Messages in this thread | | | Subject | Re: Bio pool & scsi scatter gather pool usage | From | Steve Lord <> | Date | 19 Apr 2002 10:27:22 -0500 |
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On Fri, 2002-04-19 at 03:58, Alan Cox wrote: > > But this gets you lowest common denominator sizes for the whole > > volume, which is basically the buffer head approach, chop all I/O up > > into a chunk size we know will always work. Any sort of nasty boundary > > condition at one spot in a volume means the whole thing is crippled > > down to that level. It then becomes a black magic art to configure a > > volume which is not restricted to a small request size. > > Its still cheaper to merge bio chains than split them. The VM issues with > splitting them are not nice at all since you may need to split a bio to > write out a page and it may be the last page > -
I am well aware of the problems of allocating more memory in some of these places - been the bane of my life for the last couple of years with XFS ;-)
It just feels so bad to have the ability to build a large request and use one bio structure and know that 99.9% of the time the lower layers can handle it in one chunk, but instead have to chop it into the lowest common denominator pieces for the sake of the other 0.1%.
Just looking at how my disks ended up partitioned not many of them are even on 4K boundaries, so any sort of concat built on them would have a boundary case which required such a split - I think, still working on my caffine intake this morning.
Steve
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Steve Lord voice: +1-651-683-3511 Principal Engineer, Filesystem Software email: lord@sgi.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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