Ext3 users - Mar 2014 - questions regarding file-system optimization for sortware-RAID array

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

Note that stride and stripe width only make sense for RAI-5/6 arrays. 
For RAID-1 it doesn't really matter. 

Cheers, Andreas
> On Mar 6, 2014, at 13:46, Martin T <m4rtntns@gmail.com> wrote:
> 
> 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
> _______________________________________________
> Ext3-users mailing list
> Ext3-users@redhat.com
> https://www.redhat.com/mailman/listinfo/ext3-users

Andreas,

why is it relevant only in case of RAID5 or RAID6?


regards,
Martin

On Fri, Mar 7, 2014 at 5:57 PM, Andreas Dilger <adilger@dilger.ca>
wrote:
> Note that stride and stripe width only make sense for RAI-5/6 arrays.
> For RAID-1 it doesn't really matter.
>
> Cheers, Andreas
>
>> On Mar 6, 2014, at 13:46, Martin T <m4rtntns@gmail.com> wrote:
>>
>> 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
>> _______________________________________________
>> Ext3-users mailing list
>> Ext3-users@redhat.com
>> https://www.redhat.com/mailman/listinfo/ext3-users