Why are unformatted writes broken into 512k chunks?

[crtierney42]

I have a test code that writes then subsequently reads a large file (in this case 512MB) in several different records. What I see from strace is that the write are broken into 512KB chunks, while the reads are read in as the size of the record.

Why is this? I tested with both 9.1.036 and 10.1.012.

Thanks,
Craig

program testfortranio


implicit none

integer maxsize,reccnt
parameter(maxsize=32*1024*1024,reccnt=4)

integer i
integer ar(maxsize),ar2(maxsize)
integer lun


lun=11

do i=1,maxsize
ar(i)=i
enddo
open(lun,file="/tg2/jetmgmt/ctierney/myfile.fort",form="unformatted", status="unknown")

do i=1,reccnt
write(lun) ar
enddo

close (lun)


open(lun,file="/tg2/jetmgmt/ctierney/myfile.fort", form="unformatted", status="old")
do i=1,reccnt
read(lun) ar2
enddo
close(lun)

do i=1,maxsize
if (ar(i) .ne. ar2(i)) then
write(*,*) 'The two arrays dont match at ', i
endif
enddo

stop
end

[TimP]

Documentation for the current compilers specifies that you must specify BLOCKSIZE in OPEN if you want other than the filesystem default. BLOCKSIZE was not fully supported in 9.1.

[crtierney42]

I tried blocksize, but I only tried it under 9.1 and saw no difference, so I didn't bother trying it with 10.1.

So why is there even such a setting? Since the data are already allocated, and the
call to the glibc write command takes a pointer, why bother breaking the data up?

Craig