Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cwjcc!hal!nic.MR.NET!tank!mimsy!chris From: chris@mimsy.UUCP (Chris Torek) Newsgroups: comp.unix.wizards Subject: Re: Write-Behind (was Re: Record-access libraries) Message-ID: <14122@mimsy.UUCP> Date: 23 Oct 88 16:21:59 GMT References: <287@cvbnet2.UUCP> <107@minya.UUCP> <74161@sun.uucp> Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742 Lines: 33 In article <74161@sun.uucp> pope@vatican [indeed!] (John Pope) writes: >[such a controller] will do "write-behind" in the filesystem case >as well. This could lead to ... filesystem integrity problems if >the system should crash. For whatever reasons, some hardware manufacturers either never think of this, or else do it and keep quiet about it anyway. >It's unclear that such a scheme buys you much anyway, since the kernel >already does "write-behind" for filesystem I/O (unless, as you point >out, the O_SYNC flag is set), while if you're working on the raw disk, >your application will probably want to control the data buffering itself. Not all kernels have buffering. VMS, for instance, does not (at the ODS-II level: RMS does its own buffering and no doubt uses ASTs and such). Why do you think DEC thought the 16 kB in the UDA50 was such a wonderful buffer? (Maybe it was 32k, but anyway, small enough not to matter---it does not even buffer one track of an RA81.) (It is always amusing to watch the reaction of salespersons when you tell them that your machine already uses over a megabyte of buffering, and their 64k is not interesting.) >>... maybe five years ago, intelligent controllers which defeated the >>``write straight to disk'' characteristic of raw devices were >>dismayingly commonplace. >Which ones? The UDA-50 didn't do this, did it #:-o ?? No, it does not report the transfer as done until it has in fact been written. It *may* reorder writes, however (it is hard to tell for sure). -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris