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Re: Row Device vs File System

 
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mehul_3
Regular Advisor

Row Device vs File System

Hi
What is the differance between Raw Device and File System.
We use HP-UX11i operating system.
Which is the better option from above one.

Regards,
Mehul
14 REPLIES 14
Pete Randall
Outstanding Contributor

Re: Row Device vs File System

The difference is whether I/O is buffered by the HP-UX kernel or not. With a file system, it is - with a raw device HP-UX does not do any buffering. This is typically used for a database application and the DB does any required buffering.


Pete

Pete
Pat Lieberg
Valued Contributor

Re: Row Device vs File System

I'm assuming this is in relation to a Database application. Typically, filesystems have buffering overhead from the OS involved, where the raw devices do not.
Piergiacomo Perini
Trusted Contributor

Re: Row Device vs File System

hi,
as other guys said
the difference is overhead
introduced by file system/OS.
On my past experience (with other
Unix platform) Raw Device have
better performance.

regards
Jean-Luc Oudart
Honored Contributor

Re: Row Device vs File System

Async IO are currently only available on raw device.
Lots of people claim raw device gives more performance than file system, but gain differ from application type and transaction load.

see also attachment

Regards
Jean-Luc
fiat lux
Steven E. Protter
Exalted Contributor

Re: Row Device vs File System

A. Clay Stephenson actually did some scientific tests with oracle of raw versus filesystem. The differences in performance has dwindled. HP-UX 11i v1 has very little advantage for raw filesystems on Oracle.

Of course you've not actually told us what you are going to put in these raw filesystems. The downside of raw is, your only access is via database/application utilities. Filesystems are accesible and can be backed up if the database or application is shut down.

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Steven E Protter
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Jean-Luc Oudart
Honored Contributor

Re: Row Device vs File System

From the performance point of view, you cannot say for 100% that raw is and always will be the fastest ( as I say in previous post, it depends ...) and you may want to benchmark your application to verify waht is best for you.

see also this comment form Tom Kyte :
http://asktom.oracle.com/pls/ask/f?p=4950:8:11141198565443368411::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:7931107631402

Regards
Jean-Luc
fiat lux
Piergiacomo Perini
Trusted Contributor

Re: Row Device vs File System

Jean-Luc and other,

i seriously forget to specify that
obviously it depend from use of db
:-)

regards
Bill Hassell
Honored Contributor
Solution

Re: Row Device vs File System

Something that is often overlooked in choosing raw versus file access for databases: backup and restore. Filesystems can be backed up with many tools in HP-UX, but raw has no files, no directories, nothing for a backup program to use. The only tool is dd but this is terribly unreliable. It simply reads blocks of data and writes them onto tape (or another disk). There is no timestamp as to when the backup was made, no information about source location (rlvol name), system name, nothing but raw data. What this means is that the backup is useless if incorrect information is written on the label.

So you have a couple of choices: have your databasae developers write a procedure to shutdown the database and export all the data to a set of extra disks, then backup the exported data files. Or, purchase a commercial backup program that understands and handles raw backup images. Be very careful about a cheap solution. Assuming the decision is based on important production servers, the backup program must be able to recover from tape errors, handle multiple tapes for a single backup, take care of documenting each tape, logs all activities and sends alerts when something goes wrong, etc.


Bill Hassell, sysadmin
A. Clay Stephenson
Acclaimed Contributor

Re: Row Device vs File System

There is one other thing to consider --- and this is the only time I seriously consider the use of raw i/o. It is possible for multiple hosts to access a shared raw device with r/w access allowed by all the hosts. That is simply not possible with conventional filesystems because one host will have no idea about another host's buffer cache contents. There are clustered filesystems that even manage that trick but I'm yet to be convinced of their reliability given the extreme complexity of this task. Even with raw devices there must still be a synchronization mechanism to prevent simultaneous access to the same data blocks and indices.

If it ain't broke, I can fix that.