split up the reopens equally among the ops/thread
Test Plan: make db_stress; db_stress --ops_per_thread=10 --reopens=10 => error
Reviewers: vamsi, dhruba
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D10023
Summary:
When I run db_crashtest, I am seeing lot of warnings that say db_stress completed
before it was killed. To fix that I made ops per thread a very large value so that it keeps
running until it is killed.
I also set #reopens to 0. Since we are killing the process anyway, the 'simulated crash'
that happens during reopen may not add additional value.
I usually see 10-25K ops happening before the kill. So I increased max_key from 100 to
1000 so that we use more distinct keys.
Test Plan:
Ran a few times.
Revert Plan: OK
Task ID: #
Reviewers: emayanke
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9909
Summary: The script runs and kills the stress test periodically. Default values have been used in the script now. Should I make this a part of the Makefile or automated rocksdb build? The values can be easily changed in the script right now, but should I add some support for variable values or input to the script? I believe the script achieves its objective of unsafe crashes and reopening to expect sanity in the database.
Test Plan: python tools/db_crashtest.py
Reviewers: dhruba, vamsi, MarkCallaghan
Reviewed By: vamsi
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9369
Summary:
Earlier Statistics object was a raw pointer. This meant the user had to clear up
the Statistics object after creating the database. In most use cases the database is created in a function and the statistics pointer is out of scope. Hence the statistics object would never be deleted.
Now Using a shared_ptr to manage this.
Want this in before the next release.
Test Plan: make all check.
Reviewers: dhruba, emayanke
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9735
Summary: simple sed command to replace NULL in tools directory. Was missed by the previous codemod.
Test Plan: it compiles
Reviewers: emayanke
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9621
Summary:
This patch allows an application to specify whether to use bufferedio,
reads-via-mmaps and writes-via-mmaps per database. Earlier, there
was a global static variable that was used to configure this functionality.
The default setting remains the same (and is backward compatible):
1. use bufferedio
2. do not use mmaps for reads
3. use mmap for writes
4. use readaheads for reads needed for compaction
I also added a parameter to db_bench to be able to explicitly specify
whether to do readaheads for compactions or not.
Test Plan: make check
Reviewers: sheki, heyongqiang, MarkCallaghan
Reviewed By: sheki
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9429
Summary: ldb_test.py did a lot of assertFalse checks and displayed all the failed messages on the std output making it confusing to tell a successful from a failed run. Also many empty lines used to be needlessly printed. Also added some progression-"feel-good" lines in the tests
Test Plan: python ldb_test.py
Reviewers: dhruba, sheki, dilipj, chip
Reviewed By: dilipj
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9297
Summary:
Also added some comments and fixed some bugs in
stats reporting. Now the stats seem to match what is expected.
Test Plan:
[nponnekanti@dev902 /data/users/nponnekanti/rocksdb] ./db_stress --test_batches_snapshots=1 --ops_per_thread=1000 --threads=1 --max_key=320
LevelDB version : 1.5
Number of threads : 1
Ops per thread : 1000
Read percentage : 10
Delete percentage : 30
Max key : 320
Ratio #ops/#keys : 3
Num times DB reopens: 10
Batches/snapshots : 1
Num keys per lock : 4
Compression : snappy
------------------------------------------------
No lock creation because test_batches_snapshots set
2013/03/04-15:58:56 Starting database operations
2013/03/04-15:58:56 Reopening database for the 1th time
2013/03/04-15:58:56 Reopening database for the 2th time
2013/03/04-15:58:56 Reopening database for the 3th time
2013/03/04-15:58:56 Reopening database for the 4th time
Created bg thread 0x7f4542bff700
2013/03/04-15:58:56 Reopening database for the 5th time
2013/03/04-15:58:56 Reopening database for the 6th time
2013/03/04-15:58:56 Reopening database for the 7th time
2013/03/04-15:58:57 Reopening database for the 8th time
2013/03/04-15:58:57 Reopening database for the 9th time
2013/03/04-15:58:57 Reopening database for the 10th time
2013/03/04-15:58:57 Reopening database for the 11th time
2013/03/04-15:58:57 Limited verification already done during gets
Stress Test : 1811.551 micros/op 552 ops/sec
: Wrote 0.10 MB (0.05 MB/sec) (598% of 1011 ops)
: Wrote 6050 times
: Deleted 3050 times
: 500/900 gets found the key
: Got errors 0 times
[nponnekanti@dev902 /data/users/nponnekanti/rocksdb] ./db_stress --ops_per_thread=1000 --threads=1 --max_key=320
LevelDB version : 1.5
Number of threads : 1
Ops per thread : 1000
Read percentage : 10
Delete percentage : 30
Max key : 320
Ratio #ops/#keys : 3
Num times DB reopens: 10
Batches/snapshots : 0
Num keys per lock : 4
Compression : snappy
------------------------------------------------
Creating 80 locks
2013/03/04-15:58:17 Starting database operations
2013/03/04-15:58:17 Reopening database for the 1th time
2013/03/04-15:58:17 Reopening database for the 2th time
2013/03/04-15:58:17 Reopening database for the 3th time
2013/03/04-15:58:17 Reopening database for the 4th time
Created bg thread 0x7fc0f5bff700
2013/03/04-15:58:17 Reopening database for the 5th time
2013/03/04-15:58:17 Reopening database for the 6th time
2013/03/04-15:58:18 Reopening database for the 7th time
2013/03/04-15:58:18 Reopening database for the 8th time
2013/03/04-15:58:18 Reopening database for the 9th time
2013/03/04-15:58:18 Reopening database for the 10th time
2013/03/04-15:58:18 Reopening database for the 11th time
2013/03/04-15:58:18 Starting verification
Stress Test : 1836.258 micros/op 544 ops/sec
: Wrote 0.01 MB (0.01 MB/sec) (59% of 1011 ops)
: Wrote 605 times
: Deleted 305 times
: 50/90 gets found the key
: Got errors 0 times
2013/03/04-15:58:18 Verification successful
Revert Plan: OK
Task ID: #
Reviewers: emayanke, dhruba
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9081
Summary: In light of the new option introduced by commit 806e264350 where the database has an option to compact before flushing to disk, we want the stress test to test both sides of the option. Have made it to 'deterministically' and configurably change that option for reopens.
Test Plan: make db_stress; ./db_stress with some differnet options
Reviewers: dhruba, vamsi
Reviewed By: dhruba
CC: leveldb, sheki
Differential Revision: https://reviews.facebook.net/D9165
Summary:
Store the last flushed, seq no. in db_impl. Check against it in
transaction Log iterator. Do not attempt to read ahead if we do not know
if the data is flushed completely.
Does not work if flush is disabled. Any ideas on fixing that?
* Minor change, iter->Next is called the first time automatically for
* the first time.
Test Plan:
existing test pass.
More ideas on testing this?
Planning to run some stress test.
Reviewers: dhruba, heyongqiang
CC: leveldb
Differential Revision: https://reviews.facebook.net/D9087
Summary:
Use only the counter mechanism. Do away with
incNumFileOpens, incNumFileClose, incNumFileErrors
s/NULL/nullptr/g in db/table_cache.cc
Test Plan: make clean check
Reviewers: dhruba, heyongqiang, emayanke
Reviewed By: dhruba
CC: leveldb
Differential Revision: https://reviews.facebook.net/D8841
Summary:
Currently the test tracks all writes in memory and
uses it for verification at the end. This has 4 problems:
(a) It needs mutex for each write to ensure in-memory update
and leveldb update are done atomically. This slows down the
benchmark.
(b) Verification phase at the end is time consuming as well
(c) Does not test batch writes or snapshots
(d) We cannot kill the test and restart multiple times in a
loop because in-memory state will be lost.
I am adding a FLAGS_multi that does MultiGet/MultiPut/MultiDelete
instead of get/put/delete to get/put/delete a group of related
keys with same values atomically. Every get retrieves the group
of keys and checks that their values are same. This does not have
the above problems but the downside is that it does less amount
of validation than the other approach.
Test Plan:
This whole this is a test! Here is a small run. I am doing larger run now.
[nponnekanti@dev902 /data/users/nponnekanti/rocksdb] ./db_stress --ops_per_thread=10000 --multi=1 --ops_per_key=25
LevelDB version : 1.5
Number of threads : 32
Ops per thread : 10000
Read percentage : 10
Delete percentage : 30
Max key : 2147483648
Num times DB reopens: 10
Num keys per lock : 4
Compression : snappy
------------------------------------------------
Creating 536870912 locks
2013/02/20-16:59:32 Starting database operations
Created bg thread 0x7f9ebcfff700
2013/02/20-16:59:37 Reopening database for the 1th time
2013/02/20-16:59:46 Reopening database for the 2th time
2013/02/20-16:59:57 Reopening database for the 3th time
2013/02/20-17:00:11 Reopening database for the 4th time
2013/02/20-17:00:25 Reopening database for the 5th time
2013/02/20-17:00:36 Reopening database for the 6th time
2013/02/20-17:00:47 Reopening database for the 7th time
2013/02/20-17:00:59 Reopening database for the 8th time
2013/02/20-17:01:10 Reopening database for the 9th time
2013/02/20-17:01:20 Reopening database for the 10th time
2013/02/20-17:01:31 Reopening database for the 11th time
2013/02/20-17:01:31 Starting verification
Stress Test : 109.125 micros/op 22191 ops/sec
: Wrote 0.00 MB (0.23 MB/sec) (59% of 32 ops)
: Deleted 10 times
2013/02/20-17:01:31 Verification successful
Revert Plan: OK
Task ID: #
Reviewers: dhruba, emayanke
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D8733
Summary:
Fixed a bug in the stress-test where the correct size was not being
passed to GenerateValue. This bug was there since the beginning but assertions
were switched on in our code-base only recently.
Added comments on the top detailing how the stress test works and how to
quicken/slow it down after investigation.
Test Plan: make all check. ./db_stress
Reviewers: dhruba, asad
Reviewed By: dhruba
CC: vamsi, sheki, heyongqiang, zshao
Differential Revision: https://reviews.facebook.net/D8727
Xing Jin