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fpemud
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Joined: 15 Feb 2012
Posts: 283

PostPosted: Wed Jun 18, 2014 12:18 pm    Post subject: SSD + HDD, bcache alternative? Reply with quote

I have a 128GB SSD and a 1TB HDD.

bcache suits me well, but it has an obvious disadvantage: 128GB SSD + 1TB HDD = 1TB data.
What I want is "128GB SSD + 1TB HDD = 1128GB data + all hot data in SSD", which is similar to but has fundamental difference with RAID-0.

Any suggestions?
Is my requirement doable?
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eccerr0r
Watchman
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Joined: 01 Jul 2004
Posts: 7267
Location: almost Mile High in the USA

PostPosted: Wed Jun 18, 2014 6:35 pm    Post subject: Reply with quote

The whole idea of a cache is that, except for write back cache, everything in the cache is redundant, so yes, you'll end up with only 1TB. That's how it works with Intel SRT too which was one of the first implementations of SSD caching (as well as on-chip caches).

What I'm confused about is you actually want 128G SSD+1000G HDD where it dynamically puts all the data you're working with on the SSD and anything not being worked on stored on the HDD? So if your SSD is full and you write more data, it will automatically have to flush some of the data from the SSD to the HDD, thus losing any benefits of SSD write caching once the SSD has a full 128G of data?

I think this would be a nightmare in terms of data tracking and a lot of data swapped between the disks once the initial 128GB is filled up. I don't see the disk space loss as a disadvantage at all, having the 128G as redundant to the 1TB has multiple benefits including write caching where it knows that space in the SSD hasn't been used in a while, so it can instantly blow away that data without having to write anything back because it knows (or at least it knows what is and isn't) that the data is safe on the HDD already.
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user
Tux's lil' helper
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Joined: 08 Feb 2004
Posts: 145

PostPosted: Wed Jun 18, 2014 6:46 pm    Post subject: Reply with quote

Hi fpemud,
depends on your filesystem preference.

Maybe this will fit your needs ext4 external journal on SSD?
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fpemud
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Joined: 15 Feb 2012
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PostPosted: Thu Jun 19, 2014 1:39 am    Post subject: Reply with quote

eccerr0r,

You're right. It is more complex than I imagine.
There must be some free space in SSD, and a background thread to move data from SSD to HDD.

user,

Thanks for your information, I think what I'm aftering is called tiered storage.
I'm studying this project btier.
It seems Apple has already implemented this function as Fusion Drive, I'm trying to find a linux equivalent.
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