On Fri, 2005-12-09 at 18:29 +0300, Vladislav Bolkhovitin wrote: > > Additionally, it's perfectly possible for all of this to be done zero > > copy on the data. A user space target mmaps the data on its storage > > device and then does a SG_IO type scatter gather user virtual region > > pass to the underlying target infrastructure. We already have this > > demonstrated in the SG_IO path, someone just needs to come up with the > > correct implementation for a target path. > > I'm not completely understand how it will work. Consider, there are > READ/WRITE commands with random data sizes arrive from an initiator. Are > you going to do map/unmap for each command individually or alloc data > buffers for commands from a premapped area and live with possible its > fragmentation? If map/unmap individually, then I should say that those > are very expensive operations. You do it the same way an array does: the model for SPI is read command phase, disconnect, process command (i.e. set up areas) reconnect for data transfer. map/unmap are really only necessary if you're emulating the data store, but it's a fairly cheap operation on linux: it just causes the creation of a vm_area. If it's a pass through, you can use SG_IO to pull it in and the SG_IO like output to shoot it out again, effectively using a piece of user memory as a zero copy buffer. Fragmentation isn't an issue because the I/O goes via sg lists , all that's needed is a bunch of pages. James - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
