George
Your statement
Clearly, the more channels you set, the more save sets you end up with
and thus the more save sets will be
interleaved to the tape and the longer recovers will take, so there's a
trade off between increased write speeds
versus recovers. BUT, the more save sets you break the Oracle backups
into the smaller those save sets, and that
could be a good thing, but the more filesperset, the larger those
savesets, too. Hmm ... OK, all fine and well, but:
is incorrect.
The less files allocated per channel will result in more savesets. By
default Oracle will try to divide the number of files into the number of
channels allocated as from the oracle whitepaper tuning recover manager (
RMAN )
Backup Multiplexing
One channel can simultaneously read more than one file. The level of
multiplexing
is the number of files read simultaneously on a single channel and then
written to the
same backup piece. The degree of multiplexing depends on the FILESPERSET
parameter of the BACKUP command as well as the MAXOPENFILES parameter of
the CONFIGURE CHANNEL or ALLOCATE CHANNEL commands.
The number of files in each backup set is the lesser of FILESPERSET and
the
number of files read by each channel. The level of multiplexing is the
lesser of
MAXOPENFILES and the number of files in each backup set. For example,
assume that you back up two datafiles with one channel. You set
FILESPERSET
to 3 and set MAXOPENFILES to 8. In this case, the number of files in each
backup set is 2 (the lesser of FILESPERSET and the files read by each
channel),
and so the level of multiplexing is 2 (the lesser of MAXOPENFILES and the
number of files in each backup set).
Assume a different case in which each channel reads fifteen datafiles,
FILESPERSET=10, and MAXOPENFILES=8. You can calculate the level of
multiplexing as follows:
min(min(15, 10), 8) = 8
The default values are 16 for MAXOPENFILES and 64 for FILESPERSET.
George Sinclair <George.Sinclair AT NOAA DOT GOV>
Sent by: EMC NetWorker discussion <NETWORKER AT LISTSERV.TEMPLE DOT EDU>
11/30/2006 01:09 PM
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EMC NetWorker discussion <NETWORKER AT LISTSERV.TEMPLE DOT EDU>; Please respond
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George Sinclair <George.Sinclair AT NOAA DOT GOV>
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Subject
[Networker] Oracle RMAN backup questions
Hi,
Several questions here on RMAN Oracle backups via NMO. We're running
Oracle 10g, with NW 7.2.2.
Our drive target sessions are set to 5. These are SDLT 600 drives.
Some of these questions may be moot since there's so many variables that
will determine what settings work
best for your shop but really just wanted to get an idea of what others
are doing and what, if any, bad experiences,
pit falls or suggestions you could offer.
In testing Oracle backups and recovers, we've been playing with a rather
small test database. We've tried
multiple channels, and multiple filesperset, and we don't really notice
any major performance difference, but
obviously, the more channels, the more concurrent streams we can send to
the device, so it's more efficient and
doesn't have to continually stop and then back up another like it would
if we had channels set to 1 where it would go one at a time.
Also, increasing filesperset helps in this area , too.
Clearly, the more channels you set, the more save sets you end up with
and thus the more save sets will be
interleaved to the tape and the longer recovers will take, so there's a
trade off between increased write speeds
versus recovers. BUT, the more save sets you break the Oracle backups
into the smaller those save sets, and that
could be a good thing, but the more filesperset, the larger those
savesets, too. Hmm ... OK, all fine and well, but:
1. Does more filesperset also affect recover time in the same way that
more save sets (channels) to the same tape does?
2. During normal (non-Oracle) backups, how many files does NetWorker
send in a single saveset?
3. Let's say you had a database of 100 files, maybe 70 GB total. If you
set filesperset 1, and had 1
channel then you'd end up with 100 save sets on a full which sounds
kinda dumb. On the other hand
setting filesperset 100 with 1 channel sounds ridiculous as well. So,
how about 2 channels with filesperset 5.
That would take 20 save sets on a full, maybe only 5-6 save sets on an
incremental. Does that sound reasonable
or would it be better to crank it up to 5 channels? Our devices are set
to handle 5, so it's not gonna need another drive.
We have the Oracle client's parallelism set to default 4 (NetWorker), so
I imagine we can't increase the
channels beyond 4 unless we change that value. This machine has 8 cpus,
plenty of memory, so maybe
we can increase that. Since our test database is so small, though, we
really don't have a good feel for what will and won't work so well,
but I was thinking that if we're seeing decent write speeds, and decent
recover times at say 2 channels with
filesperset 5 then maybe just leave it at that?
4. Should the channels be set to 1 when recovering?
5. Should we use regular differential backups (like level 1 every night
until the next full) or a cumulative backup?
Thanks for any horror stories or suggestions.
george
--
George Sinclair - NOAA/NESDIS/National Oceanographic Data Center
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