Re: CVS commit: [tls-maxphys] src/sys/ufs/ufs

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

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
--