linux-kernel - Re: Implementing NVMHCI...

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Message-ID: <alpine.DEB.1.10.0904111810360.28893@asgard.lang.hm>
Date:	Sat, 11 Apr 2009 18:15:36 -0700 (PDT)
From:	david@...g.hm
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Grant Grundler <grundler@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeff Garzik <jeff@...zik.org>,
	Linux IDE mailing list <linux-ide@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: Implementing NVMHCI...

On Sun, 12 Apr 2009, Alan Cox wrote:

>> We've abstract the DMA mapping/SG list handling enough that the
>> block size should make no more difference than it does for the
>> MTU size of a network.
>
> You need to start managing groups of pages in the vm and keeping them
> together and writing them out together and paging them together even if
> one of them is dirty and the other isn't. You have to deal with cases
> where a process forks and the two pages are dirtied one in each but still
> have to be written together.

gaining this sort of ability would not be a bad thing. with current 
hardware (SSDs and raid arrays) you can very easily be in a situation 
where it's much cheaper to deal with a group of related pages as one group 
rather than processing them individually. this is just an extention of the 
same issue.

David Lang

> Alternatively you go for read-modify-write (nasty performance hit
> especially for RAID or a log structured fs).
>
> Yes you can do it but it sure won't be pretty with a conventional fs.
> Some of the log structured file systems have no problems with this and
> some kinds of journalling can help but for a typical block file system
> it'll suck.
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