From: Merlin Moncure <mmoncure@gmail.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: Pgsql performance <pgsql-performance@postgresql.org>
Subject: Re: Hardware vs Software RAID
Date: Fri, 27 Jun 2008 09:16:13 -0400
Message-ID: <b42b73150806270616t6acdb5f4j26dc75902bee4509@mail.gmail.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0806271143281.4085@aragorn.flymine.org>
References: <200806251311.m5PDBNv23559@inv.it.uc3m.es>
<Pine.GSO.4.64.0806251051060.29204@westnet.com>
<Pine.LNX.4.64.0806251631520.4085@aragorn.flymine.org>
<97454AF8-315A-4B3D-8F95-D96B190C6909@khera.org>
<Pine.LNX.4.64.0806261705280.4085@aragorn.flymine.org>
<b42b73150806261035y34802356hfaf9f9a366a00c8a@mail.gmail.com>
<Pine.LNX.4.64.0806271143281.4085@aragorn.flymine.org>
On Fri, Jun 27, 2008 at 7:00 AM, Matthew Wakeling <matthew@flymine.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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To: pgsql-performance@postgresql.org
Cc: mmoncure@gmail.com, matthew@flymine.org
Subject: Re: Hardware vs Software RAID
In-Reply-To: <b42b73150806270616t6acdb5f4j26dc75902bee4509@mail.gmail.com>
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