-----Original Message-----
From: Tim Mooney [mailto:mooney AT DOGBERT.CC.NDSU.NODAK DOT EDU]
Sent: Thursday, April 17, 2003 11:39 AM
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Subject: Re: [Networker] Merits of cloning versus dual backups?
>That's correct, assuming you're backing up directly to tape. If, however,
>you use file devices for backup, you could clone from your file device(s)
to
>tape utilizing all your drives for writing clones.
---
Tim's right, of course. And there are many other reasons to backup to disk
first:
o NetWorker supports multiplexing of data written to its 'file type' and
(new in NetWorker V7.0) 'advanced file type' devices, just as multiplexing
is supported to tape devices. But the HUGE difference with the
backup-to-disk devices is that each backup stream is sent to a different
file on disk. So, when a restore (or cloning) operation is done, that data
does not need to be de-multiplexed, because it never was written in a
multiplexed format. You can't do this to a tape drive.
o when using the 'file type' device (in any version of NetWorker, including
V7.0), you can not write to and read from the 'file type' device at the same
time. So, all of your backups need to be done to this device, before you
can do a restore or a clone operation. BUT, the 'advanced file type'
supports simultaneous write and read operations. So, you could be writing
the rest of your backups to a 'advanced file type' device, while you start
cloning the backup savesets that are complete. Again, you can't to that
with a tape device. Read the fine print, though; the 'advanced file type'
device does limit the number/type of concurrent operations; see the
Administrator Guide for details.
o many disk devices currently deployed are faster than many tape devices
(note that I'm not saying that all disk drives are faster than all tape
drives; a SDLT drive will be much faster than an older IDE drive, to pick an
extreme example). Plus, RAID adapters and storage arrays support RAID,
which can both increase performance and reliability/safety.
o this was already mentioned in this mail stream, but I think it's important
to note again that doing a backup, then a clone, affects the
application/database server only once. It's common for backups to put a
heavy and long (hours) load on a server; few server administrators want to
bear that pain twice.
We've spent a lot of time talking about speed and multiplexing. The more a
NetWorker volume is multiplexed, the slower it is to restore or clone from.
Multiplexing is only valuable when it helps you to keep an output device
streaming. If you can stream your output tape (or disk) with a single
backup stream, DON'T DO MULTIPLEXING as there is no benefit during the
backup, and it hurts you during the restore/clone. In fact, in environments
where multiplexing is done, it is better to send the original backup tape
offsite and keep the cloned volume onsite. Why? Because the cloned volume
has contiguous savesets, which will restore faster than the non-contiguous,
multiplexed savesets. So, set your environment to multiplex AS LITTLE AS
NECESSARY to get your output devices streaming.
Also, remember that, when NetWorker clones, it knows the order in which
savesets (and their pieces) were written to a tape. So, it takes care to do
the minimum amount of tape positioning necessary when reading a multiplexed
source volume.
This has been a GREAT discussion. It's really nice to see so many people
with good ideas contributing!
Thanks
tl
Terry Lemons
CLARiiON Applications Integration Engineering
EMC²
where information lives
4400 Computer Drive, MS D239
Westboro MA 01580
Phone: 508 898 7312
Email: Lemons_Terry AT emc DOT com
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