Hi:
I want to ask a question about CEPH_IOC_SYNCIO flag.
I know that when using O_SYNC flag or O_DIRECT flag, write call executes in other two code paths different than using CEPH_IOC_SYNCIO flag.
And I find the comments about CEPH_IOC_SYNCIO here:
/*
* CEPH_IOC_SYNCIO - force synchronous IO
*
* This ioctl sets a file flag that forces the synchronous IO that
* bypasses the page cache, even if it is not necessary. This is
* essentially the opposite behavior of IOC_LAZYIO. This forces the
* same read/write path as a file opened by multiple clients when one
* or more of those clients is opened for write.
*
* Note that this type of sync IO takes a different path than a file
* opened with O_SYNC/D_SYNC (writes hit the page cache and are
* immediately flushed on page boundaries). It is very similar to
* O_DIRECT (writes bypass the page cache) except that O_DIRECT writes
* are not copied (user page must remain stable) and O_DIRECT writes
* have alignment restrictions (on the buffer and file offset).
*/
#define CEPH_IOC_SYNCIO _IO(CEPH_IOCTL_MAGIC, 5)
My question is:
1."This forces the same read/write path as a file opened by multiple clients when one or more of those clients is opened for write." -- Does this mean multiple clients can execute in the same code path when they all use the CEPH_IOC_SYNCIO flag? Will the use of CEPH_IOC_SYNCIO in all clients bring effects such as coherency and performance?
2."...except that O_DIRECT writes are not copied (user page must remain stable)" -- As I know when threads write with CEPH_IOC_SYNCIO flag, the write call will block until ceph osd and mds send back responses. So even with CEPH_IOC_SYNCIO flag(the user pages are not locked here, I guess), but the user cannot use these pages. How can the use of CEPH_IOC_SYNCIO flag make better use of user space memory?
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