On Tue, Feb 02, 2016 at 07:40:34PM -0800, Dilip Simha wrote: > Hi Eric, > > Thank you for your quick reply. > > Using xfs_io as per your suggestion, I am able to reproduce the issue. > However, I need to falloc for 256K and write for 257K to see this issue. > > # xfs_io -f -c "falloc 0 256k" -c "pwrite 0 257k" /srv/node/r1/t1.txt > # stat /srv/node/r1/t4.txt | grep Blocks > Size: 263168 Blocks: 1536 IO Block: 4096 regular file Fallocate sets the XFS_DIFLAG_PREALLOC on the inode. When you writing *past the preallocated area* and do delayed allocation, the speculative preallocation beyond EOF is double the size of the extent at EOF. i.e. 512k, leading to 768k being allocated to the file (1536 blocks, exactly). This is expected behaviour. > # xfs_io -f -c "pwrite 0 257k" /srv/node/r1/t2.txt > # stat /srv/node/r1/t2.txt | grep Blocks > Size: 263168 *Blocks*: 520 IO Block: 4096 regular file So pure delayed allocation, specualtive preallocation starts at 64k file size, so it would have been (((64k + 64K) + 128K) + 256k) = 768k. > I waited for around 15 mins before collecting the stat output to give the > background reclamation logic a fair chance to do its job. I also tried > changing the value of speculative_prealloc_lifetime from 300 to 10. But it > was of no use. The prealloc cleaner skips inodes with XFS_DIFLAG_PREALLOC set on them. Because the XFS_DIFLAG_PREALLOC flag is not set on the delayed allocation inode, the EOF blocks cleaner runs truncates it to EOF, and 260k (520 blocks) remains allocated to the file. i.e. you are seeing behaviour exactly as designed and intended. The way swift is using fallocate is actively harmful. You do not want preallocation for write once files - this is exactly the workload that delayed allocation was designed to be optimal for as delayed allocation sequentialises the IO from multiple files. Using preallocation means writeback of the data cannot be optimised across files as the preallocation location will not be sequential to the IO that was just issued, hence writeback will seek the disks back and forth instead of seeing a nice sequential IO stream. <sigh> Yet another way that the swift storage back end tries to be smart but ends up just making things go slow.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
