On Tue, Sep 18, 2007 at 11:00:40AM +0100, Mel Gorman wrote:
> We still lack data on what sort of workloads really benefit from large
> blocks (assuming there are any that cannot also be solved by improving
> order-0).
No we don't. All workloads benefit from larger block sizes when
you've got a btree tracking 20 million inodes and a create has to
search that tree for a free inode. The tree gets much wider and
hence we take fewer disk seeks to traverse the tree. Same for large
directories, btree's tracking free space, etc - everything goes
faster with a larger filesystem block size because we spent less
time doing metadata I/O.
And the other advantage is that sequential I/O speeds also tend to
increase with larger block sizes. e.g. XFS on an Altix (16k pages)
using 16k block size is about 20-25% faster on writes than 4k block
size. See the graphs at the top of page 12:
http://oss.sgi.com/projects/xfs/papers/ols2006/ols-2006-paper.pdf
The benefits are really about scalability and with terabyte sized
disks on the market.....
Cheers,
Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
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