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:
- Dual Pentium Pro 150 MHz
- 256 MB RAM (60 MHz EDO)
- Three IBM UltraStar 9ES 4.5 GB, SCSI U2W
- Adaptec 2940U2W
- One IBM UltraStar 9ES 4.5 GB, SCSI UW
- Adaptec 2940 UW
- Kernel 2.2.7 with RAID patches
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 4096This 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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