Re: recommended way to add ssd cache to mdraid array

[Thomas Fjellstrom <thomas@fjellstrom.ca>]

On Sat Jan 12, 2013, Stan Hoeppner wrote:
> On 1/11/2013 8:44 PM, Thomas Fjellstrom wrote:
> > On Fri Jan 11, 2013, Stan Hoeppner wrote:
> >> On 1/11/2013 12:51 PM, Thomas Fjellstrom wrote:
> >>>> It's not but as Stan writes it may not be preferred for your
> >>>> application.
> >>> 
> >>> Hm, ok, it is in my mount line, so I'll try it with that off. Though I
> >>> would be interested in hearing from Stan why it may not be good.
> >> 
> >> You should not do this.  This parameter is not a toggle switch intended
> >> to be flipped on/off at will, but changed only once and left there.
> > 
> > Makes me wonder why they are mount options if they aren't meant to ever
> > be changed?
> 
> I'll answer that question with a question:  If you were to implement a
> new (secondary) allocator on a 10 year old filesystem, by what mechanism
> would you have the user enable it?
> 
> You can't change the allocator while the filesystem is mounted, so you
> can't do this with a sysctl.  So if you must remount the filesystem to
> enable the new allocator, where do you enable it?
> 
> Make sense yet?

Could have a tool like tune2fs, if you have to remount anyway, putting the non 
option option behind a tool would make it less dangerous for people who don't 
know they aren't actually changeable.

> >> Never change XFS parameters willy-nilly without knowing the
> >> consequences.  And currently you certainly do not know them.
> >> 
> >> I don't have time for the detailed explanation.  Ask on the XFS list.
> > 
> > Alright, thanks for your help :)
> 
> And as the last few times over many months, you never post to the XFS
> list.  Which tells me you really don't care to learn this stuff.

I don't think theres a reason to be hostile. I had been meaning to join the 
XFS list, but usually, I get so far into looking into this stuff, before I run 
out of time and have to get back to work. As it is this new NAS box has been 
sitting on the floor doing nothing for a month or so waiting for me to have 
time to finish configuring it.

-- 
Thomas Fjellstrom
thomas@fjellstrom.ca