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Re: The old raw devices chestnut.

From: Andrew Hamm <>
Date: Wed, 14 Apr 2004 12:41:21 +1000
Message-ID: <c5i8fl$22chf$>

Data Goob wrote:
> Andrew,
> Fair enough considerations, and you hit the point on
> the head, the backup and restore.
> We have always set up backup of logs and data to a real
> backup, but considering all the options I might 'need'
> in the event of system failure, or migration, or cloning,
> raw devices are an added risk. We use a third-party backup
> tool to backup our SQL-Server databases, as well as EMC
> Timefinder to flash-copy databases into production environments.
> We will apply the same methods with DB2 or MySQL for that
> matter. The raw-disk paradigm adds only extra trouble/work.

OK - so you are talking about SQL-Server and various issues related to a backup tool you use? If so, we have no argument; I know nothing of your tools, and am speaking from an Informix + UNIX point of view. Our beloved symlinks on UNIX are not available to NT servers, so suggestions on this cannot help. I'm also of the opinion (only from reading) that unbuffered NTFS files are equivalent in performance and reliability to raw spaces on NT:

Even with Informix on NT (of which i have almost no experience) symlinks are not available, and further, the recommendations from Informix states that you can use normal files (O/S buffered and capable of going onto FAT), or normal files on NTFS (which will be used unbuffered) or a raw partition on NT. Further, the documentation says that on NT, the use of unbuffered NTFS files is of equal performance to NT raw spaces, therefore it's not worth using raw spaces on NT. But that's NT, something I don't play with.

> The real question is, how will you backup, much less restore
> raw device databases? Are you prepared to deal with the
> inflexibility it presents? Have you ever run Informix or
> other vendor database through a complete backup and restore,
> testing all the options with raw-device dbspaces?

Absolutely. Sometimes under great duress. The use of raw spaces has always made the restores faster too. Jonathan Leffler (c.d.i stalwart and Informix Insider) has disputed the claim someone made that raw can be 5-10 TIMES faster, and I have to agree on that. Except for two points [once again, Informix+UNIX specific]:

  1. during initialisation of an instance, if you initialise on cooked files, the creation of the dbspaces and especially the physical and logical logs takes an insufferable length of time. With raw spaces, creation of an engine is quite a few times faster. That means a great deal in the middle of a day's work.
  2. During a restore, raw spaces are a few times faster.

and 3) during a mass load of data, raw spaces are a few times faster. So is the proper use of PDQ and artificially inflated allocations of shared memory for a few unstated reasons (informix, once again...)

4) There is no point 4.

All of these points are significant when major sequential writing is taking place.

BTW I must also qualify that this experience applies to machines without fancy storage managers. If you are using a machine with SCSI disks directly connected to the SCSI bus or a straight-forward SCSI raid controller, then you'll notice performance benefits from using raw with Informix (any other engines?) If it's got a big fat storage manager, then it's implementation will hide the benefits, in which case, you need to ask "I'm using Engine E on platform P with storage manager SM, so what's the best storage model to use?"

> Most DBAs
> I've run into have never had to restore a database at any
> time in their career much less even test it. Is that amazing
> or what!

It's more tragic than anything else. By a strange coincidence I'm in a discussion on this subject in another forum, and we're swapping horror stories, such as a customer who backed up to a cleaner tape for 2 weeks, or a customer who needed a restore and discovered that their 3 year old tapes cannot be read on their 5 year old tape drive which hasn't seen a cleaner tape ever....

I've had to do restores on customer sites who could not or would not afford disk mirrors, so we really did rely on the tapes for the redundancy. I've seen power supplies blow up. All sorts of things can and do happen.

> Consider too, clustering of systems and disks, and the
> administrative challenges associated with that. Add raw
> devices to multitudes of servers and it becomes more risky
> and more to manage. It puts more opportunities for failure
> in the administration path, and nobody in their right mind
> wants to increase risk. Is the extra 10% in speed worth it?

Well, as I said, I don't understand the alleged extra administration. Clearly it's an NT thing? Or an issue for people who don't use the magic of symlinks? Pass. I'll stop asking now.

> One other thing you might find attractive to plain ole files
> instead of raw devices is the ability to clone databases. You
> can get quite creative with plain files in ways that you cannot
> with raw devices because of the lock-in of raw-devices. You
> create more work for yourself in the long run with raw disks,
> unless of course you're not as lazy as me and actually enjoy
> the extra work. '-)

This is where the YMMV slogan comes into play. I would never setup an Informix,UNIX,SCSI machine with anything else but symlinks and raw spaces. If I ever setup a machine with a high performance storage box, I'll look into the most appropriate mechanism for that. I'd probably setup any brand of engine on UNIX with symlinks, unless experience or advice shows that it's pointless.

As for administration of raw spaces, on UNIX it's trivial. Meaningless. Not a problem. Do what your engine, backup tool and storage manager works best with. Unless someone else chips in with some detailed advice about other brands, the original poster will only be lurnin' about Informix engines today. If the OP wants more advice about Informix, please we should stop cross-posting and get into more detail only on comp.databases.informix, and stop boring the other newsgroups.

Goob, I think you hang around c.d.i quite a bit, so if you wish, please further my education about the pain of raw spaces on c.d.i. Perhaps some specific stories are needed so I can undertand your experience. Received on Tue Apr 13 2004 - 21:41:21 CDT

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