[zfs-discuss] ZFS on top of iSCSI on top of ZFS

kaazoo kaazoo at kernelpanik.net
Fri Jan 8 07:01:18 EST 2016

Am 08.01.16 um 12:45 schrieb Gordan Bobic via zfs-discuss:
> On 8 Jan 2016 11:59 am, kaazoo via zfs-discuss <zfs-discuss at list.zfsonlinux.org> wrote:
>> Am 07.01.16 um 16:25 schrieb Markus Koeberl:
>>> On Thursday 07 January 2016 09:51:41 kaazoo via zfs-discuss wrote:
>>>> The iSCSI volumes have 128 KB blocksize, so ashift=17 is used.
>>>> ZFS features like compression and snapshots are used here.
>>> If I understand it right how compression is working you may not benefit with this settings.
>>> Because of ashift=17 you cannot use smaller blocks than 128 KB and because of the max blocksize of 128 KB you never get bigger blocks.
>>> So the only way to save space is if you get 128 KB of zeros I guess.
> I'm pretty sure there is no such think as ashift=17 in ZFS. The maximum value is 13 (8KB).

Hello Gordan,

I was wondering as well.

The 'Linux IO target' iSCSI device based on a zvol with blocksize=128K
will report a physical blockisze of 128K. When creating a zpool without
specifiing a value for ashift 'ZFS on Linux' seems to do autodetection
and try to match the physical blocksize which is reported by the device.
'ashift=17' was shown via 'zdb' command.

For some devices which only report 512 Byte but use 4 KB internally,
other hardcoded ashift values are used.


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