A Block-sized read will not be rboken up. But if you're reading ina size bigger than the underlying systems block sizes then it can get broken up.
So yes a sequential read will get broken up. A single read request for a block may or may not get broken up. If you're freading with set block sizes you'll see the set sizes of blocks come through, but what the underlyign OS does is undefined, same for writing. If the underlying block size is 8KB and you dump 4MB down on it, the OS may (and in many cases does) decide to write part of it, do a read ona nearby sector, then write the rest. This happens when doing long writes that end up spanning block groups because the inodes must be allocated. So the write WILL get broken up. Reads are under the same gun. IT all depends on how big. To the application you may or may not see this (probably not, unless you're set non-blocking, because the kernel will just sleep you until your data is ready). Further large read requests can of course be re-ordered by hardware. Tagged Command Queueing on SCSI drives and RAIDs. The ICP Vortex cards I use ina number of systems have 64MB on-board cache. They quite happily, and often re-order reads and writes when queueing them to keep things moving as fast as possible (Intel didn't buy them for their cards, they use the i960 as it is, Intel swiped them for their IP rights). The OS also tags commands it fires to the ICP, which can be re-ordered on block-sized chunks. Curt Sampson wrote: >On Wed, 24 Apr 2002, mlw wrote: > >>I am not arguing about whether or not they do it, I am saying it is >>not always possible. I/O requests do not remain in queue waiting for >>reordering indefinitely. >> > >It doesn't matter. When they go out to the disk they go out in >order. On every Unix-based OS I know of, and Novell Netware, if >you submit a single read request for consecutive blocks, those >blocks *will* be read sequentially, no matter what the system load. > >So to get back to the original arugment: > >>>>The supposed advantage of a sequential read over an random read, in >>>>an active multitasking system, is a myth. If you are executing one >>>>query and the system is doing only that query, you may be right. >>>> > >No, it's very real, because your sequential read will not be broken up. > >If you think it will, let me know which operating systems this >happens on, and how exactly it happens. > >cjs > ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
