02-09-2013
Physical disk IO size smaller than fragment block filesystem size ?
Hello,
in one default UFS filesystem we have 8K block size (bsize) and 1K fragmentsize (fsize). At this scenary I thought all "FileSytem IO" will be 8K (or greater) but never smaller than the fragment size (1K). If a UFS fragment/blocksize is allwasy several ADJACENTS sectors on disk (in a disk with sector=512B), all "physical disk IO" it will allways, like "Filesystem IO", greater than 1K.
But with dtrace script from DTrace Toolkit (bitesize.d) I can see IOs with 512B size.
¿What is wrong in my assumptions or what is the explanation?
Thank you very much in advance!!
Last edited by rarino2; 02-10-2013 at 08:43 AM..
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LEARN ABOUT MOJAVE
seeksize.d
seeksize.d(1m) USER COMMANDS seeksize.d(1m)
NAME
seeksize.d - print disk event seek report. Uses DTrace.
SYNOPSIS
seeksize.d
DESCRIPTION
seeksize.d is a simple DTrace program to print a report of disk event seeks by process.
This can be used to identify whether processes are accessing the disks in a "random" or "sequential" manner. Sequential is often desirable,
indicated by mostly zero length seeks.
Since this uses DTrace, only users with root privileges can run this command.
EXAMPLES
Sample until Ctrl-C is hit then print report,
# seeksize.d
FIELDS
PID process ID
CMD command and argument list
value distance in disk blocks (sectors)
count number of I/O operations
DOCUMENTATION
See the DTraceToolkit for further documentation under the Docs directory. The DTraceToolkit docs may include full worked examples with ver-
bose descriptions explaining the output.
EXIT
seeksize.d will sample until Ctrl-C is hit.
AUTHOR
Brendan Gregg [Sydney, Australia]
SEE ALSO
iosnoop(1M), bitesize.d(1M), dtrace(1M)
version 0.95 May 14, 2005 seeksize.d(1m)