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by afr0ck
1082 days ago
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msync() affects only the pages that part of the mmap area you ask for in the arguments. From the man pages: > int msync(void addr[.length], size_t length, int flags); > msync() flushes changes made to the in-core copy of a file that was mapped into memory using mmap(2) back to the filesystem |
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If you don't know why it's not possible, here's a simplified version of it: hardware protocols (s.a. SCSI) must have fixed size messages to fit them through the pipeline. I.e. you cannot have a message larger than the memory segment used for communication with the device, because that will cause fragmentation and will lead to a possibility of message being corrupted (the "tail" being lost or arriving out of order).
On the other hand, to "flush" a file to persistent storage you'd have to specify all blocks associated with the file that need to be written. If you try to do this, it will create a message of arbitrary size, possibly larger than the memory you can store it in. So, the only way to "flush" all blocks associated with a file is to "flush" everything on a particular disk / disks used by the filesystem. And this is what happens in reality when you do any of the sync family commands. The difference is only in what portion of the not-yet synced data the OS will send to the disk before requesting a sync, but the sync itself is for the entire disk, there aren't any other syncs.