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Old 8th August, 2005, 09:58 PM
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RAID Block Size

Right ive just got two Maxtor Dimondmax 10 160GB hard discs for a RAID 0 stripe for my games, question is, what sort of block size should i use? and what are the advantages of the differant block sizes>8k>16k>32k>64k>128k??? im looking at Gizmo here with is 8 disc setup!

The array is going to be for gaming, thats all nothing more.

Thanks. RF
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Old 8th August, 2005, 10:56 PM
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Ok. When you are talking about block sizes, I assume you are refering to the amount of data the RAID controller manipulates on the disk, and not the disk cluster size that Windows uses for file management.

First, you need to understand why the block size matters:

A RAID block (or stripe, as it is sometimes referred to) defines the amount of data that the RAID controller manages as a unit, in the same way that cluster size defines the amount of data that the file system manages as a unit. As with file systems, larger clusters mean more efficient data transfers and more wasted space, while smaller clusters mean less wasted space but also less efficient data transfers. For non-caching controllers like the ones built into motherboards, the cluster size isn't really too critical. For caching controllers (like mine) the block size can have a significant impact on overall performance, because the larger the block size, the more memory can be used for cache as opposed to tracking the cache blocks.

For non-caching controllers, 16k or so blocks are probably optimal. For caching controllers, 64k is usually what you want to run. There are exceptions of course. Ideally, you want to do two things:
1) Make your block size an even multiple of your file system cluster size (e.g. 4K clusters means you want to run 8K, 12K, 16K, etc. block sizes)
2) Make your block size equal to whatever your average file I/O is going to be. This usually means around 8-16k on most desktop systems, and around 32-64k on most servers. Database servers tend to benefit most from large block sizes, while web servers seem to benefit more from smaller block sizes.
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Old 9th August, 2005, 08:37 PM
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Thats great gizmo, can you explain 'wastage' for me. RF
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Old 9th August, 2005, 09:00 PM
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Simple. A filesystem has a 'cluster size'. That's the smallest item of data it can store. If you have some data that is smaller, it'll take up the whole cluster.

Hence, if you have a filesystem with a cluster size of 4K, and you store a 1K file on it, the 1K file will take up 4K of disk space, and the 3K is effectively wasted.

On the flip side, if you make your clusters as small as possible (512bytes), then other operations take much longer, as the system has to read more clusters. If you set the cluster size to 0.5K and store a 4K file on it, the filesystem has to issue 3 reads to get that 3K file, instead of the single read it'd have to do if the cluster size was 4K
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Old 9th August, 2005, 09:08 PM
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Brillient, im actually starting to understand. Thanks Gizmo and Áedán
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