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Re: SSD performance

From: david(at)lang(dot)hm
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: Greg Smith <gsmith(at)gregsmith(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: SSD performance
Date: 2009-01-25 13:16:40
Message-ID: alpine.DEB.1.10.0901250504370.16162@asgard.lang.hm (view raw or flat)
Thread:
Lists: pgsql-performance
On Sun, 25 Jan 2009, Gregory Stark wrote:

> david(at)lang(dot)hm writes:
>
>> they currently have it do a backup immediatly on power loss (which is a safe
>> choice as the contents won't be changing without power), but it then powers off
>> (which is not good for startup time afterwords)
>
> So if you have a situation where it's power cycling rapidly each iteration
> drains the battery of the time it takes to save the state but only charges it
> for the time the power is on. I wonder how many iterations that gives you.

good question.

assuming that it's smart enough to not start a save if it didn't finish 
doing a restore, and going from the timings in the article (~20 min save, 
~15 min load and 4 hour battery life)
you would get ~12 cycles from the initial battery
plus whatever you could get from the battery charging (~3 hours during the 
initial battery time)

if the battery could be fully charged in 3 hours it could keep doing this 
indefinantly.

if it takes 6 hours it would get a half charge, so 12+6+3+1=22 cycles

but even the initial 12 cycles is long enough that you should probably be 
taking action by then.

in most situations you are going to have a UPS on your system anyway, and 
it will have the same type of problem (but usually with _much_ less than 4 
hours worth of operation to start with)


so while you could loose data from intermittent power, I think you would 
be far more likely to loose data due to a defective battery or the CF card 
not being fully seated or something like that.

David Lang

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