Hello XFS developers, We are working on scylladb[1] database which is written using seastar[2] - highly asynchronous C++ framework. The code uses aio heavily: no synchronous operation is allowed at all by the framework otherwise performance drops drastically. We noticed that the only mainstream FS in Linux that takes aio seriously is XFS. So let me start by thanking you guys for the great work! But unfortunately we also noticed that sometimes io_submit() is executed synchronously even on XFS. Looking at the code I see two cases when this is happening: unaligned IO and write past EOF. It looks like we hit both. For the first one we make special afford to never issue unaligned IO and we use XFS_IOC_DIOINFO to figure out what alignment should be, but it does not help. Looking at the code though xfs_file_dio_aio_write() checks alignment against m_blockmask which is set to be sbp->sb_blocksize - 1, so aio expects buffer to be aligned to filesystem block size not values that DIOINFO returns. Is it intentional? How should our code know what it should align buffers to? Second one is harder. We do need to write past the end of a file, actually most of our writes are like that, so it would have been great for XFS to handle this case asynchronously. Currently we are working to work around this by issuing truncate() (or fallocate()) on another thread and doing aio on a main thread only after truncate() is complete. It seams to be working, but is it guarantied that a thread issuing aio will never sleep in this case (may be new file size value needs to hit the disk and it is not guarantied that it will happen after truncate() returns, but before aio call)? [2] http://www.scylladb.com/ [1] http://www.seastar-project.org/ Thanks, -- Gleb. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
