SirHaxalot
Dabbler
- Joined
- Aug 27, 2012
- Messages
- 10
I'm trying to understand how ZFS synchronous writes work. As I first understood it, adding a SSD ZIL would enable the OS to have an fast write cache, but instead my write performance goes from bad to worse when enabling the ZIL, and it only seems to affect NFS writes. My thought process was the the ZIL would enable the OS to return to synchronous writes as soon as it has written the data to the ZIL device, but my guess is that the ZIL at the moment only adds another step to the process and it still has to wait for the data to be written to the physical disks before returning.
I have 3x 2TB 7200rpm drives in a RAID-Z configuration for the main data and 2x60GB SSDs split up with a 6GB mirrored log partition and the rest assigned as L2ARC cache (no RAID). When I write to a NFS datastore synchronously, I get a write performance of ~10MB/s with sync=standard and not more than ~30MB/s with sync=disabled. Asynchronous writes are better but still bad, 30MB/s with sync=standard and 50MB/s with sync=disabled.
Random 4k writes perform a little better but there is still only an improvement of a few hundred IOPS when enabling ZIL, which is much worse than what I'm expecting.
At the same time, there's no issue at all when I'm using SMB shared. Then I can easily reach 90MB/s writes, which is what I'm expecting with the current setup (nearly hitting the 1Gbit limit).
I have 3x 2TB 7200rpm drives in a RAID-Z configuration for the main data and 2x60GB SSDs split up with a 6GB mirrored log partition and the rest assigned as L2ARC cache (no RAID). When I write to a NFS datastore synchronously, I get a write performance of ~10MB/s with sync=standard and not more than ~30MB/s with sync=disabled. Asynchronous writes are better but still bad, 30MB/s with sync=standard and 50MB/s with sync=disabled.
Random 4k writes perform a little better but there is still only an improvement of a few hundred IOPS when enabling ZIL, which is much worse than what I'm expecting.
At the same time, there's no issue at all when I'm using SMB shared. Then I can easily reach 90MB/s writes, which is what I'm expecting with the current setup (nearly hitting the 1Gbit limit).