Setting up for XCP over NFS with ZIL / SLOG

[mauzilla]

I have a Dell with HBA and currently running 12 x 6tb with OS (Core) on a SSD. I want to purchase a SLOG and ZIL for it as we want to use it as failover VM storage.

My plan is to get a PCIe NVME card and use that as the SLOG and ZIL on normal SSD storage. Given that we have limited slots on the R720, I assume I would need 2 of these for replication purposes. I believe after that I will be able to disable sync on NFS which should improve the NFS based storage in XCP?

Also, is it better to use ISCSI over NFS as XCP offers both. I'm new to ISCSI, have been using NFS for a long time. Any other recommendations considering that it would host VM's?

 

[nabsltd]

My plan is to get a PCIe NVME card and use that as the SLOG and ZIL on normal SSD storage. Given that we have limited slots on the R720, I assume I would need 2 of these for replication purposes. I believe after that I will be able to disable sync on NFS which should improve the NFS based storage in XCP?

You don't need SLOG devices to be redundant unless you want to keep the full speed until you have a chance to replace the device. If the SLOG device fails, ZFS just moves the ZIL back to the pool disks.

Also, SLOG only helps with sync writes. If you disable sync, then ZFS use RAM to buffer the write for a while, then eventually writes it to disk. The application sees the write return as soon as it is in RAM. This is the best speed you can achieve.

What SLOG does is move the ZIL from the pool disks to something faster (which means if your SLOG isn't significantly faster than your pool, it's not useful). The ZIL is used by ZFS to write the data from the RAM transaction queue. This puts the data somewhere safer than RAM as soon as possible, so a sync write can return to the application. ZFS can then use the ZIL to write out the data to the appropriate place in the pool. This protects the data from a system crash, as any writes still left in the ZIL after such a crash are dealt with by ZFS as if the crash never happened.