Hi,
We're currently evaluating Riak and we have noticed that Bitcask with
{sync_strategy, o_sync} option seems to have no effect. Instead of forcing
syncs on every writes to the disk, it does the exactly same to the default
behavior {sync_strategy, none}, which doesn't force kernel to sync the writes.
As you can see in the attached vmstat result, "bo" (blocks sent to a block
device [blocks/s]) column has many 0s even though basho_bench with 200 max
workers was constantly adding 100KB key-values to Riak.
The current behavior could be dangerous if someone rely on {sync_strategy,
o_sync} for durability.
My colleague checked Bitcask implementation and found that it uses "fcntl (2)"
system call to set O_SYNC flag to the file descriptor. Unfortunately, O_SYNC
being set by this way will be ignored by kernel and has no effect. To make it
work, Betcask should call "open" system call with O_SYNC option.
https://patchwork.kernel.org/patch/591481/
Thanks,
Tatsuya
vmstat result. See "bo" column with many 0s (bo: blocks sent to a block device
[blocks/s])
procs -----------memory---------- ---swap-- -----io---- --system--
-----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
1 0 152 31796516 4508 933452 0 0 0 0 9797 17377 4 7 89 0
0
1 0 152 31735260 4556 992524 0 0 0 0 9550 16871 4 7 89 0
0
0 5 152 31676856 4608 1045328 0 0 0 58632 9196 15189 4 7 89
1 0
0 0 152 31673880 4636 1056056 0 0 0 188868 2807 2549 1 3 79
18 0
1 0 152 31611752 4704 1111920 0 0 0 0 9332 16648 4 7 89
0 0
0 0 152 31553852 4760 1170592 0 0 0 0 9369 16949 5 7 89
0 0
0 0 152 31494068 4840 1228580 0 0 0 0 9317 16863 4 7 89
0 0
0 4 152 31432936 4908 1283836 0 0 0 47548 8794 15442 4 7 88
1 0
0 0 152 31428348 4940 1293996 0 0 0 191140 3513 2998 1 3 80
17 0
0 0 152 31367708 4988 1351964 0 0 0 0 9745 16114 4 7 89
0 0
2 0 152 31302104 5048 1412876 0 0 0 0 9637 17205 5 7 89
0 0
0 0 152 31244072 5100 1468180 0 0 0 0 9405 16214 4 7 89
0 0
0 5 152 31184428 5168 1522604 0 0 0 47856 8983 16001 4 7 88
1 0
0 0 152 31179256 5192 1533648 0 0 0 191968 3030 3099 1 3 87
10 0
2 0 152 31123316 5256 1588244 0 0 0 52 9548 16022 4 7 89
0 0
1 0 152 31061068 5308 1648228 0 0 0 0 9319 17110 5 7 88
0 0
2 0 152 30997704 5368 1708092 0 0 0 0 9423 17373 5 7 88
0 0
0 3 152 30941664 5420 1760988 0 0 0 42728 8871 15132 4 7 89
1 0
0 0 152 30934964 5440 1774016 0 0 0 196216 3282 3489 1 3 80
17 0
0 0 152 30886472 5496 1820728 0 0 0 52 7654 13882 4 6 91
0 0
0 0 152 30886472 5496 1820612 0 0 0 0 1006 107 0 0 100
0 0
0 0 152 30886208 5496 1821156 0 0 0 0 1050 150 0 0 100
0 0
--
Tatsuya Kawano (Mr.)
Tokyo, Japan
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