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Sybase devices - raw versus file system devices

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Sybase devices - raw versus file system devices

I am setting up a new L2000 with a FC60 setup with two - five disk RAID 5 arrays. We are installing Sybase ASE 12.5 64bit on HPUX 11i. Our current K460 has several 2G Jamaica drives with raw devices using async i/o where half the i/o is on the one device that has the default and system segments for the tempdb. I can put the tempdb segments on file system devices with dsync off. Should I put the production databases on raw or file system devices? If I put them on file system devices, how risky is it to use dsync off and rely on the battery backup of the FC60 and a UPS? Sybase warns that performance is degraded by using dsync on, but recovery is jeopardized by setting dsync off. Does async i/o to a raw device use the disk controllers cache?
harry d brown jr
Honored Contributor

Re: Sybase devices - raw versus file system devices

I would bet you could rely on your fc60's ups to handle any pending io.

As with any type of DB using raw devices, you have to come up with a backup strategy to back those raw volumes up. you also have to remember that they are raw, and won't show up in a bdf.

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Sridhar Bhaskarla
Honored Contributor

Re: Sybase devices - raw versus file system devices


My understanding is that FC60 doesn't care if you use raw disk or a file system. It matters only for the system. So, eventually FC60 is going to use it's cache for any operations.

It's better to have filesystems than Raw devices. You can load OnlineJFS and enable mincache/convosync options on the file systems that will do the same job as raw devices. Moreover, your data will be safe during the crash scenes.

You may be disappointed if you fail, but you are doomed if you don't try

Re: Sybase devices - raw versus file system devices

You're saying that file system devices with the dsync off will provide better throughput than raw devices with asynchronous I/O? And, that OnlineJFS will provide the recoverability from a crash?
Henrique Silva_3
Regular Advisor

Re: Sybase devices - raw versus file system devices

Hi Larry.

Maybe you have had your question anserwed, but let me add my two cents.

Beginning with ASE 12, sybase now supports database files on file systems, and garantees that you will not loose data in the event of a crash. Since we are still running 11.9.2 on hpux 10.20 for our databases, and have no need to upgrade anything yet, I am speaking from knowledge aquired by attending sybase classes and reading about this.

So, as far as performance is concerned, NOTHING beats having the database devices on raw file systems, bypassing the i/o buffering here. However, as pointed out below, you have to have some kind of backup process in order to be able to recover from a disk crash. We dump the databases to files daily and dump the transaction logs hourly, and backup is done to tape after the full daily backup.

From what I have read, dsync is on by default, and people were having performance problems on ASE 12, because master and tempdb were not raw devices, and the advantage of putting database devices on file systems is not performance, but for piece of mind that you are backing up a file and so a junior sys admin does not blow away your database by doing something on sam on these files ( raw ) that are not showing up on bdf !!!!

In any case, the thing to do there is to see what are your goals, performance as opposed to having your device files showing up on bdf and being able to back those files up without having to create a different process, and go from there, BUT, make sure you understand that if master and possibly tempdb is setup with dsync on, you might have performance problems eventhough your production database might be on raw devices.

As far as the FC60, it does not care what kind of file it is using, it will use its hardware to speed things up anytime you need to, and since you have raw file systems, it will bypass any kind of buffering too.

I hope this helps,

Henrique Silva
IT Engineer
HP Burlington
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