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Re: Concatenated disks vs RAID 0
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Re: Concatenated disks vs RAID 0

At 10:23 AM +0200 5/3/05, Guillaume Gete wrote:

There is a new option to set up concatenated disks in Disk Utility in 10.4. I am not familiar with this concept, and Apple is not giving a lot of informations out there.

So, basically, my question is : what's the main difference between concatenated disks and RAID 0, and which are the pros/cons of each ?

The concepts of logical volumes are not new, and the pros and cons of multi-drive logical volumes very well understood in the computer industry but this is a first for Mac OS X. It's really been possible before too, but this is the first time it's more mainstream.

RAID technologies are designed to solve a different class of problem, one of performance or data availability by binding multipe members into a set. In the case of RAID's 0,3,5, et al, data is striped across members so that a logical block of data is broken into stripes with part of that block stored on multiple disks. This is done so that the r/w operation can be parallelized across spindles to reduce latency.

Multi-disk logical volumes bind one or more drives or drive slices together to form a volume larger than one you'd get with a single drive. Like RAID 0,3,5, et al, you still get one large single filesystem. The difference here is that the data blocks of the filesystem aren't striped across the volume members.

Things that this immediate implies are that:

1) You get no performance gains from parallel operations, as the data blocks aren't striped across members but have affinity to a specific member

2) The largest size file you can have on the filesystem is the size of the largest member(*). That is if you have 2GB and 3GB members to form a 5GB logical volume, you can have two files of 2GB and 3GB that would fit on each member exactly (and is almost impossible to peg affinity for) but you can't have a 5GB file. (Had the volume been a striped set of some kind, then a single large file could be scattered across the entire array.)

The benefits of such multi-drive volumes is that you end up with a single contiguous filesystem. The typical alternative would have been to mount other drives into specific mountpoints in the filesystem, but the space for each mountpoint operates independent of the overall filesystem.

Hope this helps and make sense. --


(*) I haven't confirmed Apple'simplementation of this to confirm this, it's just normally the case.

Dan Shoop                                                   AIM: iWiring
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 >Concatenated disks vs RAID 0 (From: Guillaume Gete <email@hidden>)

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