Hi Ted, I wasn't sure if you missed this mail in your inbox. So I wanted to send a gentle reminder about the same. On Tue, Dec 26, 2017 at 4:07 PM, Jayashree Mohan <jayashree2912@xxxxxxxxx> wrote: > Hi, > I am Jayashree Mohan, a PhD student at the University of Texas at > Austin, working with Prof. Vijay Chidambaram. I've been working on the > Crashmonkey project, which is a test harness for file system crash > consistency[1]. > > While trying to reproduce some of the reported bugs on ext4 using > CrashMonkey we stumbled upon the following issue: > Whenever a fallocate follows a buffered write(delayed allocated), with > fallocate overlapping with a certain range of the write, the block > count reported by stat of the file is incorrect until the write is > resolved. > > Consider the following workload(The number within the braces indicate > the start and end offsets) > 1. Write (79581 - 136029) > 2. Fallocate (46385 - 92886) > > If we now stat the file, the result is as follows : > > 1. Stat after Write (79581 - 136029) > File: '/mnt/5gbdisk/only-fallocate.txt' > Size: 136029 Blocks: 120 IO Block: 4096 regular file > Device: 810h/2064d Inode: 14 Links: 1 > > 2. Stat after fallocate (46385 - 92886) > File: '/mnt/5gbdisk/only-fallocate.txt' > Size: 136029 Blocks: 216 IO Block: 4096 regular file > Device: 810h/2064d Inode: 14 Links: 1 > > 3. Stat 30 seconds later, after the checkpoint > File: '/mnt/5gbdisk/only-fallocate.txt' > Size: 136029 Blocks: 184 IO Block: 4096 regular file > Device: 810h/2064d Inode: 14 Links: 1 > > So we see here that the actual block count should be 184, as there is > an overlapping region between the buffered write(120 blocks) and > fallocate(96 blocks). Note that there are 32 overlapping blocks in the > above workload(Blocks are in units of 512B). However until the write > is resolved, the block count is shown to be 216, which is just the sum > of the blocks reserved for writes and fallocate(120+96=216), without > consideration for the overlap(120+96-32 = 184). > > So, if any program stats the file before the write is resolved (the 30 > sec checkpoint interval), the block count returned seems to be > incorrect. > > Let me know if I am missing some detail. > > [1] https://github.com/utsaslab/crashmonkey > > Thanks, > Jayashree
