[DRBD-user] Huge max latency with sequential writes

Weilin Gong wgong at alcatel-lucent.com
Fri Jan 5 23:37:28 CET 2007

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


Hi,

I ran some drbd performance tests with 'tiobench':

tiobench.pl --identifier drbd-connected --size 5000 --numruns 1 --dir 
/secroot --block 4096 --threads 4

noticed the huge max latency numbers with Sequential Writes:

Unit information
================
File size            = megabytes
Blk Size            = bytes
Rate                 = megabytes per second
CPU%             = percentage of CPU used during the test
Latency            = milliseconds
Lat%                = percent of requests that took longer than X seconds
CPU Eff           = Rate divided by CPU% - throughput per cpu load

Identifier

	

File Size

	

Blk Size

	

Num Thr

	

Rate

	

Maximum CPU%

	

Avg Latency

	

Max Latency

	

Lat% >2s

	

Lat% >10s

	

CPU Eff

Raw-device

	

5000

	

4096

	

4

	

31.24

	

26.80%

	

0.364

	

5635.06

	

0.00242

	

0.00000

	

117

Drbd-disconnected

	

5000

	

4096

	

4

	

33.18

	

26.29%

	

0.319

	

110394.58

	

0.00190

	

0.00016

	

126

Drbd-connected

	

5000

	

4096

	

4

	

20.88

	

50.18%

	

0.457

	

135712.09

	

0.00110

	

0.00008

	

42


My lab setting:

    * Two Linux nodes on 2.6.10_mvlcge401-pc_target-x86_pentium3-P3SMP.
    * Dual-core Intel Xeon 2GHz CPU, 4GB Memory, 1Gbits/s network interface.
    * 5.7GB partition on SCSI disk, ext2 file system with the "-T
      largefile4" option.
    * DRBD version is 0.7.11, rate=100M;  protocal C; sndbuf-size 1024k.

I don't believe the disk (~50MB/s) or network may be the factors here. 
You can see the big number jump even with the DRBD disconnected mode.

Any help on analyzing/identifying the problem will be greatly appreciated.

Weilin
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