questions regarding file-system optimization for sortware-RAID array

[Martin T]

Hi,

I created a RAID1 array of two physical HDD's with chunk size of 64KiB
under Debian "wheezy" using mdadm. As a next step, I would like to create
an ext3(or ext4) file-system to this RAID1 array using mke2fs utility.
According to RAID-related tutorials, I should create the file-system like
this:

# mkfs.ext3 -v -L myarray -m 0.5 -b 4096 -E stride=16,stripe-width=32
/dev/md0


Questions:

1) According to manual of mke2fs, value of the "stride" has to be the RAID
chunk size in clusters. As I use chunk size of 64KiB, then I have to use
"stride" value of 16(16*4096=65536). Why is it important for file-system to
know the size of chunk used in RAID array? I know it improves the I/O
performance, but why is this so?

2) If the "stride" size in my case is 16, then the "stripe_width=" is 32
because there are two drives in the array which contain the actual data.
Manual page of the mke2fs explain this option as "This allows the block
allocator to prevent read-modify-write of the parity in a RAID stripe if
possible when the data is written.". How to understand this? What is this
"read-modify-write" behavior? Could somebody explain this with an example?


regards,
Martin
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