Re: BBU still needed with SSD?

From: Florian Weimer
Subject: Re: BBU still needed with SSD?
Date: ,
Msg-id: 82livubix4.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 12:47, Florian Weimer wrote:
>>
>>> 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?

> smartctl -a /dev/<your disk> should show all values. If it shows
> something that looks like garbage, it means that the database of
> smartmontools doesn't have the correct information yet for these new
> drives. I know that for the recently new OCZ vertex 2 and 3 SSDs you
> need at least 5.40 or 5.41 and that's pretty new stuff. (I just
> happened to install Fedora 15 today and that has smartmontools 5.41,
> whereas e.g. Scientific Linux 6 has 5.39).

Is this "Total_LBAs_Written"?  The values appear to be far too low:

241 Total_LBAs_Written      0x0032   100   100   000    Old_age   Always       -       188276
242 Total_LBAs_Read         0x0032   100   100   000    Old_age   Always       -       116800

241 Total_LBAs_Written      0x0032   100   100   000    Old_age   Always       -       189677
242 Total_LBAs_Read         0x0032   100   100   000    Old_age   Always       -       92509

The second set of numbers are from the drive which wears more quickly.

The read asymmetry is not unusual for RAID-1 configurations (depending
on the implementation; few do "read both and compare", as originally
envisioned, but prefer the primary block device instead).  Reduced read
traffic could translate to increased fragmentation and wear if the drive
defragments on read.  I don't know if the Intel 320s do this.

--
Florian Weimer                <>
BFK edv-consulting GmbH       http://www.bfk.de/
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