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Re: md-RAID5/6 stripe_cache_size default value vs performance vs memory

To: stan@xxxxxxxxxxxxxxxxx
Subject: Re: md-RAID5/6 stripe_cache_size default value vs performance vs memory footprint
From: NeilBrown <neilb@xxxxxxx>
Date: Fri, 27 Dec 2013 09:14:07 +1100
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, Arkadiusz Mi??kiewicz <arekm@xxxxxxxx>, linux-raid@xxxxxxxxxxxxxxx, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52BBF5B0.8020206@xxxxxxxxxxxxxxxxx>
References: <52B102FF.8040404@xxxxxxxxxxx> <52B2FE9E.50307@xxxxxxxxxxxxxxxxx> <52B41B67.9030308@xxxxxxxxxxx> <201312202343.47895.arekm@xxxxxxxx> <52B57912.5080000@xxxxxxxxxxxxxxxxx> <20131226085510.GB32660@xxxxxxxxxxxxx> <52BBF5B0.8020206@xxxxxxxxxxxxxxxxx>
On Thu, 26 Dec 2013 03:24:00 -0600 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>

> On 12/26/2013 2:55 AM, Christoph Hellwig wrote:
> > On Sat, Dec 21, 2013 at 05:18:42AM -0600, Stan Hoeppner wrote:
> >> The powers that be, Linus in particular, are not fond of default
> >> settings that create a lot of kernel memory structures.  The default
> >> md-RAID5/6 stripe_cache-size yields 1MB consumed per member device.
> > 
> > The default sizing is stupid as it basically makes RAID unusable out
> > of the box, I always have to fix that up, as well as a somewhat
> > reasonable chunk size for parity RAID to make it usable.  I'm also
> > pretty sure I complained about it at least once a while ago, but never
> > got a reply.
> IIRC you Dave C. and myself all voiced criticism after the default chunk
> size was changed from 64KB to 512KB.  I guess we didn't make a strong
> enough case to have it reduced, or maybe didn't use the right approach.
> Maybe Neil is waiting for patches to be submitted for changing these
> defaults, and to argue the merits in that context instead of pure
> discussion?  Dunno.  Just guessing.  Maybe he'll read this and jump in.

Good guess.


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