linux-kernel - slow sync performance on LSI / Broadcom MegaRaid performance with battery cache

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Date:   Wed, 8 Jan 2020 07:58:13 +0100
From:   Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>
To:     Ric Wheeler <rwheeler@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Chinmay V S <cvs268@...il.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Theodore Ts'o <tytso@....edu>, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <matthew@....cx>
Subject: slow sync performance on LSI / Broadcom MegaRaid performance with
 battery cache

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