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

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


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