Hello guys, > > On Thu, Feb 12, 2015 at 05:33:41AM +0100, Kai Krakow wrote: >> 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. > > Hmm, not true in btrfs, the COW logic mentioned above is nothing but to > allocate > a NEW extent, and it's not done in background. > > Comparing to nocow logic, the main difference comes from > a) COW files' calculating checksums of the dirty data in DIO pages which > nocow files don't need to. > b) their endio handlers. > > Or am I missing something?
We did benchmark Btrfs aio/dio performance before, we noticed one big differences from COW and nocow is not only checksum but checksum cost more metadata, which will make Btrfs performance drop suddenly for a while, because of metadata reservation. > > Thanks, > > -liubo >> >> 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 > -- > 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 Best Regards, Wang Shilong -- 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
