NAME

phybs —

reveal a storage device's physical block size

SYNOPSIS

phybs [-rsw] [-l minsize] [-h maxsize] [-t total] device

DESCRIPTION

The phybs utility runs a series of I/O benchmarks designed to reveal a storage device's physical block size by comparing the performance of small and large I/O operations at various (mis-)alignments.

The phybs utility makes a series of passes with increasing block sizes. In each pass, it either reads or writes (or both) a number of non-consecutive blocks at increasing offsets relative to the ideal alignment, which is assumed to be multiples of the block size. The results are presented in terms of time elapsed, transactions per second and kB per second.

If the device's physical block size is larger than its logical block size, peak performance will be obtained when reading or writing multiples of the physical block size at offsets which are also multiples of the physical block size, while I/O operations which do not meet these criteria will be significantly slower.

The following options are available:

-h maxsize: Specify the maximum block size to test. This must be a power of two and a multiple of the minimum block size (see -l). The default is eight times the minimum block size.
-l minsize: Specify the minimum block size to test. This must be a power of two and a multiple of the device's logical block size. The default is the device's logical block size.
-r: Perform read operations. This is the default.
-s: Open the device in synchronous mode.
-t total: Specify the total amount of data to read or write in each pass. This must be a power of two and a multiple of the maximum block size (see -h). For best results, it should also be significantly larger than the device's on-board cache. The default is 128 MB.
-w: Perform write operations.

IMPLEMENTATION NOTES

The phybs utility can be used on files as well as raw devices, but the characteristics of the underlying file system will greatly influence the results, possibly to the point where they can not reveal anything of interest about the actual device.

The phybs utility reports the benchmark results, but makes no attempt at interpreting them.

EXIT STATUS

The phybs utility returns EX_OK if the benchmark completed successfully, EX_USAGE if the parameters specified on the command line were invalid, EX_PERM if the user does not have permission to open the specified device, EX_IOERR if it was interrupted by an I/O error, and EX_OSERR if it was interrupted for any other reason.

AUTHORS

The phybs utility and this manual page were written by Dag-Erling Smørgrav ⟨des@FreeBSD.org⟩.