[OpenIndiana-discuss] ZIL write cache performance

[Matt Connolly]

Yes, it is as you guess comparing ZIL on the main pool vs ZIL on an SSD. I understand that the ZIL on its own is more of an integrity function rather than a performance boost. 

However, I would have expected some performance boost by using an SSD log device since writing to the dedicated log device reduces I/O load on the main pool (or is this wrong?)

Thanks for the heads up about the bug and pending fix.. I'll take a look.

-Matt. 

On 13/01/2012, at 9:34 AM, Steve Gonczi <gonczi at comcast.net> wrote:

> Hi Matt, 
> 
> The ZIL is not a performance enhancer. (This is a common misunderstanding, 
> people sometimes view the ZIL as a write cache) . 
> 
> It is a way to simulate "sync" semantics on files where you really 
> need that, instead of the coarser ganularity guarantee that zfs gives 
> you without it. (txg level, where the in-progress transaction group may 
> roll back if you crash). 
> 
> If I am reading your post correctly, you are comparing 2 scenarios 
> 
> 1) Zil is enabled, and goes to the main storage pool 
> 2) Zil is enabled but it goes to a dedicated SSD instead. 
> Please verify that this is indeed the case.

Yes, this is the case. 

> You should not expect having an SSD based zil performing better than 
> when turning the ZIL off altogether. 
> 
> The latter of course will have better 
> performance, but you have to live with the possibility of losing some data. 
> 
> Given that the case is (1) and (2), it all depends on how much performance 
> headroom your pool has, vs. the write performance of the SSD. 
> 
> A fast SSD ( e.g.: DRAM based, and preferably dedicated to Zil and not split) 
> would work best. It does not have to be huge, just large enough to store (say) 
> 5 seconds worth of your planned peak data inflow. 
> 
> You need to be aware of a recent performance regression discovered pertaining to 
> ZIL ( George Wilson has just posted the fix for review on the illumos dev list) 
> This has been in Illumos for a while, so it is possible, that it is biting you. 
> 
> Steve 
> 
> ----- Original Message -----
> Hi, I've installed an SSD drive in my OI machine and have it partitioned (sliced) with a main slice to boot from and a smaller slice to use as a write cache (ZIL) for our data pool. 
> 
> I've noticed that for many tasks, using the ZIL actually slows many tasks at hand (operation within a qemu-kvm virtual machine, mysql loading importing a dump file, etc). I know I bought a cheap SSD to play with so I wasn't expected the best performance, but I would have expected some improvement, not a slow down. 
> 
> In one particular test, I have mysql running in a zone and loading a test data set takes about 40 seconds without the ZIL and about 60 seconds with ZIL. I certainly wasn't expecting a 50% slow down. 
> 
> Is this to be expected? 
> 
> Are there any best practices for testing an SSD to see if it will actually improve performance of a zfs pool? 
> 
> 
> Thanks, 
> Matt 
> 
> 
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