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ZFS sometimes surprises how much space it reports as available directly after creation. As this is non-trivial and usually far from the standard RAID knowledge, the following tables should hopefully help a bit.
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[[_TOC_]]
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## vdev comparisons ##
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For simplicity, the base unit for HDD sizes is 1 (for example 1 TByte), thus simply multiple the correct columns with your actual HDD size to get real numbers.
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The first column `vdev disks` shows the number of physical devices per vdev, thus in section `raidz1` the row starting with 15 would mean something akin to 14 data disks and 1 parity disk (in naive RAID terms).
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The second column `net` shows the net space in units of physical HDDs as read via `zfs get available` directly after pool creation. *Please note*: This is *only* a guesstimate as much will depend on the data which will be added into the pool.
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The following two columns (`int.loss` and `full loss`) show how much ZFS "loses" for parity and metadata. The number given is the relative loss when compared to the naive RAID picture or to the physical devices involved, respectively. The formula is always $`1-\frac{net}{basis}`$ where `basis` is either $`vdev\ disks - raidz\ level`$ or `vdev disks`.
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The final four columns show example set-ups based on typical number of disks per chassis. Each value shows how much net space one would be able to get when using the maximum number of vdevs of the given size.
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### raidz1 ###
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| vdev disks | z1 net | z1 int.loss | z1 full loss | | 12disk | 16disk | 24disk | 36disk |
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|-----------:|-------:|------------:|-------------:|---|-----:|-------:|-------:|-------:|
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| vdev disks | net | int.loss | full loss | | 12disk | 16disk | 24disk | 36disk |
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|-----------:|----:|---------:|----------:|---|-----:|-------:|-------:|-------:|
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| 4 | 2.56 | .147 | .360 | | 7.68 | 10.24 | 15.36 | 23.04 |
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| 5 | 3.52 | .120 | .296 | | 7.04 | 10.56 | 14.08 | 24.64 |
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| 6 | 4.22 | .156 | .297 | | 8.44 | 8.44 | 16.88 | 25.32 |
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... | ... | @@ -41,8 +54,8 @@ ZFS sometimes surprises how much space it reports as available directly after cr |
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### raidz2 ###
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| vdev disks | z2 net | z2 int.loss | z2 full loss | | 12disk | 16disk | 24disk | 36disk |
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|-----------:|-------:|------------:|-------------:|---|-----:|-------:|-------:|-------:|
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| vdev disks | net | int.loss | full loss | | 12disk | 16disk | 24disk | 36disk |
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|-----------:|----:|---------:|----------:|---|-----:|-------:|-------:|-------:|
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| 4 | 1.71 | .145 | .573 | | 5.13 | 6.84 | 10.26 | 15.39 |
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| 5 | 2.61 | .130 | .478 | | 5.22 | 7.83 | 10.44 | 18.27 |
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| 6 | 3.52 | .120 | .414 | | 7.04 | 7.04 | 14.08 | 21.12 |
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... | ... | @@ -79,8 +92,8 @@ ZFS sometimes surprises how much space it reports as available directly after cr |
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### raidz3 ###
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| vdev disks | z3 net | z3 int.loss | z3 full loss | | 12disk | 16disk | 24disk | 36disk |
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|-----------:|-------:|------------:|-------------:|---|-----:|-------:|-------:|-------:|
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| vdev disks | net | int.loss | full loss | | 12disk | 16disk | 24disk | 36disk |
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|-----------:|----:|---------:|----------:|---|-----:|-------:|-------:|-------:|
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| 4 | 0.88 | .120 | .780 | | 2.64 | 3.52 | 5.28 | 7.92 |
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| 5 | 1.76 | .120 | .648 | | 3.52 | 5.28 | 7.04 | 12.32 |
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| 6 | 2.48 | .174 | .587 | | 4.96 | 4.96 | 9.92 | 14.88 |
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... | ... | |