Jens Axboe <jens.axboe@xxxxxxxxxx> writes:
> On Wed, Jun 11 2008, Dmitri Monakhov wrote:
>> I've found what any non continious sequence violation result in significant
>> pefrormance drawback. I've two types of requests:
>> 1)Ideally sequential writes:
>> for(i=0;i<num;i++) {
>> write(fd, chunk, page_size*32);
>> }
>> fsync(fd);
>>
>> 2) Sequential writes with dgap for each 32'th page
>> for(i=0;i<num;i++) {
>> write(fd, chunk, page_size*31);
>> lseek(fd, page_size, SEEK_CUR);
>> }
>> fsync(fd);
>>
>> I've found what second IO pattern is about twice times slower whan the
>> first one regardless to ioscheduler or HW disk. It is not clear to me
>> why this happen. Is it linux speciffic or general hardware behaviour
>> speciffic. I've naively expected what disk hardware cat merge several
>> 31-paged requests in to continious one by filling holes by some sort
>> of dummy activity.
>
> Performance should be about the same. The first is always going to be a
> little faster, on some hardware probably quite a bit. Are you using
> write back caching on the drive? I ran a quick test here, and the second
> test is about ~5% slower on this drive.
Hmmm... it is definitly not happen in may case.
I've tested following sata drive w and w/o write cache
AHCI
ata7.00: ATA-7: ST3250410AS, 3.AAC, max UDMA/133
ata7.00: 488397168 sectors, multi 0: LBA48 NCQ (depth 31/32)
In all cases it is about two times slower.
# time /tmp/wr_test /dev/sda3 32 0 800
real 0m1.183s
user 0m0.002s
sys 0m0.079s
# time /tmp/wr_test /dev/sda3 31 1 800
real 0m3.240s
user 0m0.000s
sys 0m0.078s
Other SCSI disk is about ~1.5 times slower :
Vendor: FUJITSU Model: MAW3073NP Rev: 0104
Type: Direct-Access ANSI SCSI revision: 03
target2:0:4: Beginning Domain Validation
target2:0:4: Ending Domain Validation
target2:0:4: FAST-160 WIDE SCSI 320.0 MB/s DT IU QAS RTI WRFLOW PCOMP (6.25 ns, offset 127)
SCSI device sde: 143638992 512-byte hdwr sectors (73543 MB)
sde: Write Protect is off
sde: Mode Sense: b3 00 00 08
SCSI device sde: drive cache: write back
test source:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, char** argv)
{
char *buf;
long chunk, off, num, i;
int fd;
if (argc != 5);
chunk = atoi(argv[2])*4096;
off = atoi(argv[3])*4096;
num = atoi(argv[4]);
buf = malloc(chunk);
fd = open(argv[1], O_RDWR|O_CREAT, 0777);
for(i = 0; i < num; i++) {
write(fd, buf, chunk);
lseek(fd, off, SEEK_CUR);
}
return fsync(fd);
}
>
> --
> Jens Axboe
