December 22, 201510 yr I was thinking about getting either intel 730 or 850 pro as cache drives (I have none right now). But I was wondering how much of performance boost I will get by getting a third SSD? Based on my understanding having 2 drives will have no performance impact due to raid 1 ish, but having 3rd or 4th may impact the speed. is this true? Is it worth investing another $$ for a third one? My wallet is pretty tight, so I could "not" buy another drive, if it doesn't make much of sense...
December 22, 201510 yr Community Expert AFAIK using the default cache pool settings (raid1) won’t have any performance gains compared to single disk, but because of the way btfrs raid1 works you will however have more space with 3 devices. btrfs combines all the devices into a storage pool first, and then duplicates the chunks as file data is created. RAID-1 is defined currently as "2 copies of all the data on different devices". This differs from MD-RAID and dmraid, in that those make exactly n copies for n devices. In a btrfs RAID-1 on three 1 TB devices we get 1.5 TB of usable data. Because each block is only copied to 2 devices, writing a given block only requires exactly 2 devices to be written to; reading can be made from only one.
December 22, 201510 yr Author oh, I thought it was acting like RAID0+1, or RAID1+0 where you get an additional performance of having an additional pair of drive after the initial pair
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