For the benchmark tool itself I usually use 3 tools. The first one is good old dd. The problem with dd is that it can only do 1 type of benchmark (sequential read and write), but it can to this to and from many sources and that is its strength.
So how do you use dd for a disk benchmark. Well usually like this :
dd if=/dev/zero of=/dev/sdb bs=64k count=100kFirst a word of warning. Using dd like this will mean that any data on /dev/sdb (including filesystems) will be overwritten. The dd command does a write test and the second dd command does a read test. The write test reads from /dev/zero and writes to /dev/sdb, it uses a blocksize of 64k and it reads 102400 blocks. The read test reads from /dev/sdb and writes to /dev/null, it uses the same amount and size of blocks like the read test.
dd of=/dev/null if=/dev/sdb bs=64k count=100k
So in this case we are testing /dev/sdb, but this can be any blockdevice. Like /dev/md1 (a software RAID device) or /dev/VolGroup00/Logvol01 (a LVM logical volume.
The amount of blocks you need to read or write to have a valid is very simple. Make sure you process at least twice the amount of memory you have in your system. Why ? Because the Linux kernel does caching you need to make sure you request more data then the size of the cache to make sure you are actually testing the disk and not the Linux kernel caching.
And then the blocksize remains. Well, this depends on the type of usage you will do on the disks or RAID arrays. There are no real fixed rules for this. But generally speaking when you have a lot of small files take a small blocksize (4 to 16k), for standard fileserving and databases take a medium blocksize (64 to 128k) and for large files or applications that do a lot of sequential I/O use a large blocksize (256k and larger).
One final note on dd, since CentOS 5 dd will, when it has finished a test, tell you the speed if obtained. But on older versions of CentOS you will need to calculate this yourself. Another option there is to use ddrescue, which is a tool similar to dd but also provides the bandwidth used in its output. You can find ddrescue for CentOS in Dag's RPMForge repository.
As I said, dd only does sequential I/O but since it does not need a filesystem it is very useful to get a baseline/indication/magnitude/... for the performance of a disk. The next step is to put a filesystem on the device being tested. Then the second tool I use to test performance is iozone. You can also find prebuild rpms for CentOS in Dag's RPMForge repository.
I usually use iozone like this
iozone -e -i0 -i1 -i2 -+n -r 16k -s8g -t1Iozone has a look of options (that is part why I like it). If you do iozone -h you will get the complete list. I suggest you go over them when you start testing so you know all your options. But the example above shows enough to get started. The -e option adds the time of the flush operation to the results and the -+n disables the retest feature. The different -i options indicate the tests done by iozone, -i0 is the sequential write test (always needed), -i1 is sequential read and -i0 is random read and write. Another interresting test is the mixed read/write (-i8). The 3 remaining options are : -r 16k is the record (or block) size used, -s8g is the size of the data tested (8 GB in this case, remember the 2x RAM rule here) and -t1 is the amount of threads performing the test (in this case 1).
Depepding in the application you are planning to use you try to use iozone in a way that it mimics as close as possible the IO patterns of your real application. This means playing with the -i (the kind of test), the -r (block size) and -t (number of threads) options.
Other nice features of iozone are the autotest. Here iozone can iterate over different tests using different parameters for file size and record size. Also iozone can output its results in a Excell file, so you later make nice graphs of the tests.
The final test that I perform is a bit less scientific, but still a good indication of performance of a disk or a RAID array. The test is a copy of a directory that contains