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9. Performance

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at http://raid.wiki.kernel.org/

This section contains a number of benchmarks from a real-world system using software RAID. There is some general information about benchmarking software too.

Benchmark samples were done with the bonnie program, and at all times on files twice- or more the size of the physical RAM in the machine.

The benchmarks here only measures input and output bandwidth on one large single file. This is a nice thing to know, if it's maximum I/O throughput for large reads/writes one is interested in. However, such numbers tell us little about what the performance would be if the array was used for a news spool, a web-server, etc. etc. Always keep in mind, that benchmarks numbers are the result of running a "synthetic" program. Few real-world programs do what bonnie does, and although these I/O numbers are nice to look at, they are not ultimate real-world-appliance performance indicators. Not even close.

For now, I only have results from my own machine. The setup is:

The three U2W disks hang off the U2W controller, and the UW disk off the UW controller.

It seems to be impossible to push much more than 30 MB/s thru the SCSI busses on this system, using RAID or not. My guess is, that because the system is fairly old, the memory bandwidth sucks, and thus limits what can be sent thru the SCSI controllers.

9.1 RAID-0

Read is Sequential block input, and Write is Sequential block output. File size was 1GB in all tests. The tests where done in single-user mode. The SCSI driver was configured not to use tagged command queuing.


Chunk size Block size Read kB/s Write kB/s
4k 1k 19712 18035
4k 4k 34048 27061
8k 1k 19301 18091
8k 4k 33920 27118
16k 1k 19330 18179
16k 2k 28161 23682
16k 4k 33990 27229
32k 1k 19251 18194
32k 4k 34071 26976

>From this it seems that the RAID chunk-size doesn't make that much of a difference. However, the ext2fs block-size should be as large as possible, which is 4kB (eg. the page size) on IA-32.

9.2 RAID-0 with TCQ

This time, the SCSI driver was configured to use tagged command queuing, with a queue depth of 8. Otherwise, everything's the same as before.


Chunk size Block size Read kB/s Write kB/s
32k 4k 33617 27215

No more tests where done. TCQ seemed to slightly increase write performance, but there really wasn't much of a difference at all.

9.3 RAID-5

The array was configured to run in RAID-5 mode, and similar tests where done.


Chunk size Block size Read kB/s Write kB/s
8k 1k 11090 6874
8k 4k 13474 12229
32k 1k 11442 8291
32k 2k 16089 10926
32k 4k 18724 12627

Now, both the chunk-size and the block-size seems to actually make a difference.

9.4 RAID-10

RAID-10 is "mirrored stripes", or, a RAID-1 array of two RAID-0 arrays. The chunk-size is the chunk sizes of both the RAID-1 array and the two RAID-0 arrays. I did not do test where those chunk-sizes differ, although that should be a perfectly valid setup.


Chunk size Block size Read kB/s Write kB/s
32k 1k 13753 11580
32k 4k 23432 22249

No more tests where done. The file size was 900MB, because the four partitions involved where 500 MB each, which doesn't give room for a 1G file in this setup (RAID-1 on two 1000MB arrays).

9.5 Fresh benchmarking tools

To check out speed and performance of your RAID systems, do NOT use hdparm. It won't do real benchmarking of the arrays.

Instead of hdparm, take a look at the tools described here: IOzone and Bonnie++.

IOzone is a small, versatile and modern tool to use. It benchmarks file I/O performance for read, write, re-read, re-write, read backwards, read strided, fread, fwrite, random read, pread, mmap, aio_read and aio_write operations. Don't worry, it can run on any of the ext2, ext3, reiserfs, JFS, or XFS filesystems in OSDL STP.

You can also use IOzone to show throughput performance as a function of number of processes and number of disks used in a filesystem, something interesting when it's about RAID striping.

Although documentation for IOzone is available in Acrobat/PDF, PostScript, nroff, and MS Word formats, we are going to cover here a nice example of IOzone in action:

iozone -s 4096 
This would run a test using a 4096KB file size.

And this is an example of the output quality IOzone gives

        File size set to 4096 KB
        Output is in Kbytes/sec
        Time Resolution = 0.000001 seconds.
        Processor cache size set to 1024 Kbytes.
        Processor cache line size set to 32 bytes.
        File stride size set to 17 * record size.
                                                            random  random    bkwd  record  stride            
              KB  reclen   write rewrite    read    reread    read   write    read rewrite    read   fwrite frewrite   fread  freread
            4096       4   99028  194722   285873   298063  265560  170737  398600  436346  380952    91651   127212  288309   292633
Now you just need to know about the feature that makes IOzone useful for RAID benchmarking: the file operations involving RAID are the read strided. The example above shows a 380.952Kb/sec. for the read strided, so you can go figure.

Bonnie++ seems to be more targeted at benchmarking single drives that at RAID, but it can test more than 2Gb of storage on 32-bit machines, and tests for file creat, stat, unlink operations.


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