question about zfs doc

[marco ardito]

Hi, I was reading your zfs guide (excellent : until now, I wasn't able 
to really undertand how cool zfs is. Thanks, another point to freebsd 
(not yet user, thoough), but I found a section which could be missing 
one paragraph, maybe?

here
https://www.freebsd.org/doc/handbook/zfs-term.html#zfs-term-vdev

...is RAID-Z2 description missing?

/"RAID-Z/ - ZFS implements RAID-Z, a variation on standard RAID-5 that 
offers better distribution of parity and eliminates the "RAID-5 write 
hole" in which the data and parity information become inconsistent after 
an unexpected restart. ZFS supports three levels of RAID-Z which provide 
varying levels of redundancy in exchange for decreasing levels of usable 
storage. The types are named RAID-Z1 through RAID-Z3 based on the number 
of parity devices in the array and the number of disks which can fail 
while the pool remains operational.

In a RAID-Z1 configuration with four disks, each 1 TB, usable storage is 
3 TB and the pool will still be able to operate in degraded mode with 
one faulted disk. If an additional disk goes offline before the faulted 
disk is replaced and resilvered, all data in the pool can be lost.

In a RAID-Z3 configuration with eight disks of 1 TB, the volume will 
provide 5 TB of usable space and still be able to operate with three 
faulted disks. Sun^(TM) recommends no more than nine disks in a single 
vdev. If the configuration has more disks, it is recommended to divide 
them into separate vdevs and the pool data will be striped across them.

A configuration of two RAID-Z2 vdevs consisting of 8 disks each would 
create something similar to a RAID-60 array. A RAID-Z group's storage 
capacity is approximately the size of the smallest disk multiplied by 
the number of non-parity disks. Four 1 TB disks in RAID-Z1 has an 
effective size of approximately 3 TB, and an array of eight 1 TB disks 
in RAID-Z3 will yield 5 TB of usable space."

Thanks!
-- 
*Marco Ardito*
Responsabile Servizi Informatici

Api Formazione s.c.r.l.
Via Pianezza 123, 10151 - TORINO
Tel: +39-0114513123
Fax: +39-0114551150 - +39-0114515075
Mail : ardito at apiform.to.it <mailto:ardito at apiform.to.it>
Web : http://www.apiform.to.it


-------------------
[Ai sensi e per  gli  effetti della Legge sulla tutela della privacy
(L. 196/2003), questa  mail  è destinata unicamente alle persone sopra
indicate e le informazioni in essa contenute sono da considerarsi
strettamente riservate. E' proibito leggere, copiare, usare o
diffondere il contenuto della presente mail senza autorizzazione.
Se avete ricevuto questo messaggio per errore, siete pregati di
rispedire la stessa al mittente. Grazie]