When performing large continuous synchronous writes, the SLOG doesn't seem to be effective, and the write speed is slow

I have set up a pool using 2x2TB HDDs, with an rms200 as the SLOG.

I created a ZVOL and shared it via iSCSI with sync=always set. When I connect to it from a Windows computer and copy a 100GB file, the speed initially reaches 600MB/s for the first 12GB, but then drops significantly to around 12MB/s.

Using iostat, I observed that during the first 12GB, the SLOG shows rapid writes, and the HDD has about 120MB/s writes, which is expected. However, after 12GB, the SLOG stops receiving writes, and HDD writes drop to 30MB/s, while Windows reports 12MB/s. I’ve tried adjusting the block size, but it didn’t help. When using file-based iSCSI, there’s no such issue, and it works continuously.

What could be the problem, and how can I resolve it?

I am use TrueNAS-13.0-U6.1

The HDs can’t keep up with the ingest speed. Back pressure will be used.

This seems like expected behavior, and I’m guessing you will average 50-100MB/s over all.

If you want faster speeds you need a faster pool. Ie more spindles in mirrors. Or SSDs.

I need 16 HDs to get 10gbe.

Using file type iscsi the average speed is 80m/s.but Zvol is only 12m/s.