From: Florian Weimer <fweimer@bfk.de>
To: Yeb Havinga <yebhavinga@gmail.com>
Cc: Andy <angelflow@yahoo.com>,  pgsql-performance@postgresql.org
Subject: Re: BBU still needed with SSD?
References: <1310953399.25726.YahooMailClassic@web111304.mail.gq1.yahoo.com>
	<4E23E72B.2070508@gmail.com> <82vcuyd7p5.fsf@mid.bfk.de>
	<4E255DF9.20005@gmail.com>
Date: Tue, 19 Jul 2011 10:47:07 +0000
In-Reply-To: <4E255DF9.20005@gmail.com> (Yeb Havinga's message of "Tue, 19 Jul
	2011 12:35:37 +0200")
Message-ID: <82y5zubl90.fsf@mid.bfk.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable

* Yeb Havinga:

> On 2011-07-19 09:56, Florian Weimer wrote:
>> * Yeb Havinga:
>>
>>> The biggest drawback of 2 SSD's with supercap in hardware raid 1, is
>>> that if they are both new and of the same model/firmware, they'd
>>> probably reach the end of their write cycles at the same time, thereby
>>> failing simultaneously.
>> I thought so too, but I've got two Intel 320s (I suppose, the report
>> device model is "SSDSA2CT040G3") in a RAID 1 configuration, and after
>> about a month of testing, one is down to 89 on the media wearout
>> indicator, and the other is still at 96.  Both devices are
>> deteriorating, but one at a significantly faster rate.
> That's great news if this turns out to be generally true. Is it on
> mdadm software raid?

Yes, it is.

It's a mixed blessing because judging by the values, one of the drives
wears down pretty quickly.

> Maybe it is caused by the initial build of the array? But then a 7%
> difference seems like an awful lot.

Both drives a supposedly fresh from the factory, and they started with
the wearout indicator at 100.  The initial build should write just
zeros, and I would expect the drive firmware to recognize that.

I've got a second system against which I could run the same test.  I
wonder if it is reproducible.

> It would be interesting to see if the drives also show total xyz
> written, and if that differs a lot too.

Do you know how to check that with smartctl?

--=20
Florian Weimer                <fweimer@bfk.de>
BFK edv-consulting GmbH       http://www.bfk.de/
Kriegsstra=DFe 100              tel: +49-721-96201-1
D-76133 Karlsruhe             fax: +49-721-96201-99