Re: BBU still needed with SSD?

From: Florian Weimer
Subject: Re: BBU still needed with SSD?
Date: ,
Msg-id: 82y5zubl90.fsf@mid.bfk.de
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In response to: Re: BBU still needed with SSD?  (Yeb Havinga)
Responses: Re: BBU still needed with SSD?  (Yeb Havinga)
List: pgsql-performance

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BBU still needed with SSD?  (Andy, )
 Re: BBU still needed with SSD?  (Craig Ringer, )
  Re: BBU still needed with SSD?  (David Rees, )
   Re: BBU still needed with SSD?  (Andy, )
    Re: BBU still needed with SSD?  (Bruce Momjian, )
     Re: BBU still needed with SSD?  (Andy, )
      Re: BBU still needed with SSD?  (Greg Smith, )
 Re: BBU still needed with SSD?  (Yeb Havinga, )
  Re: BBU still needed with SSD?  (Florian Weimer, )
   Re: BBU still needed with SSD?  (Yeb Havinga, )
    Re: BBU still needed with SSD?  (Florian Weimer, )
     Re: BBU still needed with SSD?  (Yeb Havinga, )
      Re: BBU still needed with SSD?  (Florian Weimer, )
       Re: BBU still needed with SSD?  (Yeb Havinga, )
        Re: BBU still needed with SSD?  (Greg Smith, )
         Re: BBU still needed with SSD?  (Klaus Ita, )
 Re: BBU still needed with SSD?  (Greg Smith, )

* 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?

--
Florian Weimer                <>
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