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Raid 0 Chunk size

Posted on 2006-06-19
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Last Modified: 2013-12-06
I am using Red Hat Linux Enterprise 2.4

I have two 500Gb disks in a Raid 0 array. The files stored on this array will be large (at least 100Mb each).

What chunk size should I use?

Also, when I format my raid volume (as ext3) can I set block size, and is there a block size that will work better for me?

These files will be read-only.

Many thanks
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Question by:hgj1357
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8 Comments
 
LVL 16

Expert Comment

by:xDamox
ID: 16935342
Hi,

Have a look at:

http://howtos.linux.com/howtos/Software-RAID-HOWTO-9.shtml

The site gives you a table of what chunk size to use for RAID Level 0
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Author Comment

by:hgj1357
ID: 16935387
after creating the raid volume is formating like this:

mkfs -t ext3 /dev/md0

how would I specify a block size?

Thanks
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LVL 16

Expert Comment

by:xDamox
ID: 16936067
Hi,

To format the HDD you would issue:

mkfs.ext3 /dev/md0
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LVL 2

Author Comment

by:hgj1357
ID: 16936217
Actually, mkfs -t ext3 /dev/md0

but my question was how do I set the block size.
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Author Comment

by:hgj1357
ID: 16936482
Actually (again) this is what I think I need

mke2fs -b 4096 -t largefile4 ext3

right?

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LVL 22

Expert Comment

by:pjedmond
ID: 16936510
To be fair:

mkfs.ext3 /dev/md0

is equivalent to:

mkfs -t ext3 /dev/md0

Anyway, block size is set in /etc/raidtab. There is a directive:

chunk-size             16

Which would set the chunksize to 16k. Chunk size *must* be a power of 2. So 2,4,8,16,32,64,128 etc are all valid. Optimum chunk size is dependent of disc speed, disk-cache, bus speed, availability of DMA, and interface type! As a result it is extremely difficult to calculate the optimum size. However, if the system is running with a > 3GHz processor, and a very high speed disc (15000rpm SCSI) for example, then I might be tempted to go for a 64k chunksize for the large file reads that you are specifying. However, for lesser systems, I'd probably stick at something lower. For most practical applications, 8 or 16 is more than ample. Also bear in mind that there is no point in extracting the data from the hard drive(or buying the highest speed hard drive or creating the highest spec RAID array) if the bottlenet to getting the information is a 10/100 ethernet card!

HTH:)



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Accepted Solution

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pjedmond earned 2000 total points
ID: 16936559
OK - that directive would be valid....but bear in mind that 'journalling' the ext3 bit is irrelavent if the drive is going to be read only.

.....and the 'switch' is a capital T, not a small one for file type. So the command should be:

mke2fs -j -b 4096 -t largefile4
 
(You can miss out the -j to stop the file system being a journalling(ext3) fs) or

mkfs.ext3 -b 4096 -t largefile4

Try using man:

man mke2fs
man mkfs.ext3

for more info and the various options available to you).

HTH:)

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Author Comment

by:hgj1357
ID: 16936866
Got it!
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