Zil performance

[johwes]

Hi all,

My System:
Freenas 9.1
Dual six core cpu
196MB RAM
24* 2TB 7200SAS 12*mirrored vdevs
4* 120GB eMLC sm843t samsung SSD (2*mirrored vevs) ZIL
2* 240GB L2ARC sm843t emlc ssd
2* 10Gbit NIC

I have setup my zil with (4*eMLC ssd) 2*mirrored vdevs underprovision to 8GB like this:

gpart create -s GPT da0
gpart create -s GPT da1
gpart create -s GPT da2
gpart create -s GPT da3
gpart add -t freebsd-zfs -a 4k -s 8192M da0
gpart add -t freebsd-zfs -a 4k -s 8192M da1
gpart add -t freebsd-zfs -a 4k -s 8192M da2
gpart add -t freebsd-zfs -a 4k -s 8192M da3
zpool add tank log mirror da0s1 da1s1
zpool add tank log mirror da2s1 da3s1

logs - - - - - -
mirror 11.7M 7.93G 0 11 0 167K
da0p1 - - 0 11 0 167K
da1p1 - - 0 11 0 167K
mirror 11.5M 7.93G 0 12 0 104K
da2p1 - - 0 12 0 104K
da3p1 - - 0 12 0 104K

My first quotations is:

Will i get more performance with 2*mirror like my setup?
Is there any tweaks to get mote zil performance?
I only get the zil to handle about 100MB/s when i use NFS, If i disaable zil on the dataset i get about 300MB/s

/J

 

[cyberjock]

Your setup of the log drives does seem appropriate for what I think you are trying to do... maximize performance while adequately protecting your data.

Can you elaborate on your comment: I only get the zil to handle about 100MB/s when i use NFS, If i disaable zil on the dataset i get about 300MB/s

That sounds like you are comparing the ZIL/s 100MB/sec to the pools 300MB/sec. This is an apples to oranges comparison.

The ZIL doesn't handle all writable data. Writes that are over 64kbytes(I think that's the right size) are written to the pool itself. So if you were to write a 64kbyte file to the pool the file writes themselves would go to the pool but the file system updates would go to the ZIL. This is to prevent the ZIL from needing to be excessively large. Remember, the ZIL is to decrease IOPS to the pool at inopportune times and not to increase bandwidth(directly). Indirectly it does increase the pools IOPS though and bandwidth by better optimizing writes to the pool.

The pool performance should almost always be more capable than your ZIL for total throughput, so 300MB/sec isn't a good or bad thing. But, for a 24 drive mirror you should be able to do a dd write test and get FAR more than 300MB/sec.

Have you tried enabling autotune for your server? That might be something you should enable for a server of your size.