On Thu, Jan 18 2018, Theodore Ts'o wrote: > On Wed, Jan 17, 2018 at 02:07:22PM -0800, Linus Torvalds wrote: >> >> Now re-do the test while another process writes to a totally unrelated >> a huge file (say, do a ISO file copy or something). >> >> That was the thing that several filesystems get completely and >> horribly wrong. Generally _particularly_ the logging filesystems that >> don't even need the fsync, because they use a single log for >> everything (so fsync serializes all the writes, not just the writes to >> the one file it's fsync'ing). > > Well, let's be fair; this is something *ext3* got wrong, and it was > the default file system back them. All of the modern file systems now > do delayed allocation, which means that an fsync of one file doesn't > actually imply an fsync of another file. Hence... > >> The original git design was very much to write each object file >> without any syncing, because they don't matter since a new object file >> - by definition - isn't really reachable. Then sync before writing the >> index file or a new ref. > > This isn't really safe any more. Yes, there's a single log. But > files which are subject to delayed allocation are in the page cache, > and just because you fsync the index file doesn't mean that the object > file is now written to disk. It was true for ext3, but it's not true > for ext4, xfs, btrfs, etc. > > The good news is that if you have another process downloading a huge > ISO image, the fsync of the index file won't force the ISO file to be > written out. The bad news is that it won't force out the other git > object files, either. > > Now, there is a potential downside of fsync'ing each object file, and > that is the cost of doing a CACHE FLUSH on a HDD is non-trivial, and > even on a SSD, it's not optimal to call CACHE FLUSH thousands of times > in a second. So if you are creating thousands of tiny files, and you > fsync each one, each fsync(2) call is a serializing instruction, which > means it won't return until that one file is written to disk. If you > are writing lots of small files, and you are using a HDD, you'll be > bottlenecked to around 30 files per second on a 5400 RPM HDD, and this > is true regardless of what file system you use, because the bottle > neck is the CACHE FLUSH operation, and how you organize the metadata > and how you do the block allocation, is largely lost in the noise > compared to the CACHE FLUSH command, which serializes everything. > > There are solutions to this; you could simply not call fsync(2) a > thousand times, and instead write a pack file, and call fsync once on > the pack file. That's probably the smartest approach. > > You could also create a thousand threads, and call fsync(2) on those > thousand threads at roughly the same time. Or you could use a > bleeding edge kernel with the latest AIO patch, and use the newly > added IOCB_CMD_FSYNC support. > > But I'd simply recommend writing a pack and fsync'ing the pack, > instead of trying to write a gazillion object files. (git-repack -A, > I'm looking at you....) > > - Ted [I didn't find an ideal message to reply to in this thread, but this seemed to probably be the best] Just an update on this since I went back and looked at this thread, GitLab about ~1yr ago turned on core.fsyncObjectFiles=true by default. The reason is detailed in [1], tl;dr: empty loose object file issue on ext4 allegedly caused by a lack of core.fsyncObjectFiles=true, but I didn't do any root cause analysis. Just noting it here for for future reference. 1. https://gitlab.com/gitlab-org/gitlab-foss/-/issues/51680#note_180508774
