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List: linux-xfs
Subject: Re: howto preallocate to minimize fragmentation
From: Eric Sandeen <sandeen () sgi ! com>
Date: 2005-09-23 12:58:09
Message-ID: 4333FBE1.2080400 () sgi ! com
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Ying-Hung Chen wrote:
> when I run the sample code, and created "testfile"
>
> [yhchen@fc3 ~]$ ./xfs_fcntl testfile
> [yhchen@fc3 ~]$ ls -l testfile
> -rwx------ 1 yhchen yhchen 0 Sep 23 14:17 testfile
> [yhchen@fc3 ~]$ du testfile
> 20480 testfile
> [yhchen@fc3 ~]$ du -h testfile
> 20M testfile
>
> I see the filesize is 0, but du does show 20MB
>
> However, when I try to determine the file size via fseek, it returns 0.
>
> so, what does this kind of allocation mean? does it really mean that it
run xfs_bmap on the resulting file and you'll see...
> so, if I write anything within this 20MB
> range, it will be writing to the allocated space and hence no
> fragmentation?
right.
> what happens if I go over? my guess is if i go over, xfs
> allocation algorithm will be and there will be fragamentation?
well it depends on what else is going on; if there is plenty of free
space after that 20M, then no. but if you have parallel streams
writing, then fragmentation past the amount of space you allocated is
likely.
-Eric
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