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Is there software for Linux to use an SSD as disk cache? I believe that Sun does something like this with ZFS, though not sure. A quick search provides nothing suitable. The goal would be to put frequently requested files on the SSD on-the-fly. Since the SSD has more capacity than RAM for less money and better performance than hard disk, this should provide an efficient performance boost.

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Are you sure SSD is nice for caching? As far as i know, SSD have rewrite limit of about 10000 cycles... –  kolypto Dec 7 '09 at 0:05
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Good grief, modern flash SSDs are good for allegedly millions of cycles of writes. See serverfault.com/questions/14189/reliability-of-ssd-drives for some further comments. Furthermore, the server will likely be serving up a similar set of files frequently. –  casualcoder Dec 7 '09 at 2:59
    
See also serverfault.com/questions/239808/ssd-for-swap-on-ubuntu-server/…, which is about swap, but the same caveats apply. –  mattdm Feb 25 '11 at 17:34
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4 Answers

up vote 6 down vote accepted

Facebook recently released a module for the Linux kernel called 'FlashCache' which can do exactly this:

http://github.com/facebook/flashcache

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Have you looked into Bcache? http://bcache.evilpiepirate.org/

I realize this is an old topic, hope this helps anyone else who might have arrived here like me with an ssd cache question.

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I don't think there's a simple way to do this on Linux yet. ZFS is available as a userspace filesystem, but it's not very good. Some ZFS clones are on their way, but as far as I know nothing is ready for production.

Perhaps you could consider a BSD with ZFS?

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With SUN's ZFS, SSDs can be used as L2ARC cache [1], using the zpool add cache command:

cache

A device used to cache storage pool data. A cache device cannot be cannot be configured as a mirror or raidz group. For more information, see the “Cache Devices” section.

I know nothing similar with Linux filesystems. I don't know if it would be suitable, but one thing you may try would be to add swap on SSD and tune the `swapiness' Linux kernel parameter (sysctl vm.swapiness)

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Swap on SSD won't help file performance - the kernel never uses swap space for page cache... –  James Dec 8 '09 at 9:28
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