Message-ID: <5140D5B3.7080104@denninger.net>
Date: Wed, 13 Mar 2013 14:38:27 -0500
From: Karl Denninger <karl@denninger.net>
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To: Steve Crawford <scrawford@pinpointresearch.com>
CC: pgsql-performance@postgresql.org
Subject: Re: New server setup
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On 3/13/2013 2:23 PM, Steve Crawford wrote:
> On 03/13/2013 09:15 AM, John Lister wrote:
>> On 13/03/2013 15:50, Greg Jaskiewicz wrote:
>>> SSDs have much shorter life then spinning drives, so what do you do
>>> when one inevitably fails in your system ?
>> Define much shorter? I accept they have a limited no of writes, but
>> that depends on load. You can actively monitor the drives "health"
>> level...
>
> What concerns me more than wear is this:
>
> InfoWorld Article:
> http://www.infoworld.com/t/solid-state-drives/test-your-ssds-or-risk-ma=
ssive-data-loss-researchers-warn-213715
>
>
> Referenced research paper:
> https://www.usenix.org/conference/fast13/understanding-robustness-ssds-=
under-power-fault
>
>
> Kind of messes with the "D" in ACID.
>
> Cheers,
> Steve

One potential way around this is to run ZFS as the underlying filesystem
and use the SSDs as cache drives.  If they lose data due to a power
problem it is non-destructive.

Short of that you cannot use a SSD on a machine where silent corruption
is unacceptable UNLESS you know it has a supercap or similar IN THE DISK
that guarantees that on-drive cache can be flushed in the event of a
power failure.  A battery-backed controller cache DOES NOTHING to
alleviate this risk.  If you violate this rule and the power goes off
you must EXPECT silent and possibly-catastrophic data corruption.

Only a few (and they're expensive!) SSD drives have said protection.  If
yours does not the only SAFE option is as I described up above using
them as ZFS cache devices.

--=20
-- Karl Denninger
/The Market Ticker =AE/ <http://market-ticker.org>
Cuda Systems LLC

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    <br>
    <div class="moz-cite-prefix">On 3/13/2013 2:23 PM, Steve Crawford
      wrote:<br>
    </div>
    <blockquote cite="mid:5140D226.4060609@pinpointresearch.com"
      type="cite">On 03/13/2013 09:15 AM, John Lister wrote:
      <br>
      <blockquote type="cite">On 13/03/2013 15:50, Greg Jaskiewicz
        wrote:
        <br>
        <blockquote type="cite">SSDs have much shorter life then
          spinning drives, so what do you do when one inevitably fails
          in your system ?
          <br>
        </blockquote>
        Define much shorter? I accept they have a limited no of writes,
        but that depends on load. You can actively monitor the drives
        "health" level...
        <br>
      </blockquote>
      <br>
      What concerns me more than wear is this:
      <br>
      <br>
      InfoWorld Article:
      <br>
<a class="moz-txt-link-freetext" href="http://www.infoworld.com/t/solid-state-drives/test-your-ssds-or-risk-massive-data-loss-researchers-warn-213715">http://www.infoworld.com/t/solid-state-drives/test-your-ssds-or-risk-massive-data-loss-researchers-warn-213715</a>
      <br>
      <br>
      Referenced research paper:
      <br>
<a class="moz-txt-link-freetext" href="https://www.usenix.org/conference/fast13/understanding-robustness-ssds-under-power-fault">https://www.usenix.org/conference/fast13/understanding-robustness-ssds-under-power-fault</a>
      <br>
      <br>
      Kind of messes with the "D" in ACID.
      <br>
      <br>
      Cheers,
      <br>
      Steve
      <br>
    </blockquote>
    <br>
    One potential way around this is to run ZFS as the underlying
    filesystem and use the SSDs as cache drives.&nbsp; If they lose data due
    to a power problem it is non-destructive.<br>
    <br>
    Short of that you cannot use a SSD on a machine where silent
    corruption is unacceptable UNLESS you know it has a supercap or
    similar IN THE DISK that guarantees that on-drive cache can be
    flushed in the event of a power failure.&nbsp; A battery-backed
    controller cache DOES NOTHING to alleviate this risk.&nbsp; If you
    violate this rule and the power goes off you must EXPECT silent and
    possibly-catastrophic data corruption.<br>
    <br>
    Only a few (and they're expensive!) SSD drives have said
    protection.&nbsp; If yours does not the only SAFE option is as I
    described up above using them as ZFS cache devices.<br>
    <br>
    <div class="moz-signature">-- <br>
      -- Karl Denninger<br>
      <a href="http://market-ticker.org"><i>The Market Ticker &reg;</i></a><br>
      Cuda Systems LLC</div>
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