Re: recommendations for stripe/chunk size

[Neil Brown <neilb@suse.de>]

On Wednesday February 6, davidsen@tmr.com wrote:
> Keld Jørn Simonsen wrote:
> > Hi
> >
> > I am looking at revising our howto. I see a number of places where a
> > chunk size of 32 kiB is recommended, and even recommendations on
> > maybe using sizes of 4 kiB. 
> >
> >   
> Depending on the raid level, a write smaller than the chunk size causes 
> the chunk to be read, altered, and rewritten, vs. just written if the 
> write is a multiple of chunk size. Many filesystems by default use a 4k 
> page size and writes. I believe this is the reasoning behind the 
> suggestion of small chunk sizes. Sequential vs. random and raid level 
> are important here, there's no one size to work best in all cases.

Not in md/raid.

RAID4/5/6 will do a read-modify-write if you are writing less than one
*page*, but then they often to read-modify-write anyway for parity
updates.

No level will every read a whole chunk just because it is a chunk.

To answer the original question:  The only way to be sure is to test
your hardware with your workload with different chunk sizes.
But I suspect that around 256K is good on current hardware.

NeilBrown
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