On my local file server I have raid-6 on 7x HDD drives.
dd if=/dev/zero of=tempfile bs=1M count=2048 conv=fdatasync
Local speed test gives me 349 MB/s write speed.
Remote writes to Samba from SSD (>2Gb/s read speed) gives me 259 MB/s writes. But remote writes to iSCSI drive (on Win10 iSCSI initiator) gives me mere 151 Mb/s writes.
raid6 config - 128K chunk size, stripe_cache_size = 8191. Write intent bitmap is on SSD (Samsung 860 PRO, 4096K bitmap chunk).
Array mounted with options:
open-iscsi setup: target is based on a 4Tb file.
Any hints why iSCSI is slower than Samba on writes? Any hints on how to improve iSCSI writes speed?
I assume it has something to do with the desire of open-iscsi to flush writes to disk after each operation, which increases write amplification on raid6 due to excessive parity rewrites. But I am not sure how to fix it. Speed it more important than safety of currently written data in case of power outage.
As a side note older ietd iSCSI target had the ability to enable write-back mode (using
IOMode=wb) and the sustained write speed was much faster. Unfortunately it seems to be currently unmaintained.