Xiang Wang wrote: > 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. Multiple parallel extending DIOs in a single dir is a tough case for a filesystem - it has no hints about what to do, and can't use delalloc to wait to see what's happening; it just has to allocate things as they come, more or less. > For comparison, I did the same experiment on an ext2 partition, > resulting in each file having only 1 extent. Interestinng, not sure I would have expected that. > 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. delayed allocation at work I suppose. > 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? Is this representative of a real workload? -Eric > Thanks, > Xiang -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
