SSD cache questions

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Chirag K

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Feb 6, 2017
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Hello

I am new to Freenas - like a week into it now... my server is an old repurposed Dell R710 with dual Xeon 48gb ram and 10GigE network... my current limitation is 6gbps backplanes instead of the new 12gbps ones. But that is not an issue.

A bit background, So I was experimenting with various setups to familiarize myself - striped disks, mirrors, mirror of stripes etc. i am not going to do raid5 as everyone says don't do it...
Anyhow, I tested out with 2 SSD in stripe and I was very pleased - getting 550MB/s across network on my XenServer VM using crystal diskmark. Also tested with large file copy paste and it looked good.

Next I tried a single 5400rpm disk. Of course the performance maxes out at about 120MB/s. Then I setup 1 SSD for cache and 1 for Log, and oh my god... I essentially get SSD performance out of my spinning disk. Needless to say I was very impressed.

That brings me to my question-
If I have a database setup on the spinning disk which has SSD for log and cache, can I expect the writes to be SSD like? (Because the log would store it in SSD and flush it to disk slowly).
How about reads? I am guessing unless the read data is cached, the database query would be just like spinning disk performance?

My plan is to have 4x4TB HDD in mirror stripe (Raid 10), put the 2 SSD for cache and log and that should give me a decent performance database?

I cannot afford to have the 8TB worth of SSDs so with budget in mind, what's your opinion?
 

joeschmuck

Old Man
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I think there is some reading you need to do about ZIL/LOG caches. The first thing to understand is there is no substitute for RAM. If you have a 64GB+ RAM then you would be fine installing these devices and yes, they effectively give you that speed you are seeing. In the L2ARC you will only see that speed if the data resides in the SSD. You should only use specific SSD hardware for a ZIL as well. Do some reading, it's all over these forums.
 
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