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

[Richard Elling]

On Nov 12, 2014, at 9:24 PM, Nicholas George <headlesscow at gmail.com> wrote:

> 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.

This information is not quite right. ZFS has several algorithms at play and recall that there 
is not one ZIL, there is one ZIL per dataset. In practice, a busy server will see work spread
out across all slogs and adding more can improve performance. There is always a point of
marginal return, but these are often determined by physical choices.

The best approach is to measure and experiment on your hardware under your workload.
 -- richard

> 
> 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> wrote:
> http://www.fusionio.com/products/iodrive
> 
> 160GB SLC
> 
> -----Original Message-----
> From: Bob Friesenhahn [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
> 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, http://www.simplesystems.org/users/bfriesen/
> GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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