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I have a few oldish servers running virtual machines but they are limited to 16GB RAM and that limits the number of VMs I can run and the options for RDS.

I've added a RAID1 array of two Intel 311s: 20GB SLC SSDs, and configured them as swap on the host, in the hope that I can now more confidently over-allocate the available memory to the VMs.

So far so good in testing, but before I go any further I want to ask whether I'm missing any big reason why this is going to be a bad idea?


I should perhaps have mentioned that several of the VMs are used very intermittently but I can't reduce the memory they have as when they are active, it is needed.

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possible duplicate of SSD for swap on Ubuntu server –  Michael Hampton Nov 3 '12 at 19:43
    
Thanks Michael, the answers there are certainly helpful. –  Jack Douglas Nov 3 '12 at 20:04

1 Answer 1

up vote 2 down vote accepted

No, it's not a good substitute for real RAM. SSD drives are an order of magnitude slower than RAM... But you know that :).

You also have issues with write-endurance of the drives under constrained memory conditions.

If the solution seems to work well in testing, then try using it... But know that the right approach is to equip your systems with the requisite amount RAM. If your systems cannot accommodate that amount of RAM, it's time to upgrade to modern hardware.

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+1, thanks, write-endurance is my main concern. I am wondering if I should under-partition the array by, say, 4GB to improve endurance. –  Jack Douglas Nov 3 '12 at 19:30
    
SLC is a good step. Underprovisioning is also an okay idea if its your only option. Try not to swap :) Consider a RAID level with data protection as well. –  ewwhite Nov 3 '12 at 19:33
    
sorry, I meant RAID1 not RAID0—question edited. –  Jack Douglas Nov 3 '12 at 19:59
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Write-endurance for SLC is much less of a concern than MLC due to the lack of write-amplification SLC allows. That combined with underprovisioning should give multi-year endurance with an SLC drive purchased today. –  sysadmin1138 Nov 4 '12 at 2:11
    
But if the system is old enough to not accommodate > 16GB RAM, we all know that newer hardware will be the best bang-for-buck performance option. An SLC drive is wasted in that scenario. –  ewwhite Nov 4 '12 at 2:24

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