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Re: in 3.7 kernel, how does 1GB page tables for kernel pagetables affect

To: Linda Walsh <xfs@xxxxxxxxx>
Subject: Re: in 3.7 kernel, how does 1GB page tables for kernel pagetables affect XFS?
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Sun, 20 Jan 2013 11:46:38 +1100
Cc: xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <50FB3265.8060506@xxxxxxxxx>
References: <50FAF860.3000702@xxxxxxxxx> <20130119231644.GX2498@dastard> <50FB3265.8060506@xxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Sat, Jan 19, 2013 at 03:55:17PM -0800, Linda Walsh wrote:
> 
> 
> Dave Chinner wrote:
> >On Sat, Jan 19, 2013 at 11:47:44AM -0800, Linda Walsh wrote:
> >>Given that XFS relies on the page size for it's maximum block size,
> >>does switching to 1GB pages for kernel pagetables allow for larger
> >>page-size use in XFS -- and therefore, larger blocksize?
> >
> >No, it doesn't. The page cache can't use large pages so there's no
> >path to allowing XFS to use larger blocks with them.
> >
> >As it is, you'd need a system that supports 16k or 64k page sizes
> >for this to be useful, otherwise you just waste a *lot* of memory...
> >
> >>Is it something that might be in the not too distant future? ;-)
> >
> >I have no plans to do this any time soon - there's no compelling
> >reason to do so. I can't speak for anyone else, though...
> 
> ---
>       No prob.. I admit a 1GB page size might be a tad large for
> my purposes...  I actually was reading 1MB though even that would be
> large...but a GB/disk sector...um...-- at least I'd be likely to
> get full speed read & writes if I could fill them...(1GB/R or W
> gives me about the fastest overall throughput to my disks).
> 
>       Can't test higher (i.e. >=2GB) than that due to getting
> partial block read/writes (on a 64 bit machine??)  Sounds like

I think that the read/write interface are limited to 2GB buffer
size, regardless of the platform.

> my LSI controller is only 32bits...but that doesn't seem right...
> Maybe it only supports consecutive read/writes up to the limit
> of the memory on the controller card..
> 
>       All that talk about RAIDs recently, got me depressed a bit
> when I realize that while I can get fast speeds, type speeds in seeking
> around are about 1/10-1/20th the speed...sigh.
> 
>       Might that indicate that I should go with smaller RAIDS with more
> spindles?  I.e. instead of 3 groups of RAID5 striped as 0, go for 4-5 groups
> of RAID5 striped as a 0?  Just aligning the darn things nearly takes a rocket
> scientist!  But then start talking about multiple spindles and optimizing
> IOP's...ARG!...;-)  (If it wasn't challenging, I'd find it boring...)...

Somebody on the list might be able to help you with this - I don't
have the time right now as I'm deep in metadata CRC changes...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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