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Forum Discussion
Striping granularity of RAID0
Merry x-mas, dear community!
On my RN104 guineapig I have created a RAID0 array and would like to know what chunk size determines the striping granularity. Researching turns out, that "normally...
Merry Christmas!
Retired_Member wrote:
On my RN104 guineapig I have created a RAID0 array and would like to know what chunk size determines the striping granularity.
There are a couple of ways to find this:
- It's in the log zip file (mdstat.log).
- # cat //proc/mdstat
- # mdadm --detail /dev/md127
It is set to 64K for RAID-5, so it likely is the same for RAID-0. Let us know what you find out.
Merry Christmas!
Retired_Member wrote:
On my RN104 guineapig I have created a RAID0 array and would like to know what chunk size determines the striping granularity.
There are a couple of ways to find this:
- It's in the log zip file (mdstat.log).
- # cat //proc/mdstat
- # mdadm --detail /dev/md127
It is set to 64K for RAID-5, so it likely is the same for RAID-0. Let us know what you find out.
Yup, 64K it is for RAID0, StephenB. For RAID1 it is also 64K, as I can observe.
What is your opinion on the following:
Assume you have a system doing nothing else than streaming HD videos from the concerned volume/share. A backup of all data is conducted on a regular base, of course.
Would it make sense to use RAID0 and a higher chunksize than 64K on that volume to improve the responsiveness of the nas in case lets say multiple people try to watch movies on different clients (sometimes it might even be the same movie)?
And, ...If you would want to go with a higher chunkzise on that volume, though, what would you do to achieve this?
Retired_Member wrote:
What is your opinion on the following:
Assume you have a system doing nothing else than streaming HD videos from the concerned volume/share. A backup of all data is conducted on a regular base, of course.
Would it make sense to use RAID0 and a higher chunksize than 64K on that volume to improve the responsiveness of the nas in case lets say multiple people try to watch movies on different clients (sometimes it might even be the same movie)?
It's not something I've ever benchmarked, so I can only guess.
In the specific case of streaming video, I think it would depend on the disk i/o read size(s) used by the streaming server. With multiple sequential reads going on simultaneously, the reads likely will end up out of phase automatically (e.g, disk 1's stripe being read for stream 1, with disk 2's stripe being read at about the same time for stream 2).
Retired_Member wrote:
And, ...If you would want to go with a higher chunkzise on that volume, though, what would you do to achieve this?
There is an mdadm command you can try - not sure if it will work:
# mdadm --grow --chunk=128 /dev/md127would change it to 128KB (if it works).
StephenB wrote: "There is an mdadm command you can try"
If it would work, will there be some kind of "re-chunking"? Should say, data in 64K chunks will be redistributed to newly created 128K chunks, or is it keeping the occupied 64K chunks and only grows the empty 64K chunks into 128K chunks. Could we understand that as some kind of "resyncing" a RAID0 array?
I understand, that RAID0 is not truely RAID, because of the missing redundancy, but growing the chunksize to me would require some reorganising of chunks. Your input is most welcome, I just want to manage my expectations as much as possible :-)
I will wait for a few hours for your answer, StephenB, before I start. Thanks in advance.
Retired_Member wrote:
If it would work, will there be some kind of "re-chunking"?
Yes, if the command is allowed it would need to re-chunk.
Retired_Member wrote:
is it keeping the occupied 64K chunks and only grows the empty 64K chunks into 128K chunks.
Empty/Occupied is a file system concept, and RAID runs below the file system (providing raw block storage on a virtual disk). MDADM has no idea if chunks are "occupied" or "empty".
Retired_Member wrote:
RAID0 is not truely RAID, because of the missing redundancy
Technically it is just RAID without parity blocks. Personally I would still call it RAID, even though the "R" in the acronym ("Redundant") doesn't apply.
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