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

Rune Tipsmark rt at steait.net
Fri Nov 14 23:06:04 UTC 2014

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    I get only about half the bandwidth with Sync=Always compared to Sync=Disabled.

    Using an SLC device that should perform better, its rated as 750 MB/sec, I only get something like 60% of that at best of times. If there is a way to stripe a ZIL it would be great. I have enough to mirror two stripes and get the speed I want.

    Copying ~60GB from one LUN to another on same ZFS box.

    Sync=Always:

    [cid:image001.png at 01D0001C.7E3289F0]

    Sync=Disabled

    [cid:image002.png at 01D0001C.7E3289F0]

    -----Original Message-----
    From: Rune Tipsmark
    Sent: Friday, November 14, 2014 11:53 AM
    To: Rune Tipsmark; Bob Friesenhahn
    Cc: omnios-discuss at lists.omniti.com
    Subject: RE: [OmniOS-discuss] slog limits write speed more than it should

    Is there a way to stripe two block devices and use them as log?

    I tried following this one https://blogs.oracle.com/bilke/entry/raid_0_stripe_on_solaris

    But I cannot use the device in ZFS - getting the following error: cannot use '/dev/md/rdsk/zil1d0': must be a block device or regular file

    Br,

    Rune

    -----Original Message-----

    From: OmniOS-discuss [mailto:omnios-discuss-bounces at lists.omniti.com] On Behalf Of Rune Tipsmark

    Sent: Friday, November 14, 2014 9:47 AM

    To: Bob Friesenhahn

    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

    Well that sucks... I guess one more reason to move to NV-Dimms to replace slow SLC cards.

    Br,

    Rune

    -----Original Message-----

    From: Bob Friesenhahn [mailto:bfriesen at simple.dallas.tx.us]

    Sent: Friday, November 14, 2014 6:48 AM

    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 Fri, 14 Nov 2014, Rune Tipsmark wrote:

    >

    > 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.

    This could be a mistaken impression.  The slog commits each write before proceeding to the next write.  If the involved SSDs have a fixed minimum write latency, then this would limit the maximum transaction rate regardless of the number of SSDs involved.  There could be an advantage to more SSDs if the additional time between writes allows the SSD to more effectively prepare for the next write and reduce the write latency.

    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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