From: Greg Smith <gsmith@gregsmith.com>
To: 'pgsql-performance' <pgsql-performance@postgresql.org>
Subject: Re: With 4 disks should I go for RAID 5 or RAID 10
Date: Wed, 26 Dec 2007 17:40:06 -0500 (EST)
Message-ID: <Pine.GSO.4.64.0712261733390.10252@westnet.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0712261534530.11785@asgard.lang.hm>
References: <0f8201c847f6$08e15090$8f01010a@iptel.com.ar>
<Pine.LNX.4.64.0712261317320.11785@asgard.lang.hm>
<87zlvxnpkl.fsf@mid.deneb.enyo.de>
<4772CA58.5050406@mark.mielke.cc>
<Pine.LNX.4.64.0712261504310.11785@asgard.lang.hm>
<4772CE34.2040608@mark.mielke.cc>
<Pine.LNX.4.64.0712261534530.11785@asgard.lang.hm>
On Wed, 26 Dec 2007, david@lang.hm wrote:
> yes, the two linux software implementations only read from one disk, but I
> have seen hardware implementations where it reads from both drives, and if
> they disagree it returns a read error rather then possibly invalid data (it's
> up to the admin to figure out which drive is bad at that point).
Right, many of the old implementations did that; even the Wikipedia
article on this subject mentions it in the "RAID 1 performance" section:
http://en.wikipedia.org/wiki/Standard_RAID_levels
The thing that changed is on modern drives, the internal error detection
and correction is good enough that if you lose a sector, the drive will
normally figure that out at the firmware level and return a read error
rather than bad data. That lowers of the odds of one drive becoming
corrupted and returning a bad sector as a result enough that the overhead
of reading from both drives isn't considered as important. I'm not aware
of a current card that does that but I wouldn't be surprised to discover
one existed.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
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To: pgsql-performance@postgresql.org
Cc: gsmith@gregsmith.com
Subject: Re: With 4 disks should I go for RAID 5 or RAID 10
In-Reply-To: <Pine.GSO.4.64.0712261733390.10252@westnet.com>
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