January 11, 201610 yr I'm interested in knowing how btrfs mirroring in the cache pool affect performance. For example, currently I run a SSD as my cache. If I were to change my cache to a pair of HDDs, would the btrfs use both drives simultaneously to give the effect of lower seek times? I know Windows' RAID1 does this, where it reads from both drives to give a pseudo striping effect. My issue is I'm loading more VMs on the cache, and am running out of space, so I thinking of stuffing in a pair of 2TB WD Blacks. So, what's the story with btrfs mirroring?
January 11, 201610 yr I'm interested in knowing how btrfs mirroring in the cache pool affect performance. For example, currently I run a SSD as my cache. If I were to change my cache to a pair of HDDs, would the btrfs use both drives simultaneously to give the effect of lower seek times? I know Windows' RAID1 does this, where it reads from both drives to give a pseudo striping effect. My issue is I'm loading more VMs on the cache, and am running out of space, so I thinking of stuffing in a pair of 2TB WD Blacks. So, what's the story with btrfs mirroring? I am about to do this too over the next couple of days with 2 x SSD's. Based on what I have read over the past week on the Internet / Forum I have come to the conclusion that on average (across various types of reads, writes etc) I can expect performance of btrfs RAID1 over 2 disks will offer equal to slightly better performance than a single disk AND under no scenario have I seen it performing worse.
January 11, 201610 yr Also curious as to performance expectations for moving to cache pool over single drive cache. Same scenario as you, with VM's filling my cache up, as well as dockers etc. I however, am not interested in moving to spinning drives at all. Seek times are horrid, even on Black drives, compared to SSD's. And pooling/raid etc does not help seek times, that is still and always is the bottleneck between SSD and HDD, the problem is made exponentially worse when talking VM's, and multiple of the. SSD is the only way to go for those cases. What are you doing that you need so much space? My VM's dont use the SSD as anything more than host OS storage, for the most part.
January 11, 201610 yr AFAIK, default Unraid cache pool (btrfs raid1) will have same performance with 1 or 2 disks. 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.
January 12, 201610 yr Well, I have just set this up like I mentioned I would be doing above! So far I see no different at all in performance but I have only tested several GB! Great to have the protected Cache disk now though!
January 29, 201610 yr is there a way to do btrfs raid-10 with 4 disks? Go to cache webpage and on balance status type: -dconvert=raid10 -mconvert=raid10 Press balance, this will also take some time depending on data size but you can use the pool in the meantime.
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