[CentOS] SSD for Centos SWAP /tmp & /var/ partition

Mon May 23 19:45:39 UTC 2011
Keith Roberts <keith at karsites.net>

On Mon, 23 May 2011, John R Pierce wrote:

> To: centos at centos.org
> From: John R Pierce <pierce at hogranch.com>
> Subject: Re: [CentOS] 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?



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