PFC, On 2/1/06 1:01 AM, "PFC" <lists@xxxxxxxxxx> wrote: > 3- Read one block of A, then one block of B, then one block of A, etc. > Essentially this is the same as the threaded case, except there's only one > thread. > 53 seconds total (with heavy seeking noise from the hdd). > > I was half expecting 3 to take the same as 2. It simulates, for > instance, > scanning a table and its index, or scanning 2 sort bins. Well, maybe one > day... This is actually interesting overall - I think what this might be showing is that the Linux SW RAID1 is alternating I/Os to the mirror disks from different processes (LWP or HWP both maybe?), but not within one process. > It would be nice if the Kernel had an API for applications to tell it > "I'm gonna need these blocks in the next seconds, can you read them in the > order you like (fastest), from whatever disk you like, and cache them for > me please; so that I can read them in the order I like, but very fast ?" More control is always good IMO, but for now there's I/O reordering in the SCSI layer and readahead tuning. There is POSIX fadvise() also to tell the underlying I/O layer what the access pattern looks like. - Luke
