Message-ID: <4E255DF9.20005@gmail.com>
Date: Tue, 19 Jul 2011 12:35:37 +0200
From: Yeb Havinga <yebhavinga@gmail.com>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB;
	rv:1.9.2.18) Gecko/20110616 Lightning/1.0b2 Thunderbird/3.1.11
MIME-Version: 1.0
To: Florian Weimer <fweimer@bfk.de>
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>
In-Reply-To: <82vcuyd7p5.fsf@mid.bfk.de>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

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?

I searched a bit in the mdadm manual for reasons this can be the case. 
It isn't the occasional check (echo check > 
/sys/block/md0/md/sync_action) since that seems to do two reads and 
compare. Another idea was that the layout of the mirror might not be 
different, but the manual says that the --layout configuration directive 
is only for RAID 5,6 and 10, but not RAID 1. Then my eye caught 
--write-behind, the maximum number of outstanding writes and it has a 
non-zero default value, but is only done if a drive is marked write-mostly.

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

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

regards,
Yeb Havinga