Duncan <[email protected]> schrieb: > P. Remek posted on Tue, 10 Feb 2015 18:44:33 +0100 as excerpted: > >> In the test, I use --direct=1 parameter for fio which basically does >> O_DIRECT on target file. The O_DIRECT should guarantee that the >> filesystem cache is bypassed and IO is sent directly to the underlaying >> storage. Are you saying that btrfs buffers writes despite of O_DIRECT? > > I'm out of my (admin, no claims at developer) league on that. I see > someone else replied, and would defer to them on this.
I don't think that O_DIRECT can work efficiently on COW filesystems. It probably has a negative effect and cannot be faster as normal access. Linus itself said one time that O_DIRECT is broken and should go away, and instead cache hinting should be used. Think of this: For the _unbuffered_ direct-io request to be fulfilled the file system has to go through its COW logic first which it otherwise had buffered and done in background. Bypassing the cache is probably only a side-effect of O_DIRECT, not its purpose. At least I'd try with a nocow-file for the benchmark if you still have to use O_DIRECT. -- Replies to list only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
