On 2/3/16 2:30 AM, Dave Chinner wrote: > On Tue, Feb 02, 2016 at 11:09:15PM -0800, Dilip Simha wrote: >> Hi Dave, >> >> On Tue, Feb 2, 2016 at 10:37 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: >> >>> 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). >>> >> >> Thank you for the details. >> This is exactly where I am a bit perplexed. Since the reclamation logic >> skips inodes that have the XFS_DIFLAG_PREALLOC flag set, why did the >> allocation logic allot more blocks on such an inode? > > To store the data you wrote outside the preallocated region, of > course. I think what Dilip meant was, why does it do preallocation, not why does it allocate blocks for the data. That part is obvious of course. ;) IOWS, if XFS_DIFLAG_PREALLOC prevents speculative preallocation from being reclaimed, why is speculative preallocation added to files with that flag set? Seems like a fair question, even if Swift's use of preallocation is ill-advised. I don't have all the speculative preallocation heuristics in my head like you do Dave, but if I have it right, and it's i.e.: 1) preallocate 256k 2) inode gets XFS_DIFLAG_PREALLOC 3) write 257k 4) inode gets speculative preallocation added due to write past EOF 5) inode never gets preallocation trimmed due to XFS_DIFLAG_PREALLOC that seems suboptimal. Never doing speculative preallocation on files with XFS_DIFLAG_PREALLOC set, regardless of file offset, would seem sane to me. App asked to take control via prealloc; let it have it, and leave it at that. (Of course now I'll go read the code to see if I understand it properly...) -Eric _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
