Re: LVM performance

[Oliver Martin <oliver.martin@student.tuwien.ac.at>]

Michael Guntsche schrieb:
> That's exactly what I ment sorry for not being clear enough.
> If you have a chunk size of 128KB it makes sense to align the beginning 
> of the PV to this as well.

I was talking about stripe size, not chunk size. That 128KB stripe size 
is made up of n-1 chunks of an n-disk raid-5. In this case, 3 disks and 
64KB chunk size result in 128KB stripe size.

I assume if you tell the file system about this stripe size (or it 
figures it out itself, as xfs does), it tries to align its structures 
such that whole-stripe writes are more likely than partial writes. This 
means that md only has to write 3*64KB (2x data + parity).

If a write touches both (data_chunk_1 + offset) and (data_chunk_2 + 
offset), you can calculate (parity_chunk + offset) without reading 
anything. If it doesn't change all data chunks, you have to read either
* the current parity
* the data chunk(s) to be changed
* all other data chunks
to calculate parity.

So, if this 128KB write is offset by half a stripe, md has to read one 
of the chunks from each stripe prior to writing so it can update parity. 
Also, there are two parity chunks to write. So that's 2*64KB read + 
4*64KB write.

That's what I meant with stripe-aligning the PV (and thus the LV and 
thus the file system).

-- 
Oliver