zfs performance degradation

Dmitrijs war at dim.lv
Thu Sep 24 16:17:17 UTC 2015

2015.09.24. 18:44, Paul Kraus пишет:
> On Sep 24, 2015, at 11:11, Dmitrijs <war at dim.lv> wrote:
>> Nope, no compression, no deduplication, only pure zfs. Even no prefetch, as it is not recommended for machines 4Gb RAM and below.
> I am very surprised that ZFS is CPU limited on that system. My N54L has less CPU performance than that and I easily get 60 MB/sec via CIFS (Samba) from a Mac or Windows client.

I also get about 60-70MB/sec via CIFS or ftp, but my aim is to be 
limited by network, so 100MB is wanted. Or, to understand why it is not 
possible on my config :)
But simple dd of=/dev/null in the console shows me 110MB/sec...
iozone gives me the same 100+Mb/sec both on read and write.

That's one of the reasons I'm seeking advice in 
freebsd-questions at freebsd.org

>> Now I'm not sure what configuration will make better performance for 4 HDD - raid10 or raid-z2? Or two separate mirrors? Need directions for scale things up in the future.
> Of all the questions you have asked that one is the easiest to answer … a zpool which has 2 vdevs each of which is a 2-way mirror will have roughly double the performance of a zpool that has one vdev that is a 4 drive RAIDz2. Performance scales with the number of vdevs, not the number of drives. I know that is not obvious at first, but when you look at the design of ZFS (all top level vdevs are striped across) it makes perfect sense.
> So a 2 x 2-way mirror will be faster than a 4 drive RAIDz2. At a cost, the MTTDL (Mean Time To Data Loss) will be better for the RAIDz2 than the 2 x 2-way mirror. See Richard Ellings post here http://blog.richardelling.com/2010/02/zfs-data-protection-comparison.html for a comparison of relative MTTDL for ZFS configurations.
> Note that I use 3-way mirrors where I need _both_ performance and reliability and RAIDz2 where I need mostly reliability and performance is secondary. But … back when I was managing lots of data (2007 - 2012), I did use RADIz2 in production for critical data, but we had 22 top level vdevs, each a 5 drive RAIDz2 and 10 hot spares. Striping data across 22 RAIDz2 gave us the performance we needed with the reliability.

Thanks, noted. Will educate myself in the given direction.

>> Thought it would be sufficient, but now I'm in doubt.
> I think that 4 GB is slightly low for a file server, but it should not be too bad. The CPU should be fine. What are the drives themselves ? [Because with only 4 GB RAM you _will_ feel the effect of drive performance, and it is random I/Ops that really matter for ZFS]

2x HGST HDN724040ALE640, 4Tb, 64Mb, 7200.
I even ordered 8Gb RAM for tests, but they mistakenly delivered me 4Gb!..

>> I can live with reduced performance for my 1st NAS, but would be nice to have clear performance requirements in mind for planing future storage boxes.
>> I see QNAPs and Synology NAS, they use like 1Ghz CPU and 1Gb of RAM for 4 HDD, so either I'm doing it wrong, either those NASes don't have performance (or safety?) at all.
> Do they calculate checksums for end-to-end data integrity ?
> What is their performance like ?
> The data integrity and reliability features of ZFS do come at a cost.
For example, yesterday I explored QNAP TS-451
official site: 
https://www.qnap.com/i/en/product/model.php?II=143&event=2 (Intel® 
Celeron® 2.41GHz dual-core processor, 1GB DDR3L, etc)
and review: http://www.storagereview.com/qnap_ts451_nas_review

Promised performance of the models is about 100Mb/sec, even up to 
200Mb/sec but ok, it's marketing and pretty diagrams ;)
I have no personal experience with them, so no idea about checksums and 

best regards,

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