[zfs-discuss] ZFS on RAID-5 array

Edward Ned Harvey (zfsonlinux) zfsonlinux at nedharvey.com
Fri Apr 13 08:53:16 EDT 2018

> From: zfs-discuss <zfs-discuss-bounces at list.zfsonlinux.org> On Behalf Of
> Gordan Bobic via zfs-discuss
>> Allow zfs to toggle the disk's cache on (give zfs the whole disk, not just a
>> partition).
> This may be a little misleading. Even when zpool is told to use a whole disk as
> a vdev it will create two partitions, 1 and 9. Partition 9 is 8MB, and not used,
> but it is useful when you discover that the replacement disk has a few
> sectors fewer than the original one.

I've never seen zpool create a partition on a disk. I give zpool the whole disk, and it uses the whole disk. Not sure how it can end up the way you've described... But even assuming that's correct and not a mistake... The point is for zpool to know it has control of the whole disk, so zpool will enable the on-disk cache. When the situation happens that you've described, it sounds like zpool should still know it has control over the whole disk and enable the cache, right?

I neglected to mention... If you're using HDD's, and you don't have a log device, and you don't set sync=disabled, then the HBA write back is very helpful. But assuming at least one of those conditions doesn't apply, you're better off to disable HBA write-back.

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