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Re: [HELP] RAID chunk-size - alternatives

On Wed, 3 Apr 2002 00:29, Alvin Oga wrote:
> > Chunk size does not matter for RAID-1, but does matter for other RAID
> > levels.
> humm ..thought was the otehr way ... time for me to go look at some
> raid source code i suppose .. when time permits

The chunk size determines physical location of the data for RAID-0 and RAID-5.
For RAID-1 both disks will have exactly the same data, which is the same as 
what you would have if you only had one disk.  So chunk size is not an issue.

> > > if your disk was partitioned as... 2K bytes/inode...
> >
> > You probably mean 2K blocks.  The number of bytes per inode just
> > determines the size of the inode tables.
> yuppers ... and a block is 512bytes ( aka a sector )

No.  A block is the allocation unit for the file system.  For ext2 that can 
be 1, 2, or 4K (but generally no-one uses 2K).  For ReiserFS it's always 4K.  
Other file systems have different options.

> > If you have lots of little files then if you want good write performance
> > then you want RAID-1 or RAID-10.  RAID-5 is the cheap alternative.
> hummm ..... thinking outloud....
> "cheap" is relative???
> 	- $$$ for disks vs "(usable) disk space lost to raid"

Cheap is when you cut corners to save money.

> typically a minimum of 2 disks used for raid0 or raid1...
> 	raid1(mirroring) protects against one disk failure
> 	( one disk's capacity is used as a redundant copy and not for user)
> 		( 50% lost of space )
> 	raid0(stripping) does not help for disk failures

Yes.  Also RAID-0 increases the probability of data loss unless you take 
other protective measures.  If during a particular time period the 
probability of one disk failing is 0.1, then the probability of a RAID-0 
failing is 1-(1-0.1)*(1-0.1) = 0.19.

> typically 5 disks for raid5 ...
> 	( 3 disks mininum -- 1/3 of your disks lost to parity
> 	( 4 disks .......... 1/4 of your disks lost to parity
> 	( 5 disks .......... 1/5 of your disks lost to parity

Another thing, you should have a separate cable for each disk you want to be 
independant.  So for RAID-1 you should have two cables so that a cable 
failure won't lose your data.  For a RAID-5 with 5 disks you want 5 cables.

One setup I've seen had 30 disks with 5 RAID-5 arrays.  There were 6 cables, 
each RAID-5 array had 5 data disks and one spare disk on separate cables.

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