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Wed Jan 14 05:09:21 PST 2009

From: =?ISO-8859-1?Q?Bj=F8rn_Vermo?= <bv at>
Subject: Re: RAID 1 or Hammer
Date: Wed, 14 Jan 2009 14:05:46 +0100
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On 14. jan.. 2009, at 12.17, Jasse Jansson wrote:

> First of all, thanks Simon for the links, It sure was interesting =20
> reading.
> On Jan 14, 2009, at 11:45 AM, Bj=F8rn Vermo wrote:
>> I have toyed with the idea that one drive in a mirrored pair ought =20=

>> to have the addresses inverted, so sector 1 on one drive is mapped =20=

>> to sector MAX -1 on the other.
> I read it like the locality (radius) of the errors occur on the same =20=

> disc,
> so I don't see the benefit from your "reversed mirror strategy".

He clearly stated that some drives models had error spikes at certain =20=

cluster numbers.
This should not really surprise anybody who is familiar with the way =20
modern drives work internally. The cylinder where you change to a =20
different data density is a border case, and any border cases open =20
extra bug opportunities.

> You will also get some interesting performance problems to tackle, =20
> when
> one disc works on the inner tracks and the other one is on the outer =20=

> tracks.

A smart controller could make that into an opportunity. It is hardly =20
any more difficult than staggered stripes, which is widely implemented.

In most mirroring controllers I have seen the system will get the read =20=

data from the first drive that has them ready. Two "opposite" drives =20
should be able to offer a better average read performance.

Bj=F8rn Vermo
Core networking
Opera Software ASA

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