# [OmniOS-discuss] slog limits write speed more than it should

Rune Tipsmark rt at steait.net
Fri Nov 14 07:58:34 UTC 2014

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    Looking at the first link he clearly states that ZIL is written to in a round robin fashion, I was under the impression that 2 log devices would then be faster than 1…unless mirrored of course.

    If this is not true, what is the point of allowing more than 1 vdev as log device at all?

    How can I test the max sped of my log device directly in OmniOS?

    Br,
    Rune

    From: OmniOS-discuss [mailto:omnios-discuss-bounces at lists.omniti.com] On Behalf Of Nicholas George
    Sent: Wednesday, November 12, 2014 9:24 PM
    To: omnios-discuss at lists.omniti.com
    Subject: Re: [OmniOS-discuss] slog limits write speed more than it should

    From what I understand, ZFS writes to the intent log in a single threaded synchronous fashion. Adding three drives just means that the first block is written to the first drive synchronously, then the second block to the second drive synchronously, then the third to the third drive and then back around to the first. You don't actually increase your throughput beyond the performance of a single drive.

    http://nex7.blogspot.com/2013/04/zfs-intent-log.html
    http://www.nexentastor.org/boards/5/topics/6179

    On Wed, Nov 12, 2014 at 6:21 PM, Rune Tipsmark <rt at steait.net<mailto:rt at steait.net>> wrote:
    http://www.fusionio.com/products/iodrive

    160GB SLC

    -----Original Message-----
    From: Bob Friesenhahn [mailto:bfriesen at simple.dallas.tx.us<mailto:bfriesen at simple.dallas.tx.us>]
    Sent: Wednesday, November 12, 2014 3:17 PM
    To: Rune Tipsmark
    Cc: omnios-discuss at lists.omniti.com<mailto:omnios-discuss at lists.omniti.com>
    Subject: Re: [OmniOS-discuss] slog limits write speed more than it should

    On Wed, 12 Nov 2014, Rune Tipsmark wrote:
    >
    > each SLOG device can easily handle much more than 6000 IOPS

    Where may we find the specifications for the SSDs you are using?

    6000 IOPS sounds like it might be quite a lot, depending on the SSD used.

    Bob
    --
    Bob Friesenhahn
    bfriesen at simple.dallas.tx.us<mailto:bfriesen at simple.dallas.tx.us>, http://www.simplesystems.org/users/bfriesen/
    GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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