ZIL and Cache on SSD

[tuaris]

I am building a second NAS4Free server and this one is going to have a little more capable hardware in it. 64-bit version, 8GB RAM, 3Ghz Dual Core Xeon. It will run on a Lenovo RS110 server I have sitting around. I'd like to mirror two 1TB SATA II drives using ZFS.

I have installed the embedded version of NAS4Free on a 1GB IDE Flash module I plugged into the server's motherboard. I also have space to plug in a 32GB 2.5inch SATA SSD. Finally, I also have a PCI Express card installed that provides an internal USB 2.0 connection.

I've read about how the "log" aka (ZIL) and the "cache" can be placed on separate devices. What would the recommendation be to achieve the best possible performance. Would it make any noticeable difference with my hardware? I currently get about 50 MB/sec read and write performance. Over SMB the performance is close to 35MB/sec on a 1Gbit connection. This server will mostly be used to store music, videos, photos, documents, backups, and maybe the occasional iSCSI target for VMware experimentation.

[ku-gew]

I don't think you need either ZIL cache or L2ARC. The first one is used when you have a lot of synchronous writes (and usually, if you have them, you know you have them: if you ask, you don't need it). L2ARC is used when *a lor* of random reads of static data are requested (-> databases, for examples). Your usage doesn't seem to be the case.
Better use the same money for more RAM.
If you really want to try, given the expected performances, you can use the same disk for both ZIL and cache. Keep in mind that the ZIL is usually 5*maximum thoughput of the hard disks combines. In your case, 1,5 GB is already overkill. Keep also in mind that the L2ARC uses some RAM as well: if you don't need it, you are just losing precious RAM.

[BeNooL]

Yes, you can very well reach 80-120MB/s over gigabit links without any ZIL or L2ARC.

[ku-gew]

I tried GbE transfer between two macs over a Netgear WNDRMAC: maximum 70 MB/s, I wouldn't expect more from normal routers, you really need a dedicated switch to go past 70 MB/s.

[raulfg3]

92MB/s PC to NAS and 70MB/s from NAS to PC using SAMBA ( without ZIL and/or SSD)

see graphs: viewtopic.php?f=63&t=31&p=42#p42

[daoyama]

If you want to use SSD for both ZIL and L2ARC, you must use over 100MB/s writing. Not reading, 200MB/s is recommend.
Most of 64GB SSD drives does not have enough performance. Look the spec of SSD you want to use.

For example,
Samsung SSD 830 series,
64GB Read 520MB/s Write 160MB/s < it's slow writing!
128GB Read 520MB/s Write 320MB/s
256GB Read 520MB/s Write 400MB/s

In this case, 128GB or 256GB model is strongly recommended.

Don't forget L2ARC is used after ARC is full. So, you can't see difference with smaller test than your physical memory.
Also, using 16GB or more physical memory is recommended.

Daisuke Aoyama