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Re: Hardware vs Software RAID

From: "Merlin Moncure" <mmoncure(at)gmail(dot)com>
To: "Matthew Wakeling" <matthew(at)flymine(dot)org>
Cc: "Pgsql performance" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Hardware vs Software RAID
Date: 2008-06-27 13:16:13
Message-ID: b42b73150806270616t6acdb5f4j26dc75902bee4509@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Fri, Jun 27, 2008 at 7:00 AM, Matthew Wakeling <matthew(at)flymine(dot)org> wrote:
> On Thu, 26 Jun 2008, Merlin Moncure wrote:
>>
>> In addition there are many different types of flash (MLC/SLC) and the
>> flash cells themselves can be organized in particular ways involving various
>> trade-offs.
>
> Yeah, I wouldn't go for MLC, given it has a tenth the lifespan of SLC.
>
>> The main issue is lousy random write performance that basically makes them
>> useless for any kind of OLTP operation.
>
> For the mentioned device, they claim a sequential read speed of 100MB/s,
> sequential write speed of 80MB/s, random read speed of 80MB/s and random
> write speed of 30MB/s. This is *much* better than figures quoted for many
> other devices, but of course unless they publish the block size they used
> for the random speed tests, the figures are completely useless.

right. not likely completely truthful. here's why:

A 15k drive can deliver around 200 seeks/sec (under worst case
conditions translating to 1-2mb/sec with 8k block size).   30mb/sec
random performance would then be rough equivalent to around 40 15k
drives configured in a raid 10.  Of course, I'm assuming the block
size :-).

Unless there were some other mitigating factors (lifetime, etc), this
would demonstrate that flash ssd would crush disks in any reasonable
cost/performance metric.  It's probably not so cut and dry, otherwise
we'd be hearing more about them (pure speculation on  my part).

merlin

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