[Lustre-discuss] Removing an OST

[Thomas Roth]

Andreas, thank you very much for this log sought-after explanation!

As far as I have read manuals and the mailing list, no one ever put these two commands side by side.
 One would only find the advice to "make the OST read-only" - and the how-to side I also only found
the "lctl --conf_param..." version.

I think anyhow the meaning and syntax of the various set_param commands is never really explained in
the manual?

Regards,
Thomas

Andreas Dilger wrote:
> On Jan 16, 2009  13:45 +0100, Heiko Schroeter wrote:
>> the manual (May 2008) describes on page 4-14 how to remove an OST from LUSTRE.
>> Its says "deactivate the OST (make it read-only).
>>
>> Hm, i'am a bit puzzled here.
>> When i deactivate the OST on the MDS using
>> 'lctl --device 18 conf_param foo-OST000d.osc.active=0' the device is 
>> deactivated but i cannot read any files from it any longer. Quite logical.
> 
> Ah, it looks like you are confusing two different commands here.
> By using "lctl conf_param ..." you are storing this setting in the
> filesystem-wide configuration file, permanently deactivating the device
> on all nodes.
> 
> If you just want to deactivate the OST only on the MDS you can use:
> 
> 	lctl set_param osc.foo-OST000d.active=0
> 
> This means the MDS will not allocate any new objects on this OST.  It will
> still be active on all of the clients, so they can read or write to this
> file, or delete it.
> 
>> On the other hand using lctl "readonly" option is specified as "dangerous".
> 
> Yes, this isn't something you should be using - it is for regression
> testing only and doesn't do what you want.
> 
>> So what is the correct approach of replacing an OST and/or what is the
>> correct syntax for lctl (or whatever command) putting a device in
>> read-only mode ?
>>
>> When the OST is in read-only mode and i "recopy" the file over the original 
>> one what happens to the directory structure?
> 
> If the OST is deactivated on only the MDS, but not the clients, then the
> deleted file will be cleaned up correctly on the OST.
> 
>> Is it necessary to recreate 
>> it? Or is it enough to do something like this (mentioned under 3. on the 
>> above lustre manual page):
> 
> 
>> cp /lustrefs/dir1/foo/dat.file    /tmp/.
>> cp /tmp/dat.file     /lustrefs/dir1/foo/.
> 
> Using "cp" likely isn't what you want to do, because "cp" doesn't change
> the inode (object) allocation (even on local filesystems).  More likely
> you want to do:
> 
> 	cp .../dat.file .../dat.file.tmp
> 	mv .../dat.file.tmp .../dat.file
> 
> This will create a new file (not on the OST deactivated at the MDS),
> and delete the old one when the new one is moved over top of it.
> 
> Once you have moved all of your files off of this OST then it is safe
> to remove and the "conf_param" command can be used to deactivate it
> permanently.
> 
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
> 
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> Lustre-discuss mailing list
> Lustre-discuss at lists.lustre.org
> http://lists.lustre.org/mailman/listinfo/lustre-discuss

-- 
--------------------------------------------------------------------
Thomas Roth
Gesellschaft für Schwerionenforschung
Planckstr. 1                -         64291 Darmstadt, Germany
Department: Informationstechnologie
Location: SB3 1.262
Phone: +49-6159-71 1453  Fax: +49-6159-71 2986

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