creating/modifying sparse files on linux

[draghuram]

Hi,

Is there any special support for sparse file handling in python? My
initial search didn't bring up much (not a thorough search). I wrote
the following pice of code:

options.size = 6442450944
options.ranges = ["4096,1024","30 000,314572800"]
fd = open("testfile" , "w")
fd.seek(options .size-1)
fd.write("a")
for drange in options.ranges:
off = int(drange.spli t(",")[0])
len = int(drange.spli t(",")[1])
print "off =", off, " len =", len
fd.seek(off)
for x in range(len):
fd.write("a")

fd.close()

This piece of code takes very long time and in fact I had to kill it as
the linux system started doing lot of swapping. Am I doing something
wrong here? Is there a better way to create/modify sparse files?

Thanks,
Raghu.

[Mike Meyer]

"dr*******@gmai l.com" <dr*******@gmai l.com> writes:

My goal is very simple. Have a mechanism to create sparse files and
modify them by writing arbitratry ranges of bytes at arbitrary offsets.
I did get the information I want (xrange instead of range, and a simple
way to generate 1Mb string in memory). Thanks for pointing out about
using "len" as variable. It is indeed silly.

My only assumption from underlying OS/file system is that if I seek
past end of file and write some data, it doesn't generate blocks for
data in between. This is indeed true on Linux (I tested on ext3).

This better be true for anything claiming to be Unix. The results on
systems that break this aren't pretty.

<mike
--
Mike Meyer <mw*@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.