Re: recommended way to add ssd cache to mdraid array

[Roy Sigurd Karlsbakk <roy@karlsbakk.net>]

> > uh… how can the filesystem chosen be relevant to the disk elevator?
> 
> It's the other way round. The chosen elevator can cause problems with
> the filesystem. You should find this relevant conversation amongst the
> lead XFS developers educational:
> 
> http://oss.sgi.com/archives/xfs/2011-07/msg00464.html

This article is about using CFQ with RAID controllers with write cache, which isn't something I'd recommend. CFQ is, if I'm not mistaken, good if Linux has local access to each disk, and the disks are spinning (!SSD).

> > CFQ will try to optimize access to reduce seeks
> 
> "Completely Fair Queuing" -- The name alone tells you how it works. It
> most certainly does not do what you state. Please read the brief
> Wikipedia article: http://en.wikipedia.org/wiki/CFQ

It's how it's designed, for spinning rust.

> > and should be completely independent on the filesystem used on top.
> 
> Operative word: "should"
> 
> The USA should not be $16 Trillion in debt, but it is. By
> international
> law whales should not be killed, but they still are. Etc.

Well, most things don't work as they are intended to do, but I guess CFQ works pretty well for spinning rust directly attached to a Linux box.

The default scheduler was changed in kernel, but only because SSDs are getting more common. Would those kernel developers know their things?

Vennlige hilsener / Best regards

roy
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Roy Sigurd Karlsbakk
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