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Re: File Channel performance and fsync
Hi Denny

Thanks for the inputs.
Btw when you say you tested another case without 'fsync'; I think
you changed the file channel code to comment out 'flush' part of it.
And if we rely on OS flushing then still it can be reasonably reliable.
Is that right?


On 10/22/2012 07:08 PM, Denny Ye wrote:
> hi Jagadish,
>    I have tested performance of FileChannel recently. Here I can
> support the test report to you for your thinking and questions at this
> thread.
>     Talking about the comparison between FileChannel and File Sink.
> FileChannel supports both sequential writer and random reader, there
> have so many times shift of magnetic head, it's slow than the
> sequential writing much more.
>     'fsync' command has consuming much time than writing, almost
> 100times/sec, same as number mentioned from Brock. Also, I didn't know
> why there have such difference between your two servers. I think it
> might be related with OS version (usage between fsync and fdatasync
> instruction) or disk driver (RAID, caching strategy, and so on).
>     Throughput of single FileChannel is almost 3-5MB/sec in my
> environment. Thus I used 5 channels with 18MB/sec. It's hard to
> believe the linear increasing with more channels. Meanwhile, it look
> like the limit of throughput with 'fsync' operation. I tested another
> case without 'fsync' operation after each batch, almost
> 35-40MB/sec(Also, I removed the pre-allocation at disk writing in this
> case).
>     Hope useful for you.
>    PS : I heard that OS has demon thread to flush page cache to
> disk asynchronously with second latency, does it's effective for
> amount of data with tolerant loss?
> -Regards
> Denny Ye
> 2012/10/22 Jagadish Bihani <[EMAIL PROTECTED]
> <mailto:[EMAIL PROTECTED]>>
>     Hi
>     I am writing this on top of another thread where there was
>     discussion on "fsync lies" and
>     only file channel used fsync and not file sink. :
>     -- I tested the fsync performance on 2 machines  (On 1 machine I
>     was getting very good throughput
>     using file channel and on another almost 100 times slower with
>     almost same hardware configuration.)
>     using following code
>     #define PAGESIZE 4096
>     int main(int argc, char *argv[])
>     {
>             char my_write_str[PAGESIZE];
>             char my_read_str[PAGESIZE];
>             char *read_filename= argv[1];
>             int readfd,writefd;
>             readfd = open(read_filename,O_RDONLY);
>             writefd = open("written_file",O_WRONLY|O_CREAT,777);
>             int len=lseek(readfd,0,2);
>             lseek(readfd,0,0);
>             int iterations = len/PAGESIZE;
>             int i;
>             struct timeval t0,t1;
>            for(i=0;i<iterations;i++)
>             {
>                     read(readfd,my_read_str,PAGESIZE);
>                     write(writefd,my_read_str,PAGESIZE);
>     *gettimeofday(&t0,0);**
>     **                fsync(writefd);**
>     **              gettimeofday(&t1,0);*
>                     long elapsed = (t1.tv_sec-t0.tv_sec)*1000000 +
>     t1.tv_usec-t0.tv_usec;
>                     printf("Elapsed time is= %ld \n",elapsed);
>              }
>             close(readfd);
>             close(writefd);
>     }
>     -- As expected it requires typically 50000 microseconds for fsync
>     to complete on one machine and 200 microseconds
>     on another machine it took 290 microseconds to complete on an
>     average. So is machine with higher
>     performance is doing a 'fsync lie'?
>     i
>     -- If I have understood it clearly; "fsync lie" means the data is
>     not actually written to disk and it is in
>     some disk/controller buffer.  I) Now if disk loses power due to
>     some shutdown or any other disaster, data will
>     be lost. II) Can data be lost even without it ? (e.g. if it is
>     keeping data in some disk buffer and if fsync is being
>     invoked continuously then will that data can also  be lost? If