send raidz2 ZIL/SLOG to boot mirror (enterprise SSDs w/PLP)?

[VinceO]

I'm new to Proxmox and ZFS and have my first question. I have a 2u server with 2x 480Gb Intel Datacenter SSDs (s4510 w/PLP) and 6x 4TB SAS enterprise HDDs. The system has 128GB of RAM.

I would like to set up the SSDs in a mirror for boot and VM storage. The 6x HDDs will be a raidz2 for larger VMs and a PostgreSQL LXC. Should/can I set the the logs for the raidz2 pool to the boot mirror? My searches return conflicting answers, mainly in regards to people using non-enterprise drives which I am not doing.

 

[Dunuin]

With "logs" you mean the ZIL/SLOG for write caching?

That is theoretically working but not really recommended. A slog drive isn't made to last forever. Sooner or later it will fail because of all the writes and if it fails you don't want to have important stuff (boot partition, your root folder) on that drive. And a SLOG should be super low latency and if you run VMs of the same drive these are going up.

You can test it and use two 32GB or 64GB partitions as a mirrored slog, monitor the SSD wear and remove the SLOG later from the pool if the wear is too high.

 

[VinceO]

Thanks for the fast response. Yes, the ZIL/SLOG write caching from the raidz2. The SSDs have a rated 1.2 PBW and the VMs I would have on there would only be lightweight app servers. I suppose I could have those VMs on the raidz2 pool instead so the only thing on the SSDs would be boot ZIL/SLOG.

 

[Dunuin]

Keep in mind that a SLOG is only caching sync writes. So it will help with DBs but most stuff will use async writes which won't be cached at all.
Would be more usefull to just run all VMs and DBs of SSDs and use the HDDs as a cold storage. For VMs and DBs you want alot of IOPS and HDDs can't handle them well, especially if you are using any kind of raidz. A striped mirror (raid10) would be way better as a VM storage (2x 3-way mirror for better data integety or 3x 2-way mirror for better performance and capacity).