Date: Fri, 10 Jun 2005 10:10:47 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Jeff Roberson <jroberson@chesapeake.net> Cc: arch@FreeBSD.org, Pawel Jakub Dawidek <pjd@FreeBSD.org> Subject: Re: simplify disksort, please review. Message-ID: <17437.1118391047@critter.freebsd.dk> In-Reply-To: Your message of "Fri, 10 Jun 2005 04:05:52 EDT." <20050610040302.S16943@mail.chesapeake.net>
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In message <20050610040302.S16943@mail.chesapeake.net>, Jeff Roberson writes: >On Thu, 9 Jun 2005, Poul-Henning Kamp wrote: > >> In message <20050609193008.GB837@darkness.comp.waw.pl>, Pawel Jakub Dawidek writes: >> >> >The one example of how the order can be broken (write(offset, size)): >> > >> > write(1024, 512) >> > write(0, 2048) >> >> If you issue these two requests just like that, you get no guarantee >> which order they get written in. >> >> It's not just disksort which might surprise you, tagged queuing and >> write caches may mess up your day as well. > >Anyway, it's really not important to solve at the disk queue layer, >because it's only an issue with raw devices, and it's very irregular to >have multiple processes opening the same raw device to issue a lot of io >of different sizes. It will not be solved at any level, because it would require the GEOM layer to either make all writes strictly synchronous, a total no-go, or keep very expensive housekeeping of which requests are outstanding so we can block conflicting writes etc. This is something that belongs 100% in the code issuing the I/O requests and if some of that code does something stupid, it looses and that is how it should be. Jeff is correct that the buffer-cache mostly will prevent people from shooting their feet off, but nothing prevents a GEOM class from making a fool of itself. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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