DBMS Cache

theholysyrup

I'm installing Navision 3.7 Server on a new computer.

2 Xeon dual-core processors
4 GB PC4200 RAM
4 SCSI 320 HD's in a RAID-10 config

Setup recommends that I set the DBMS cache to 2 GB. I've been told that Navision doesn't run well with >1GB cache. But the reasons behind this were never explained.

Can/Should I set it to 2 GB?

themave

Why raid 10 with only four drives, you will have the operating system and database on the same drive. and no way of splitting the database.

You would be better (in my opinion) going with two raid 1 mirriors, that way you still have redundency and you would double your storage, you could then have the operating system and part of the database on one mirror and the rest of the database on the second mirror. splitting the database across the mirror should get a little better performance, not that you will have to worry much about that with your server, sounds pretty nice. 8)

theholysyrup

RAID-10 with our drives. Data is striped across the first two drives. Then the striped set is mirrored to the other two drives. So the OS and the DB will be distributed across two drives.

When performance decreases, over the next couple years, I figure I can put in a second SCSI controller and a second RAID-10 set of 4 drives.

From what I read about SCSI configurations, there are, of course, advantages and disadvatages of each setup. RAID-10 is purportedly good for a server running distributed databases that sees frequent read/writes of small amounts of data and considerable client-side processing.

I never put much stock in the "don't go bigger than 1 GB of DBMS cache" advice, because there was no elaboration. Why would Navision recommend that I set a cache size that was too large for effective use?

themave

your four drives in a raid 10 configuration will be the equivilant of one drive.

the first drive is mirrored to the second, which gives you two copies of your data, one copy on each drive. the actual data is not split between then with part of the data on one and part on the other. it is two copies of the same data.

then to add the second mirror in raid 10 configuration your are making a second copy of the first mirror.

so, for example if you have a file name file1, it's primary location is on drive 1, then when it is written to disk, a second copy is made on drive 2, and since you have a raid 10 second set of mirror drives, what is written to drive 1 and 2 is also written to drive 3 and 4.

so you will end up with four copies of file1, one on each physical hard drive. Nothing will be distrubuted across two drives. You can confirm this by the fact that your raid 10 configuration with only have a capacity of one of the 320 hd's

Here is the wikipedia description

RAID 10

Diagram of a RAID 10 setup.A RAID 10, sometimes called RAID 1+0, or RAID 1&0, is similar to a RAID 0+1 with exception that the RAID levels used are reversed—RAID 10 is a stripe of mirrors. Below is an example where three collections of 120 GB level 1 arrays are striped together to add up to 360 GB of total storage space:

RAID 0
       /-----------------------------------\
       |                 |                 |
     RAID 1            RAID 1            RAID 1
   /--------\        /--------\        /--------\
   |        |        |        |        |        |
120 GB   120 GB   120 GB   120 GB   120 GB   120 GB
  A1       A1       A2       A2       A3       A3
  A4       A4       A5       A5       A6       A6
  B1       B1       B2       B2       B3       B3
  B4       B4       B5       B5       B6       B6

[Edited by Administrator to make it more readable]

Note: A1, B1, et cetera each represent one data block; each column represents one disk.
All but one drive from each RAID 1 set could fail without damaging the data. However, if the failed drive is not replaced, the single working hard drive in the set then becomes a single point of failure for the entire array. If that single hard drive then fails, all data stored in the entire array is lost.

Extra 120 GB hard drives could be added to any one of the level 1 arrays to provide extra redundancy. Unlike RAID 0+1, all the "sub-arrays" do not have to be upgraded simultaneously.

RAID 10 is often the primary choice for high-load databases, because the lack of parity to calculate gives it faster write speeds.

RAID 10 Capacity: (Size of Smallest Drive) * (Even Number of Drives ) / 2

theholysyrup

So, I want a RAID 0+1?

themave

theholysyrup wrote:

So, I want a RAID 0+1?

, that's what I would do.

If you wanted to optimize the set up a little more, you could add 4 small drives, either 18 or 36gig drives, you could use those to mirror the os, and use the 4 320hd to mirror the database. This would separate the os, from the database completly, and not cost much, since the small drives are very in expensive. and the os, doesn't take up much space.

kriki

theholysyrup wrote:

So, I want a RAID 0+1?

I think rather this:
-1 RAID1-pair for the system
-1 RAID10 with your 4 current disks for the DB.

Navision server does not use more than 1 GB of DB-cache (but I read that it is better not go over 850MB DBCache.
Navision server uses only 1 processor and it generally remains under 10% usage even under stress-conditions.
So the second processor and the extra memory can be handy for something else like a NAS.