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Date:      Sat, 28 Jul 2012 20:22:11 +0200 (CEST)
From:      Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To:        Vladimir Videscu <vladimir.videscu@gmail.com>
Cc:        questions@freebsd.org
Subject:   Re: FreeBSD on SSD
Message-ID:  <alpine.BSF.2.00.1207282016110.9859@wojtek.tensor.gdynia.pl>
In-Reply-To: <CAMxhPysfghi-0ufy0OU0nonqha5mFeRacPVXM0MBDe6fqxmjUg@mail.gmail.com>
References:  <CAMxhPyt-gF228nX0DNrdsFVXKVx6YjZVa7obfov5vf+dtV71zw@mail.gmail.com> <alpine.BSF.2.00.1207281229260.6801@wojtek.tensor.gdynia.pl> <CAKB6gVgEyvVYaUvLkvE_AU-aViJxHAiLu6drcodLjOOBeNEYLg@mail.gmail.com> <CAMxhPysfghi-0ufy0OU0nonqha5mFeRacPVXM0MBDe6fqxmjUg@mail.gmail.com>

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> The read-cache idea is very sound, mainly because by using it this way Seagate would not have to create a special set of
> instructions for installing and using the HDD.

I don't think that this drive cache is smart enough to really cache needed 
things and not flush that cache with useless data too often.

i personally would prefer that drive to show up as 8GB disk and 750 GB 
disk.

It would be easy to fit most of /usr in 8GB with even large set of 
software installed.

when out of space then use softlinks and move not very used things (eg. 
documentation, non-yours locales, examples, some linarly accessed big 
files etc.) to disk.

> My final question would be :
> 
> Seeing as the HDD only has a SATA connector, this would mean that the SSD part already has a memory control device that regulates
> access to that sector, whether it is a plain read-cache or not. This would imply that FreeBSD could communicate with the HDD
> normally, through the SATA connector, just like any regular HDD.

indeed.

Such a drive is a good idea, but complete lack of documentation (how it 
operate) is not.

You have to guess how this SSD-cache works because it is not documented.

the other thing is erasing data. You want to sell that drive and clear 
your data by

dd if=/dev/zero of=/dev/disk bs=1m

but does it clear SSD cache? i don't think so. Someone sophisticated 
enough would perform raw read of cache chips and get cached data, which 
can actually be the most important part (things you work on often).



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