SOLVED Slog performacnce

[Caffe Mocha]

First of all, Happy New Year everyone
Right now I am using Intel Dc S3700 800G and underprovisioned it to 16G as Slog
Freenas is backing up Nfs files from Esxi,

initially, without slog and write sync=standard, the backup speed was around 30MB/s
then , to prove to my IT ppl that the lack of performances was due to design of Esxi and freenas
I turned the write sync=disable and the performances jumped to around 880Mbits/s to 900Mbits/s (Max rate)
Finally, I got my SSD and set it to 16G and sync=always, the performace dropped to around 780Mbits/s.
and the write speed to slog during backup is around 96MB/s
I suppose the performances dropped is normal and within my acceptable range but still I would love to know if that speed is normal for ssd, because ssd should be able to hit over 100MB/s easily.
Anyway please let me know, if I made a mistake along the way or any suggestions are much appreciated.

Cheers Everyone

 

[depasseg]

96MB/s = 780 Mbits/s (well, it's actually 12 Mbits/s slower, but close enough).

What's the issue?

 

[Caffe Mocha]

Actually no issue at all, just curious if it is normal to see a slight drop in performance .
I suppose it's the price to pay for proper backup : )

 

[jgreco]

I suppose the performances dropped is normal and within my acceptable range but still I would love to know if that speed is normal for ssd, because ssd should be able to hit over 100MB/s easily.
Anyway please let me know, if I made a mistake along the way or any suggestions are much appreciated.

Why would you think that the speed that an SSD can hit has anything to do with this?

Please refer to https://forums.freenas.org/index.php?threads/some-insights-into-slog-zil-with-zfs-on-freenas.13633/

In particular, be sure to read "What is a good choice for a SLOG device?"

and even more particularly,

The other thing a SLOG should have is low latency. When a sync write request comes in, everything comes to a halt. ZFS has to put the write request out to the SLOG, then wait for confirmation. For a SATA SSD on a HBA port, this involves invoking a device driver, which talks over PCIe to the HBA, which serializes the request onto a SATA cable, which is deserialized by the SSD's controller, which then has to Do Something, and then responds back over the SATA, up through the HBA, and back through the device driver, at which point ZFS is now aware that the data's been committed to stable storage, and now ZFS can process the next block.

Your SSD isn't being fed a wire-speed bunch of sequential data to write. It's being fed small blocks of data in a synchronous fashion, and your choice of CPU and how the SSD is attached to the host play a large role in how fast the system can possibly process that. In the case of NFS, what's actually happening is that a sync request initiated by the ESXi NFS client has to go over the ethernet, through the networking stack on FreeNAS, through the NFS client on FreeNAS, then into ZFS, which notes a sync write is requested, so it then pushes the block over to the SSD, through a SATA device driver, and a port, which then has to serialize the data, over to the SSD, where the controller reassembles the data, and then the controller has to actually store it in some stable manner, at which time it acknowledges that, back out through the SATA channel, through the device driver, at which point ZFS knows it's been committed, and continues on to the next NFS transaction, which then bubbles back through more layers to the ESXi NFS client, at which point the ESXi NFS client can then write the next block.

Doing this at 780Mbps is *AWESOME*.

 

[Caffe Mocha]

Thank Depasseg & Jgreco for your feedback,
I actually read your Zil/Slog post a couple of times, it's a must read especially for beginner such as me.
I suppose I like to leave no stone unturned hence the post.
as for my choice of CPU..... due to company budget constrains that's all I can do, it turns out to be well within acceptable range. and it actually is faster than the Synology we have. and the price per bay is way lower. :)
there are much to learn about freenas and feel free to lecture me.. lol

 

[jgreco]

I know all about budget constraints.

Stick around and there's almost always someone willing to help set you right.