On Mon, 23 May 2011, John R Pierce wrote:
> To: centos@xxxxxxxxxx
> From: John R Pierce <pierce@xxxxxxxxxxxx>
> Subject: Re: SSD for Centos SWAP /tmp & /var/ partition
>
> On 05/23/11 9:54 AM, Gordon Messmer wrote:
>> On 05/23/2011 07:23 AM, Michael Schumacher wrote:
>>> As far as I understand, SSD are fast at reading and slow at writing.
>> A good SSD will be substantially faster at writes than a disk drive, as
>> well. Because there's no head seeking around a platter, latency is
>> vastly better, which provides a massive performance advantage in many
>> (if not most) use patterns.
>
> yes,butt.... SSD has to erase and write a LARGE block all at once, so
> they don't do so well with the sorts of 8k random writes that write
> intensive applications like relational databases commonly perform. To
> write a single 8K block would require reading the whole flash block
> (something like 64k or 256k), flash erasing that block, then rewriting
> the whole thing. This would be painfully slow. So, what the drives
> do instead is remap blocks rather randomly onto 'new' space, caching
> them in drive-local buffer memory, then flashing whole blocks at once.
> once the new space is used up, performance tends to degrade as they have
> to scavenge scattered free blocks.
Would a defrag program work on a SSD?
Keith
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