Re: Strange prformance degradation when COW writes happen at fixed offsets

[Nik Markovic]

I noticed a few errors in the script that I used. I corrected it and
it seems that degradation is occurring even at fully random writes:


#!/bin/bash

mode=$1
if [ -z "$mode" ]; then
        echo "Usage $0 <incremental|random|fixed>"
        exit -1
fi
mode=$1

src=`pwd`/test/src
dst=`pwd`/test/dst
srcfile=$src/test.tar
dstfile=$dst/test.tar

mkdir -p $src
mkdir -p $dst

filesize=100MB

#build a 10GB file from a smaller download. You can tweak filesize and
the loop below for lower bandwidth
if [ ! -f $srcfile ]; then
        cd $src
        if [ ! -f $srcfile.dl ]; then
                wget http://download.thinkbroadband.com/${filesize}.zip
--output-document=$srcfile.dl
        fi
        rm -rf tarbase
        mkdir tarbase
        for  i in {1..100}; do
                cp --reflink=always $srcfile.dl tarbase/$i.dl
        done
        tar -cvf $srcfile tarbase
        rm -rf tarbase
fi

cat <<END > $src/FileTest.java
import java.io.IOException;
import java.io.RandomAccessFile;
public class FileTest {
    public static final int BLOCK_SIZE = 50000;
    public static final int MAX_ITERATIONS = 40000;
    public static void main(String args[]) throws IOException {
        String mode = args[0];
        RandomAccessFile f = new RandomAccessFile(args[1], "rw");
        //int offset = 0;
        int i;
        int offset = new java.util.Random().nextInt(BLOCK_SIZE); //
initializer ONLY for incremental mode
        for (i=0; i < MAX_ITERATIONS; i++) {
            try {
                int writeOffset;
                if (mode.equals("incremental")) {
                    writeOffset = new
java.util.Random().nextInt(offset + i * BLOCK_SIZE);
                }  else if (mode.equals("fixed")) {
                    writeOffset = i * BLOCK_SIZE;
                    offset = writeOffset; // for reporting it at the end
                } else { // mode.equals random
                    writeOffset = new
java.util.Random().nextInt(((int)f.length() - 100));
                    offset = writeOffset; // for reporting it at the end
                }
                if (writeOffset > (f.length() - 100)) {
                        break;
                }
                f.seek(writeOffset);
                f.writeBytes("DEADBEEF");
            } catch (java.io.IOException e) {
                System.out.println("EOF");
                break;
            }
        }
        System.out.print("Last offset=" + offset);
        System.out.println(". Made " + i + " random writes.");
        f.close();
    }
}
END

cd $src
javac FileTest.java


/usr/bin/time --format 'rm: %E' rm -rf $dst/*
cp --reflink=always $srcfile $dst/1.tst
cd $dst
for i in {1..100}; do   
        echo -n "$i."
        i_plus=`expr $i + 1`
        /usr/bin/time --format 'write: %E' java -cp $src FileTest $mode $i.tst
        /usr/bin/time --format 'cp:    %E' cp --reflink=always $i.tst 
$i_plus.tst
        /usr/bin/time --format 'rm:    %E' rm $i.tst
        /usr/bin/time --format 'sync:  %E' sync
        sleep 1
done
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