On Fri, Sep 09, 2011 at 04:04:07PM +0200, Kevin Wolf wrote: > > need to read in the existing 4 KB, modify the 512 bytes in place, and > > write out the 4 KB block again. This is read-modify-write. In this > > worst-case scenario a 512 byte write turns into a 4 KB read followed > > by a 4 KB write. > > But that should only happen with a 4k sector size, otherwise there's no > reason for RMW.
The might not be a need for RMW, but if you're doing 512 byte writes to a sparse file on 4k filesystem that filesystem will have to serialize the I/O to prevent races from happening during block allocation.
