Hi All,
On recent kernels we noticed a way lower throughput to our SAN system
than before.
While on pre 4.18 kernels we had 400-700MB/sec read speed, on 4.18+ we
only had 70-120MB/sec.
The SAN is connected via iSCSI over a 10G network (ixgbe/X520 NICS if it
matters).
After some debugging, I tried to bisect between 4.17 and 4.18 to see
what commit caused the slowdown.
It showed that the addition of the SACK compression
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5d9f4262b7ea41ca9981cc790e37cca6e37c789e)
was the cause.
And indeed, if I set net.ipv4.tcp_comp_sack_nr to 0 on 4.19 for example,
the throughput is (almost) back to normal again.
So it seems like this change causes quite some performance issues.
Any ideas?
Thanks
Jean-Louis
