Gary R. Van Sickle wrote:
[cc'ing you per your request]
From: Vladimir Dergachev
Sent: Wednesday, August 02, 2006 5:33 PM
Subject: NTFS fragmentation
Hi all,
I have encountered a rather puzzling fragmentation
that occurs when writing files using Cygwin.
What happens is that if one creates a new file and
writes data to it (whether via a command line redirect or
with a Tcl script - have not tried C
yet) the file ends up heavily fragmented.
In contrast, native Windows utilities do not exhibit
this issue.
Someone suggested to me that Windows requires an
expected file length to be passed at the time of open, thus I
searched on Google and found "fsutil" program that allows to
reserve space on the filesystem.
I attached a small Tcl script that, when run, creates
two 30 MB files - one using regular open/write pair (and
which is fragmented into about 300 pieces on my system) and
one using fsutil/open in append mode/seek 0 method.
To see the problem defragment your system, run the test
script and then run analyze and ask to view report. You will
see a.dat at top of the list, while b.dat never appears in
the report.
Despite the workaround, it is still kinda hard for me
to believe that anyone has designed a filesystem that needs
to know what is the file size going to be - especially for a
single program writing on an almost empty disk. Perhaps there
is some sort of environment variable that I need to set ?
Any suggestions and comments would be greatly
appreciated.
Please CC me - I am not on the list.
thank you very much
Vladimir Dergachev
I'll try your test case when I get a chance, but my WAG is that you're
seeing the effects of Cygwin's creation of sparse files by default for any
file beyond a certain size. I unfortunately do not recall what that size
is. What happens as you change FILE_SIZE and/or BUFFER_SIZE in your script,
to maybe a small multiple of your cluster size?
This clicked with me as well. I was thinking first though to try it with
straight C, to remove the possibility of some TCL pollution. Otherwise a
quick Google of "sparse" for cygwin dot com turns up lots of relevant hits,
utilities, and technical details. Should be pretty easy to determine if the
resulting files are sparse or not with this info.
--
Larry Hall http://www.rfk.com
RFK Partners, Inc. (508) 893-9779 - RFK Office
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