Re: Setup about Disk RAID 5

In article <79fj21$bgf$1_at_news-2.news.gte.net>,   "coa" <coaster1_at_gte.net> wrote:
> For RAID 5, there is any important issue about partitions. How to balance
> between number of mount points and partition size?

One important thing to remember is that for every group of disks that make up a RAID volume, you're going to lose the storage of one of the disks. Yet, the more disks you include in a RAID volume, the bigger the volume will get.

> We have a Sun 4500 disk RAID with 15x9GB hard disks. Our applications are
> Oracle 8 and Oracle Financials.

With 15 identical disks, one method would be to have three groups of five disks each. Then, you would have three 36 GB volumes. Another reasonable approach would be to have three groups of four disks (27 GB each) and one of three disks (18 GB).

However, there is another issue. If you cannot hot swap failed disks without shutting down the machine (which may or may not be the case on your hardware and OS level), you will probably want to have unused ("hot spare") disks in the cabinet so that you can remove a dead disk from a raid volume and add one of your hot spares without shutting down the machine. In fact, this may be a good idea anyway, since I believe Veritas can be configured to automatically switch over from a dead disk to a hot spare. This means that when that disk does fail in the middle of the night, Veritas can not only use RAID to prevent data loss, but can also reestablish redundancy so that you don't have to worry about your system being in a riskier state and you can fix it at your leisure.

Also, on some machines, replacing disks is possible without shutting the machine off, but a whole group of disks (such as all the ones on a single SCSI controller) must be not in use to do this. This will impact your layout as well.

Another thing to consider is your controller configuration. If you have 3 SCSI busses and five disks on each, then a natural way to do it would be to have one RAID volume per set of 5 disks. An alternate way to do it would be to have RAID volumes composed of one disk on each SCSI bus to spread the load and to aid in case a whole controller fails or needs to be shut down to replace a dead drive.

Then there is the matter of backups. If you have filesystems on your RAID volumes and a tape drive that can only back up 20 GB of data, you might find it convenient to limit the size of a RAID volume to less than 20 GB. Or, you might be comfortable with having the backup software span tapes so you might not care.

• Logan

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