![]() In your case a bs that huge is nonsensical with odirect as your "disk"'s maximum transfer block size (let alone the optimal size) is almost certainly smaller. When you specify oflag=odirect you are saying "trust that all my parameters are sensible and turn off as much kernel buffering as you can". Since you are sending only one giant write this will put you into near enough the same scenario as doing conv=fdatasync above. Oflag=dsync is still allowed to make use of kernel buffering - it just causes a flush+wait for completion after each submission. ![]() Only when dd has finished sending ALL the data will it have to wait for anything still only in cache to be flushed to disk (and with fsync that includes any metadata). This gives the kernel a big opportunity to merge I/Os, create parallel submission out of I/O that has yet to be destaged and generally decouples I/O submission to the kernel from I/O acceptance by the disk (to the extent that buffering will allow). Note that directio is not supported on HP-UX.įor the tempdb devices you should use neither dsync nor directio (as you do not need the recoverability at all).Dd with oflag=dsync or conv=fdatasync/fsync is around 10 times faster than dd with oflag=directĬonv=fdatasync / conv=fsync still mean I/O is initially queued to the kernel cache and destaged to disk as the kernel sees fit. ![]() So transaction log devices are very good candidates for directio (or for raw devices).Īlso on newer Linux kernels dsync provides awful performance and you should then rather use directio than dsync. On the other hand, dsync is faster on devices for read operations. But it since the OS buffer caches is bypassed, it does provide a pretty good recoverability.ĭirectio provides better write performance than sync (especially if the device is stored on a SAN). the OS buffer caches are bypassed and data are written directly to disk.īut directio does not guarantee that the writes will only return after all data have been stored on disk (just that data will not go into caches). directioĭirectio is basically a way to get a way to perform I/O on file system devices in a similar way to raw devices i.e. On the other hand, it is common to turn off dsync on devices of databases which do not need to be recovered like the tempdb. Please also note that a cache and a buffer are different.ĭsync is always on for the master device: the performance of writes there is not critical and it’s important that it can be fully recovered. It just means that when you write synchronously or check for whether the asynchronous I/O was performed, you’ll only get the response that the write is completed once the data are effectively on the physical disk. It should be noted that dsync doesn’t mean that there is not asynchronous I/O. The drawback of dsync is that it costs performance (because the writes, even if buffered by the OS, are guaranteed to go to the disk before the operation finishes). The data could still be in the disk write cache and get lost… Of course, this only handles OS level buffering. ![]() This allows for a better recoverability of the written data in case of crash: If the writes are buffered by the OS and the system crashes, these writes are lost. First, dsync has no effect on raw devices (i.e., a device on a raw partition) and on devices on Windows operating system (i.e., it only affects Unix/Linux operating systems).ĪSE opens a database device file of a device with the dsync setting on, using the operating system dsync flag.With this flag, when ASE writes to the device file, both the written data must be physically stored on disk before the system call returns.
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