On 2012-01-25, at 9:34, Eric Sandeen [off-list ref] wrote:

On 1/24/12 11:19 PM, Amit Sahrawat wrote:

quoted

In XFS we can write parallel (i.,e we can make use of allocation
groups for writing process). If the files are kept in individual
directories, there is a possibility that first the blocks for that
files be used from individual allocation groups. If I start ‘4’
writing process(cp 100MB_file /<dirnum>/) – after writing is finished
– if I check the bmap – it does shows that initial allocation was from
individual allocation groups.
Even though in Ext4 also we do have groups – but I am not able to get
behavior similar to XFS.
If I check the file extents – the extents are in mixed form, the
allocation pattern is also very fragmented.

Please share more on this. Also, if there is a possible exact test
case to check for parallel writes support.

It seems that you are asking more about allocation policy than parallelism
in general?  With either filesystem, you could use preallocation to wind
up with more contiguous files when you write them in parallel, though
that requires some idea of the file size ahead of time.

ext4 doesn't have that exact dir::group heuristic that xfs uses,
but it does have other mechanisms and heuristics to try to get good
file and directory layout.

In general, ext4 tries to put new root directories into new groups, see
comments above find_group_orlov().  Other directories tend to stay near
their parent directory.  So it's really the roots of dir trees that
get spread across the disk in general.  New non-dir inodes also tend to
stay close to their parent.  (I think I have that all right ...)

Note that this policy of new subdirectory placement (and indirectly the placement of files created therein) can be tuned by userspace by setting the "topdir" flag via setattr on the parent directory. 

This will push new subdirs created in that dir to a new group, which would In turn allow the files to allocate from that group. It is intended to be set on directories like /home where files within the subdirs are unrelated.

Whether that is enough for your usage is unclear. 

The test you describe above does result in more contiguous allocation on
xfs than on ext4, though - a quick check on kernel 3.2 yielded 2-3
extents per file on ext4, 1 extent per file for xfs.

-Eric

quoted

Thanks & Regards,
Amit Sahrawat
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