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Date: Mon, 20 Jul 2009 18:41:03 -0700 From: Xiang Wang <xiangw@...gle.com> To: linux-ext4@...r.kernel.org Subject: Using O_DIRECT in ext4 Hi, Recently I've been experimenting with O_DIRECT in ext4 to get a feeling of how much file fragmentation will be generated. On a newly formatted ext4 partition(no-journal), I created a top-level directory and under this top-level directory I ran a test program to generate some files. The test program does the following: -- create multiple threads(in my test case: 16 threads) -- each thread creates a file with the O_DIRECT flag and keeps extending the file to 1MB Since these threads run concurrently, they compete in block allocation. After the program ran to a completion, I ran filefrag on each file and measure how many extents there are in the file. And here is a sample result: file0: 6 extents found file1: 20 extents found file2: 7 extents found file3: 6 extents found file4: 6 extents found file5: 5 extents found file6: 6 extents found file7: 20 extents found file8: 20 extents found file9: 20 extents found file10: 20 extents found file11: 20 extents found file12: 20 extents found file13: 19 extents found file14: 19 extents found file15: 19 extents found Looks like these files are quite heavily fragmented. For comparison, I did the same experiment on an ext2 partition, resulting in each file having only 1 extent. I also did the experiments of using buffered writes(by removing the O_DIRECT flag) on ext2 and ext4, both resulting in each file having only 1 extent. I am wondering whether this kind of file fragmentation is already a known issue in ext4 when O_DIRECT is used? Is it something by design? Since it seems like ext2 does not have this issue under my test case, is it necessary that we make the behavior of ext4 similar to ext2 under situations like this? Thanks, Xiang -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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