Understanding IOZone Tests

Note: This is a copied version from IOZone official document IOzone_msword_98.pdf.

Definitions of the tests

  • Write: This test measures the performance of writing a new file. When a new file is written not only does the data need to be stored but also the overhead information for keeping track of where the data is located on the storage media. This overhead is called the “metadata” It consists of the directory information, the space allocation and any other data associated with a file that is not part of the data contained in the file. It is normal for the initial write performance to be lower than the performance of re- writing a file due to this overhead information.
  • Re-write: This test measures the performance of writing a file that already exists. When a file is written that already exists the work required is less as the metadata already exists. It is normal for the rewrite performance to be higher than the performance of writing a new file.
  • Read: This test measures the performance of reading an existing file.
  • Re-Read: This test measures the performance of reading a file that was recently read. It is normal for the performance to be higher as the operating system generally maintains a cache of the data for files that were recently read. This cache can be used to satisfy reads and improves the performance.
  • Random Read: This test measures the performance of reading a file with accesses being made to random locations within the file. The performance of a system under this type of activity can be impacted by several factors such as: Size of operating system’s cache, number of disks, seek latencies, and others.
  • Random Write: This test measures the performance of writing a file with accesses being made to random locations within the file. Again the performance of a system under this type of activity can be impacted by several factors such as: Size of operating system’s cache, number of disks, seek latencies, and others.
  • Random Mix: This test measures the performance of reading and writing a file with accesses being made to random locations within the file. Again the performance of a system under this type of activity can be impacted by several factors such as: Size of operating system’s cache, number of disks, seek latencies, and others. This test is only available in throughput mode. Each thread/process runs either the read or the write test. The distribution of read/write is done on a round robin basis. More than one thread/process is required for proper operation.
  • Backwards Read: This test measures the performance of reading a file backwards. This may seem like a strange way to read a file but in fact there are applications that do this. MSC Nastran is an example of an application that reads its files backwards. With MSC Nastran, these files are very large (Gbytes to Tbytes in size). Although many operating systems have special features that enable them to read a file forward more rapidly, there are very few operating systems that detect and enhance the performance of reading a file backwards.
  • Record Rewrite: This test measures the performance of writing and re-writing a particular spot within a file. This hot spot can have very interesting behaviors. If the size of the spot is small enough to fit in the CPU data cache then the performance is very high. If the size of the spot is bigger than the CPU data cache but still fits in the TLB then one gets a different level of performance. If the size of the spot is larger than the CPU data cache and larger than the TLB but still fits in the operating system cache then one gets another level of performance, and if the size of the spot is bigger than the operating system cache then one gets yet another level of performance.
  • Strided Read: This test measures the performance of reading a file with a strided access behavior. An example would be: Read at offset zero for a length of 4 Kbytes, then seek 200 Kbytes, and then read for a length of 4 Kbytes, then seek 200 Kbytes and so on. Here the pattern is to read 4 Kbytes and then Seek 200 Kbytes and repeat the pattern. This again is a typical application behavior for applications that have data structures contained within a file and is accessing a particular region of the data structure. Most operating systems do not detect this behavior or implement any techniques to enhance the performance under this type of access behavior. This access behavior can also sometimes produce interesting performance anomalies. An example would be if the application’s stride causes a particular disk, in a striped file system, to become the bottleneck.
  • Fwrite: This test measures the performance of writing a file using the library function fwrite(). This is a library routine that performs buffered write operations. The buffer is within the user’s address space. If an application were to write in very small size transfers then the buffered & blocked I/O functionality of fwrite() can enhance the performance of the application by reducing the number of actual operating system calls and increasing the size of the transfers when operating system calls are made. This test is writing a new file so again the overhead of the metadata is included in the measurement.
  • Frewrite: This test measures the performance of writing a file using the library function fwrite(). This is a library routine that performs buffered & blocked write operations. The buffer is within the user’s address space. If an application were to write in very small size transfers then the buffered & blocked I/O functionality of fwrite() can enhance the performance of the application by reducing the number of actual operating system calls and increasing the size of the transfers when operating system calls are made. This test is writing to an existing file so the performance should be higher as there are no metadata operations required.
  • Fread: This test measures the performance of reading a file using the library function fread(). This is a library routine that performs buffered & blocked read operations. The buffer is within the user’s address space. If an application were to read in very small size transfers then the buffered & blocked I/O functionality of fread() can enhance the performance of the application by reducing the number of actual operating system calls and increasing the size of the transfers when operating system calls are made.
  • Freread: This test is the same as fread above except that in this test the file that is being read was read in the recent past. This should result in higher performance as the operating system is likely to have the file data in cache.

Specialized tests

  • Mmap: Many operating systems support the use of mmap() to map a file into a user’s address space. Once this mapping is in place then stores to this location in memory will result in the data being stored going to a file. This is handy if an application wishes to treat files as chunks of memory. An example would be to have an array in memory that is also being maintained as a file in the files system. The semantics of mmap files is somewhat different than normal files. If a store to the memory location is done then no actual file I/O may occur immediately. The use of the msyc() with the flags MS_SYNC, and MS_ASYNC control the coherency of the memory and the file. A call to msync() with MS_SYNC will force the contents of memory to the file and wait for it to be on storage before returning to the application. A call to msync() with the flag MS_ASYNC tells the operating system to flush the memory out to storage using an asynchronous mechanism so that the application may return into execution without waiting for the data to be written to storage. This test measures the performance of using the mmap() mechanism for performing I/O.
  • Async I/O: Another mechanism that is supported by many operating systems for performing I/O is POSIX async I/O. The application uses the POSIX standard async I/O interfaces to accomplish this. Example: aio_write(), aio_read(), aio_error(). This test measures the performance of the POSIX async I/O mechanism.

Refer to our former post Measure Filesystem I/O Performance With iozone for usage examples on IOZone.

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