Re: Alignment of disk-I/O from userland.

From: Ben Mesander (ben_at_timing.com)
Date: 10/07/03

  • Next message: Bruce M Simpson: "Re: Alignment of disk-I/O from userland." 
    Date: Tue, 7 Oct 2003 09:41:17 -0600
To: "Poul-Henning Kamp" <phk@phk.freebsd.dk>

    Poul-Henning Kamp writes:
    > 1: Not "on all i/o operations going to a device", but rather "on i/o
    > operations which take the physread/write fast-path to avoid a copyin/out
    > overhead." (disks and tapes mostly). Ttys, /dev/null and all the
    > "typical" devices are unaffected.

    How about a flag for open(2), such as O_DIRECT, which would indicate
    if the fd in question is such a device, and you did not sufficiently
    align, etc. your buffers, then you get an error when you attempt the
    I/O operation.

    If you did not specify this flag when opening the device, then do the
    bounce buffer dance for the I/O operation, and lose efficiency.

    --Ben

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