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Re: Hardware vs Software RAID

From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
To: "Matthew Wakeling" <matthew(at)flymine(dot)org>
Cc: "Vivek Khera" <vivek(at)khera(dot)org>, "Pgsql performance" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Hardware vs Software RAID
Date: 2008-06-26 17:35:09
Message-ID: b42b73150806261035y34802356hfaf9f9a366a00c8a@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Thu, Jun 26, 2008 at 12:14 PM, Matthew Wakeling <matthew(at)flymine(dot)org> wrote:
>> None of these manufacturers rates these drives for massive amounts of
>> writes. They're sold as suitable for laptop/desktop use, which normally is
>> not a heavy wear and tear operation like a DB.  Once they claim suitability
>> for this purpose, be sure that I and a lot of others will dive into it to
>> see how well it really works.  Until then, it will just be an expensive
>> brick-making experiment, I'm sure.
>
> It claims a MTBF of 2,000,000 hours, but no further reliability information
> seems forthcoming. I thought the idea that flash couldn't cope with many
> writes was no longer true these days?

Flash and disks have completely different failure modes, and you can't
do apples to apples MTBF comparisons.  In addition there are many
different types of flash (MLC/SLC) and the flash cells themselves can
be organized in particular ways involving various trade-offs.

The best flash drives combined with smart wear leveling are
anecdotally believed to provide lifetimes that are good enough to
warrant use in high duty server environments.  The main issue is lousy
random write performance that basically makes them useless for any
kind of OLTP operation.  There are a couple of software (hacks?) out
there which may address this problem if the technology doesn't get
there first.

If the random write problem were solved, a single ssd would provide
the equivalent of a stack of 15k disks in a raid 10.

see:
http://www.bigdbahead.com/?p=44
http://feedblog.org/2008/01/30/24-hours-with-an-ssd-and-mysql/

merlin

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