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Re: CVS commit: [tls-maxphys] src/sys/ufs/ufs
On Sun, Oct 14, 2012 at 02:33:32PM +0000, Thor Lancelot Simon wrote:
> Module Name: src
> Committed By: tls
> Date: Sun Oct 14 14:33:32 UTC 2012
> Modified Files:
> src/sys/ufs/ufs [tls-maxphys]: ufs_readwrite.c
> Log Message:
> In the FFS writebehind code (ufs_readwrite.c:WRITE()), use division
> and multiplication instead of shifts, to accomodate devices with
> MAXPHYS that is a multiple of the page size, but not a power of 2.
> MegaRAID neatly writes out 192k chunks now.
> An open question: is is really a good idea to always writebehind
> at the largest size supported by the device? Likely not, as this
> could have a major impact on I/O fairness. OS X and Solaris both
> seem to limit transfers to 128k likely for this reason (the same
> problem exists with the readahead code but since it is adaptive,
> it will not *always* do huge transfers).
> However, simply imposing a smallish limit like 128k here seems
> like a bad idea because then we cannot accomodate greedy devices
> like RAID, for which you want something like 128k * number of
> data components. Hmmmmmm.
I wonder if we could reuse bits from the read-ahead code for write-behind ?
Maybe this could allow to move the write-behind code out of ufs_readwrite.c
to ubc (maybe ubc_uiomove()) ?
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
NetBSD: 26 ans d'experience feront toujours la difference
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