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Hardware

59 bytes added, 15:44, 2 September 2015
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Hardware RAID controllers
** There can be sector alignment problems on ZFS when a drive misreports its sector size. Such drives are typically NAND-flash based solid state drives and older SATA drives from the advanced format (4K sector size) transition before Windows XP EoL occurred. This can be [[Performance_tuning#Alignment_Shift_.28ashift.29 | manually corrected]] at vdev creation.
** It is possible for the RAID header to cause misalignment of sector writes on RAID 1 by starting the array within a sector on an actual drive, such that manual correction of sector alignment at vdev creation does not solve the problem.
 
* If multiple RAID arrays are formed by the same controller and one fails, the identifiers provided by the arrays exposed to the OS might become inconsistent. Giving the drives directly to the OS allows this to be avoided via naming that maps to a unique port or unique drive identifier.
** e.g. If you have arrays A, B, C and D; array B dies, the interaction between the hardware RAID controller and the OS might rename arrays C and D to look like arrays B and C respectively. This can fault pools verbatim imported from the cachefile.
** Not all RAID controllers behave this way. However, this issue has been observed on both Linux and FreeBSD.
* Controller failures can require that the controller be replaced with the same model, or in less extreme cases, a model from the same manufacturer. Using ZFS by itself allows any controller to be used.
* The controller's firmware is an additional layer of complexity that cannot be inspected by arbitrary third parties. The ZFS source code is open source and can be inspected by anyone.
 
* If multiple RAID arrays are formed by the same controller and one fails, the identifiers provided by the arrays exposed to the OS might become inconsistent. Giving the drives directly to the OS allows this to be avoided via naming that maps to a unique port or unique drive identifier.
** e.g. If you have arrays A, B, C and D; array B dies, the interaction between the hardware RAID controller and the OS might rename arrays C and D to look like arrays B and C respectively. This can fault pools verbatim imported from the cachefile.
** Not all RAID controllers behave this way. However, this issue has been observed on both Linux and FreeBSD when system administrators used single drive RAID 0 arrays.
One might be inclined to try using single-drive RAID 0 arrays to try to use a RAID controller like a HBA, but this is not recommended for many of the reasons listed for other hardware RAID types. It is best to use a HBA instead of a RAID controller, for both performance and reliability.
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