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** Changed in: linux (Ubuntu)
Status: New => Incomplete
** Tags added: bionic
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https://bugs.launchpad.net/bugs/1785020
Title:
fsync is slow on later kernels with ext4 filesystms
Status in linux package in Ubuntu:
Incomplete
Bug description:
So this is a simple one, if I run the following code (see below) on Ubuntu
18.04 (4.15.0-29-generic ) and against Ubuntu 16.04 ( 4.4.0-87-generic) I get
two different results ; The question is why? It seems that fsync may be broken
in the latest kernel and may well be broken as early as 4.10 ? This has huge
implications around performance sensitive applications like mysql.
It also so worth noting that I'm running this against an ext4 filesystem with
the following mount options
rw,noatime,nodiratime,nobarrier,data=journal
4.4.0-87-generic - # time ./fsync
real 0m7.537s <-----
user 0m0.000s
sys 0m1.060s <-----
4.15.0-29-generic - # time ./fsync
real 1m38.299s <-----
user 0m0.013s
sys 0m0.893s <-----
Linux buntu-1804 4.15.0-29-generic
no sync() seconds:0.024438 writes per second:0.000000
sync() seconds:1.705764 writes per second:0.000000
real 3m6.475s
user 0m0.006s
sys 0m1.725s
Linux ubuntu 4.4.0-87-generic
no sync() seconds:0.012887 writes per second:0.000001
sync() seconds:0.736563 writes per second:0.000000
real 0m8.945s
user 0m0.016s
sys 0m0.732s
#!/usr/bin/python
import os, sys, mmap
# Open a file
fd = os.open( "/a/testfile", os.O_RDWR|os.O_CREAT|os.O_DIRECT )
m = mmap.mmap(-1, 512)
for i in range (1,10000):
os.lseek(fd,os.SEEK_SET,0)
m[1] = "1"
os.write(fd, m)
os.fsync(fd)
# Close opened file
os.close( fd )
#define _GNU_SOURCE
#include <stdio.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
void withSync() {
int f = open( "/a/t8" , O_RDWR | O_CREAT |O_DIRECT );
lseek (f, 0, SEEK_SET );
int records = 10*1000;
clock_t ustart = clock();
for(int i = 0; i < records; i++) {
write(f, "012345678901234567890123456789" , 30);
fsync(f);
}
clock_t uend = clock();
close (f);
printf(" sync() seconds:%lf writes per second:%lf\n",
((double)(uend-ustart))/(CLOCKS_PER_SEC),
((double)records)/((double)(uend-ustart))/(CLOCKS_PER_SEC));
}
void withoutSync() {
int f = open( "/a/t10" , O_RDWR | O_CREAT | O_DIRECT );
lseek (f, 0, SEEK_SET );
int records = 10*1000;
clock_t ustart = clock();
for(int i = 0; i < records; i++) {
write(f, "012345678901234567890123456789" , 30 );
}
clock_t uend = clock();
close (f);
printf("no sync() seconds:%lf writes per second:%lf \n",
((double)(uend-ustart))/(CLOCKS_PER_SEC),
((double)records)/((double)(uend-ustart))/(CLOCKS_PER_SEC));
}
int main(int argc, const char * argv[])
{
withoutSync();
withSync();
return 0;
}
Linux buntu-1804 4.15.0-29-generic
no sync() seconds:0.024438 writes per second:0.000000
sync() seconds:1.705764 writes per second:0.000000
real 3m6.475s
user 0m0.006s
sys 0m1.725s
Linux ubuntu 4.4.0-87-generic
no sync() seconds:0.012887 writes per second:0.000001
sync() seconds:0.736563 writes per second:0.000000
real 0m8.945s
user 0m0.016s
sys 0m0.732s
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