Subject: Re: Slow backup performance...
To: Kevin P. Neal <firstname.lastname@example.org>
From: Justin T. Gibbs <email@example.com>
Date: 07/29/1998 22:54:35
>AlphaStation 200 4/233, 64Mb, Ross' 1.3F snap.
>I'm trying to use Amanda to backup my system (in a few weeks my then-new
>roommate will have his boxes online, so no Amanda isn't overkill). I'm
>getting performance on the order of 35KB/sec on my dumps.
>Investigation shows that I'm only able to do backups at about 65KB/s from a
>local disk to a networked null device.
>Any ideas on what's wrong?
It's probably the same problem that Bill Fenner addressed in FreeBSD
a while ago:
date: 1998/05/17 06:39:38; author: fenner; state: Exp; lines: +3 -6
Turn on TCP_NODELAY on the remote socket, to turn off sender silly window
syndrome avoidance. The combination of SWS avoidance and ack-every-other
causes low throughput if the block size divided by the MSS is odd (which
is true with the default block size and MSS).
Turning on TCP_NODELAY disables the Nagle algorithm and sender SWS avoidance.
The rdump request/response protocol can not invoke Nagle and cannot cause
SWS, so this has no negative effects.
Dump has other performance problems that prevent you from being able to
dump more than perhaps 1MB/s anyway. It uses the raw device to access data
and thus prevents the system from doing any read ahead. The solution to
this is to convert dump to use the aio interface so that it can take
advantage of drive level queuing and gain some performance from overlapped
I/O. Some day when I have some time on my hands (ha! time!) I plan to do
this under FreeBSD as we have aio already.