Re: Discrepancy between amrestore and dd speeds
2007-12-18 17:45:54
Jeremy,
If I can make the process more efficient, then that's what I want to
do. In the event of a catastrophe, time is money.
I have tracked down what I believe to be a bug; that is, if no blocksize
is defined for amrestore, it defaults to 32K, even though the man page
says "Amrestore should normally be able to determine the blocksize for
tapes on its own and not need this parameter" - I wrote to the tape in
256K blocks. I straced it both with and without the explicit blocksize
parameter, and I can see the read/write blocksizes differ in between the
runs. This really ought to be fixed, both in amrestore's behavior, and
in the man pages. (Version 2.5.1p1-2.1, the latest in Debian Etch. The
source files do not differ from the original releases.)
However, setting that option seems (disappointingly) to not make any
difference. I ran strace with the -t option, and saw that in both
cases, the data transfer rate was about the same - between 5 and 6 MB/s,
even though the block sizes were different. This kind of makes me think
that's not my bottleneck, but any informed opinions on the subject are
welcome.
I'm not sure if this is of any consequence or not, but 'mt -f /dev/nst0
status' tells me my 'Tape block size' is 1024 bytes; the max is just shy
of 256K. I'll try setting that to both 0 and the max, and see if that
generates any different results. Again, informed opinions welcome.
TIA for any insight,
Ryan
--
Ryan Steele
Systems Administrator
The Archer Group
Jeremy Mordkoff wrote:
I know this is no answer, so apologies in advance.
Why are you worried about restore speeds? Hopefully you will never need
it and if you do, will anyone really complain that it took a little
longer than the tape drive is capable of? My users are usually just
grateful that they can get anything back. Plus, if I'm too quick,
they'll ask more and more often :)
JLM
-----Original Message-----
From: owner-amanda-users AT amanda DOT org [mailto:owner-amanda-
users AT amanda DOT org] On Behalf Of Ryan Steele
Sent: Tuesday, December 18, 2007 1:16 PM
To: amanda-users AT amanda DOT org
Subject: Discrepancy between amrestore and dd speeds
Hello list,
I've been performance testing Amanda, trying to get the setup ready
for
prime-time, but I'm having an issue getting my read speeds from tape
to
get even to 6MB/s. My writes are about 12.7MB/s, which is what the
tape
drive boasts (12-24MB/s normal and 2:1 compression, respectively).
But,
my reads seem to hover around 5.5MB/s. I ran some dd tests, which
show
the data being pulled off at 11.7MB/s, and I'm not quite sure how to
tell what amrestore is doing that is causing the performance drop off.
Here are the tests I ran; any help is appreciated.
backup@amandaserver# dd if=/dev/nst0 of=/home/restores/foobar bs=256K
count=10000G
10308+0 records in
10307+0 records out
2701918208 bytes (2.7 GB)
copied,
230.539 seconds, 11.7 MB/s
backup@amandaserver# amrestore -r /dev/nst0 hostname /foo/bar
backup@amandaserver# stat
/home/restores/hostname._foo_bar.20071216.0.1.RAW
File:
`home/restores/hostname._foo_bar.20071216.0.1.RAW'
Size: 2836922368 Blocks:
5546275 IO Block: 131072 regular file
Device: 908h/2312d Inode:
13 Links: 1
Access: (0640/-rw-r-----) Uid:
( 34/ backup) Gid: ( 34/ backup)
Access: 2007-12-18
12:50:15.000000000 -0500
Modify: 2007-12-18
12:58:40.000000000 -0500
Change: 2007-12-18
12:58:40.000000000 -0500
So, ~8 minutes for a 2.7GB file = ~5.7MB/s. I saw similar
performance
when retrieving a DLE that spanned 6 chunks (~5.3MB/s) No errors or
anything in my amrestore logs, just start and stop times. I'm kind of
at a loss as to what would be causing this, unless my calculations (
size / time ) aren't valid? Thanks.
Best Regards,
Ryan
--
Ryan Steele
Systems Administrator
The Archer Group
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