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That's why you use bcache.

Sure. BTRFS is, arguably, a disappointment. It has a lot of the features of ZFS, but it doesn't seem to be anything like as user friendly or logical. When I played with it for the first time about a year ago, I found no out-of-the-box automated snapshot features, and had to install a package to handle this, which didn't seem to work reliably. It's also an enormous missed opportunity that BTRFS doesn't have the ability to natively do RAID-6, or the ability to use an SSD as a fast block cache in front of standard HDDs. ZFS has done all of this for many years now.

I'm sure someone will reply with details of how to do these things. But with ZFS it was obvious and I didn't need to spend a lot of time googling.

That said, there's nothing that says that ZFS on Linux is automatically stable either. It's been necessary to extensively modify it in order to make it work. Running either of these two filesystems in production on Linux would be a risky proposition. It's no wonder the major enterprise vendors haven't switched to use them yet.

Unfortunately at the moment it looks like the short term future of filesystems on Linux is based around XFS. Longer term, bcachefs looks interesting.

Linux already supports SSD-HDD caching with normal drives so if anything, it will probably already work or work with little changes. Otherwise, just pick up a tiny SSD and ignore this solution. http://bcache.evilpiepirate.org/

Nice troll, btw.

It's been around in the form of bcache for ages now and bcache is considered stable -- it is already in production-use. It's a pretty nifty thing, can be configured to your needs to quite a large degree and it's smart enough not to cache large, sequential reads/writes. If you're interested check out http://bcache.evilpiepirate.org/ and http://atlas.evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt?h=bcache-dev

It's been around in the form of bcache for ages now and bcache is considered stable -- it is already in production-use. It's a pretty nifty thing, can be configured to your needs to quite a large degree and it's smart enough not to cache large, sequential reads/writes. If you're interested check out http://bcache.evilpiepirate.org/ and http://atlas.evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt?h=bcache-dev

As I understand it, this is the current recommended setup for SSD drives.

Actually, I think even better than that is to use an SSD as a caching disk, backed by regular disks.

Then everything can benefit from the speed boost since the computer is better than you at deciding which files need a speedup. You don't want to move so many high write files off of the SSD that you make it pointless.

If you are on linux see: http://bcache.evilpiepirate.org/

For Linux users: http://bcache.evilpiepirate.org/

Please help mainline this, please.

For Linux users: http://bcache.evilpiepirate.org/

Lets you use any SSD as a cache in front of another filesystem.

Solaris and Windows have been shipping with production ready L2 FS cache for years already, L2ARC/ReadyBoost. I'll give Apple a pass because their systems are mostly not designed for adding drives, and they were apparently betting on high capacity SSDs coming down in price by now. Desktops have less of a need for caches in the tens of GB anyway. Linux, as a server OS doesn't have much of a good excuse, why wasn't L2 cache worked out years ago when everyone was racing for TRIM support? Using smaller cheaper SSD drives as L2 cache almost makes too much sense. It covers up the short write cycle lifetime and poor sequential read performance. 60 some odd GB of cache starts to look pretty dang good for a lot of server workloads.

I feel I should point this out because these cheesy Linux +1 MeToo posts are _really_ aggravating to people who use it professionally. It's a tool. We're not in love with it.

http://arstechnica.com/civis/viewtopic.php?f=21&t=1114013
The developer apparently didn't even know what the ARC algorithm is... which is just bizarre, like developing a race car without knowing what variable valve timing is. Not saying it is needed, but what level of quality do you expect out of this?

See also bcache: http://bcache.evilpiepirate.org/MoreInformationAboutBcache

(though I'm not sure why you wouldn't just put your fs journal on the ssd directly if you could)

Intel is doing the very same thing on their most recent "enthusiast" desktop chipsets.

For systems using the Linux kernel, there are software implementations of the very same block-level-caching-concept available - one I stumbled over is http://bcache.evilpiepirate.org/

... Make no mistake, this should be the job of the operating system. Solaris has zfs l2arc, and Windows has ReadyBoost that is similar. Windows and Mac systems both ship with the big disk little SSD pattern. MS and Apple need to get off their asses and ship 'SSD as cache' software like Sun did. Only reason I can think of for not doing this is patent disputes, some angle that makes this not feasible for desktop use, or an intentional long term strategy to drive SSD costs down by poising them as spinning disk replacements rather than complements. Ok, for Apple that last one actually makes sense.. but Microsoft? They already have ReadyBoost.. I don't get it.

Intel is doing the very same thing on their most recent "enthusiast" desktop chipsets.

For systems using the Linux kernel, there are software implementations of the very same block-level-caching-concept available - one I stumbled over is http://bcache.evilpiepirate.org/

I know that bcache exists but why isn't there a mainline kernel SSD cache available yet? I could use it for my whitebox SANs where there are dozens of terabytes but only the need for a few gigs to be written on a given day.

Am I supposed to just run out and buy SSDs for the whole load?

http://bcache.evilpiepirate.org/ you mean ?

with your own hard drive and your own SSD. It'd probably cost less, too. http://bcache.evilpiepirate.org/

And Linux has bcache.
And Solaris has L2ARC for ZFS (zpool add XXX cache cXdXtX).

Both of these technologies allow the OS to uses a fast SSD as a general cache device for data on slower HDDs.