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As we all know, Solid State Drives (SSD) are expensive ($/Gb) but they are quick to access. I'm wondering if there exists a sort of software-kernel-level 'cache swap' filesystem technology; the idea of such filesystem is that it keeps a cache of frequently accessed files on the main HDD, so;

  • you get the benefits of SSD speed access for 90% of circumstances
  • you get the benefits of cheap long-term storage for files that have not been used in a while
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You should read this article: the best of SSD and HDD worlds

Basically the simplest and cheapest approach for getting SSD speed benefits without breaking your bank is to use a small SSD (16 ~ 32Gb should be more than enough to reap 80% of performance benefits) disk to keep the / partition where all system and applications files are kept (those are what mostly affect the startup performance I/O bottleneck anyways) and /users, where all the personal data is stored, in the HDD.

This has an additional side-benefit; system files are updated less often than user files, so it means the average lifespan of the SSD is largely extended by confining it to a read-mostly-write-rarely filesystem

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Your idea motivated me. Well I have got Dragonfly OS that maybe suits your need. As stated there:

DragonFly is uniquely positioned to take advantage of the wide availability of affordable Solid Storage Devices (SSDs), by making use of swap space to cache filesystem data and meta-data. This feature, commonly referred to as "swapcache", can give a significant boost to both server and workstation workloads, with a very minor hardware investment.

This is a online documentation of the " swapcache" functionality which might help you get further.

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Its a BSD OS. As stated there: DragonFly is uniquely positioned to take advantage of the wide availability of affordable Solid Storage Devices (SSDs), by making use of swap space to cache filesystem data and meta-data See the cacheswap documentation here and view full site here –  ashutosh Apr 30 '12 at 21:42
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ZFS L2ARC is precisely doing that. Although there is a native ZFS implementation for Linux and a FUSE based one, this kernel might not be the optimal choice here.

I would recommend OSes with native ZFS support like Solaris 11, OpenSolaris/Illumos based distributions or FreeBSD.

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Intel's newest chipsets support caching for an installation on a hard drive with a small SSD (in fact, they're selling 20GB SSDs intended for this use). They call it 'Smart Response Technology'. There are a few vendors who've been putting proprietary versions of this on motherboards as well for the last year or so, but I can't speak for the specifics of those.

There are also some manufacturers making 'hybrid hard drives', which are essentially a variant of this. They're a platter drive with some NAND onboard, but I think it's primarily used for buffering more than caching. They're probably a little more cost effective, but nowhere near as fast as using a real SSD for it. (They're still faster than normal hard drives, though.)

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