Archive for April, 2010

directio,dsync & sync , async IO

April 20th, 2010 3 comments


I came across very conceptual article about the sybase device io.

If you have any questions and suggestions please let me know. Thanks.

Source: Sybase Blogs, www, sybooks, sybase white paper for direct io.


  • Asynchronous I/O allows the process issuing the I/O to continue executing while OS completes the request.
  • Dataserver process does not block on this request till its completion.


  • Synchronous I/O blocks the I/O issuing process from continuing executing while OS completes the request.
  • The process is blocked till the request is completed. This generally results in poor throughput.


  • In 12.0 sybase introduced the dsync flag – shorthand for “Data Synchronous.”
  • When the dsync setting is on, Adaptive Server opens a database device file using the UNIX dsync flag.
  • The dsync flag in ASE directly translates to the O_DSYNC open(2) flag. That is to say, when ASE opens a device that has the dsync flag set, ASE will pass O_DSYNC to open(2).
  • This flag tells the file system that a write to that file must pass though the cache and be written to disk before the write is considered complete.
  • In other words, for writes we throw away the cache efficiency and make sure the data goes to disk.
  • This way if the system crashes, everything that we thought had been written to disk has in fact been written to disk.

Is DSYNC “synchronous”??

  • This is not true.
  • This synchronous / asynchronous conflict is at a different level. With async i/o we are talking about the context in which the i/o is executed, i.e. whether the i/o blocks the caller or if it is done in a different context.
  • With dsync we are talking about when the write() is considered complete.
  • These are not mutually exclusive, and you can asynchronously do a data synchronous i/o.
  • The async portion is as always: the application issues an i/o and later polls (or is notified) for completion. The dsync portion means that the application won’t be told that the I/O has completed until the data has made it to disk.


  • Direct I/O is another kind of file system i/o, introduced in ASE15.
  • In the direct i/o model the file system cache is completely bypassed.
  • Using direct i/o, writes are naturally safe because they bypass the cache and go straight to disk.
  • Direct i/o is very, very similar to raw i/o.
  • The main difference is that in direct i/o the structure of a file system still exists, easing manageability.
  • With raw i/o, no file system exists. The performance of raw and direct i/o should be very similar.

Also like to add :

  • The dsync and directio are mutually exclusive. Both cannt be turn on same time on the device. Both dsync and directio provide the full recoveribility.
  • If you gone through the full article, the next question is, which would be best in raw device and filesystem device with direct io, both are bypassing the file system cache.
Categories: ASE Tags: ,