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Re: 4k sector drives

To: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Subject: Re: 4k sector drives
From: Matthias Schniedermeyer <ms@xxxxxxx>
Date: Fri, 23 Jul 2010 15:38:43 +0200
Cc: xfs@xxxxxxxxxxx
In-reply-to: <4C4994CB.2050200@xxxxxxxxxxxxxxxxx>
References: <201007211333.48363.eye.of.the.8eholder@xxxxxxxxx> <201007230809.28111@xxxxxx> <20100723095832.GA23174@xxxxxxxxxxxxx> <201007231259.11714@xxxxxx> <4C4994CB.2050200@xxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
On 23.07.2010 08:10, Stan Hoeppner wrote:
> Michael Monnerie put forth on 7/23/2010 5:59 AM:
> > On Freitag, 23. Juli 2010 Christoph Hellwig wrote:
> >> -b size=4096 is the default anyway, for 4k sector drivers you also
> >>  want -s size=4096, which you do not want for 512 byte sector disks.
> > 
> > Thanks for clarification. Should I use "-s size=4096" despite the drive 
> > saying 512b sectors? It's a "hidden" 4K sector drive, so maybe declaring 
> > it extra for XFS helps performance? Or does it not matter at all?
> >  
> >> What values do the files
> >>
> >>         /sys/block/<device>/queue/logical_block_size
> >>         /sys/block/<device>/queue/physical_block_size
> >>         /sys/block/<device>/alignment_offset
> >>
> >> say about your disk?
> > 
> > 512, 512, 0 for a Western Digital D20EARS-00MVWB0 (2TB) which has 4K 
> > sectors but obviously hides it.
> The WD20EARS-00MVWB0 is definitely a 4k sector drive:
> http://products.wdc.com/Library/Flyer/ENG/2178-771123.pdf
> If you're currently seeing somewhere between 60-120MB/s per drive with hdparm
> or dd then you don't need to further tweak anything.  If alignment is off,
> from all I've read, performance will be abysmal, down in the sub 30MB/s range.

Only for writes, reads are for practically unaffected.

The problem for write is the "Read Modify Write" Cycle that is needed 
when you don't change all 8 512byte "sub-"sectors contained in a 4096 
byte sector.

Bis denn

Real Programmers consider "what you see is what you get" to be just as 
bad a concept in Text Editors as it is in women. No, the Real Programmer
wants a "you asked for it, you got it" text editor -- complicated, 
cryptic, powerful, unforgiving, dangerous.

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