# [OmniOS-discuss] iSCSI poor write performance

Steffen Wagner mail at steffenwagner.com
Wed Sep 23 13:51:30 UTC 2015

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    Hi Hanno,

    how do you calculate the best ashift value?

    Thanks,
    Steffen

    -----Original Message-----
    From: OmniOS-discuss [mailto:omnios-discuss-bounces at lists.omniti.com] On Behalf Of Hanno Hirschberger
    Sent: Mittwoch, 23. September 2015 14:43
    To: omnios-discuss at lists.omniti.com
    Subject: Re: [OmniOS-discuss] iSCSI poor write performance

    Hi Martin,

    On 23.09.2015 10:51, Martin Truhlář wrote:
    > Tests revealed, that problem is somewhere in disk array itself.

    are you familiar with the ashift problem on 4k drives? My best guess
    would be that the 1 TB WD drives are emulating a block size of 512 bytes
    while using 4k sectors internally. OmniOS is using a ashift value of 9
    then to align the data efficiently (on 512 byte sectors!). This slows
    the whole pool down - I had the same problem before. The ashift value
    has to be 12 on 4k drives!

    Try the command 'zdb' to gather the values for your drives. Look for
    'ashift: 9' oder 'ashift: 12'.

    Regards,

    Hanno
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