slow sync performance on LSI / Broadcom MegaRaid performance with battery cache

[Stefan Priebe - Profihost AG]

Hello list,

while we used adaptec controller with battery cache for years we
recently switched to dell hw using the perc controllers which are
rebranded lsi/broadcom controllers.

We're running btrfs subvolume / snapshot workloads and while those are
very fast on btrfs using a btrfs raid 0 on top of several raid 5 running
on adaptec (battery backed up) in write back mode.

The performance really sucks on those LSI controllers even the one i
have has 8GB cache instead of just 1GB at adaptec.

Especially sync / fsync are awfully slow taking sometimes 30-45 minutes
while btrfs is doing snapshots. The workload on all machines is the same
and the disks are ok.

Is there a way to disable FLUSH / sync at all for those devices? Just to
test?

I'm already using nobarrier mount option on btrfs but this does not help
either.

Thanks!

Greets,
Stefan