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In one of the answers on SO (I forgot which one) I've seen a suggestion to make a RAID-1 array composed of a RAM disk and a physical partition. By adding the physical partition with --write-mostly and enabling --write-behind the system should read everything instantly from the RAM disk but still save all data to the physical partition so that the data are preserved and the RAID array can be assembled again after reboot.

Is such a setup reasonable? By reasonable I mean if it will perform any better in some scenario than using a SSD disk or than having just the physical partition and perhaps tweaking the kernel to favor disk cache (swappiness and vfs_cache_pressure)?

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If this question doesn't get any answers here, or looks like it's going to get closed, I'm going to suggest sending it over to superuser.com - it's certainly an interesting question, but falls a bit outside the scope of this site. –  Mark Henderson Nov 25 '12 at 22:05
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It doesn't seem like you're chasing data redundancy, but to be absolutely clear: you'd only have one copy of your data in permanent storage. If the disk fails, all your data is lost if there's any interruption to the server's RAM. I think you're chasing speed however. It seems as though an array of SSDs (or specialty devices like IOFusion cards) would provide more speed than you'd need at less cost than a system with dozens of gigabytes of RAM. I think you'd have to better define "reasonable" to get any sort of reasonable answer to this Question though. –  Chris S Nov 25 '12 at 22:39
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If all you are wanting to do is speed up the reads then it might be worth looking at something like Flashcache (https://github.com/facebook/flashcache/) in write around or write through mode. Instead of pointing to an SSD you can point the cache device to the ram disk which will speed up the reads.

It would also save a lot on costs as you won't need to buy enough ram for a complete copy of the physical partition and you wouldn't be adding extra wear to the physical disk as the raid1 mirrors the whole disk at boot.

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