Date: Mon, 27 Nov 1995 08:01:48 +0000 () From: Michael Smith <msmith@atrad.adelaide.edu.au> To: sysseh@devetir.qld.gov.au (Stephen Hocking) Cc: current@FreeBSD.org Subject: Re: Prioritised disk requests Message-ID: <199511270801.IAA03995@genesis.atrad.adelaide.edu.au> In-Reply-To: <199511270628.GAA00550@netfl15a.devetir.qld.gov.au> from "Stephen Hocking" at Nov 27, 95 04:28:50 pm
next in thread | previous in thread | raw e-mail | index | archive | help
Stephen Hocking stands accused of saying: > Has anyone thought of a mechanism (similar to stuff I've seen on > Masscomp machines) where one could indicate that a particular stream > of disk I/O had a higher priority than normal? Masscomp used to have > a B_URGENT flag for these blocks in the buffer cache (for > semi-realtime I/O when acquiring or transmitting data). CD burners > could do with this, or people trying to keep tapes streaming on > backups. As far as CD burners are concerned, going on the datasheets I have here, most would be pushing to beat 350K/sec, and have buffers in the >1M range. I doubt very much that anything other than a spammed-to-the-wall system is going to have the multi-second latency or sub-350K/sec net available SCSI bandwidth. (ref. phillips CDD522 glossy; 153.6/176.4/307.2/352.8 selectable xfer rates, standard buffer 2M, expandable to 8 or 32M.) The problem with old tapes is just the opposite - their buffers are too small 8) You'd have to tag both data going to the tape, and data for the backup process coming off the disk as urgent, and keep the disk requests short so that the tape could come back for more data. Not easy 8( (also, newer tape units have bigger buffers, so the problem is going away) > Stephen -- ]] Mike Smith, Software Engineer msmith@atrad.adelaide.edu.au [[ ]] Genesis Software genesis@atrad.adelaide.edu.au [[ ]] High-speed data acquisition and (GSM mobile) 041-122-496 [[ ]] realtime instrument control (ph/fax) +61-8-267-3039 [[ ]] "Who does BSD?" "We do Chucky, we do." [[
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199511270801.IAA03995>