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Re: separate log and structure from user data device?

To: Jan Wagner <jwagner@xxxxxxxxxxx>
Subject: Re: separate log and structure from user data device?
From: Nathan Scott <nathans@xxxxxxx>
Date: Mon, 5 Jun 2006 20:55:29 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <Pine.LNX.4.58.0606050943180.13176@kurp.hut.fi>; from jwagner@kurp.hut.fi on Mon, Jun 05, 2006 at 09:49:10AM +0300
References: <Pine.LNX.4.58.0606050943180.13176@kurp.hut.fi>
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On Mon, Jun 05, 2006 at 09:49:10AM +0300, Jan Wagner wrote:
> Hi,
> 
> I saw with the logdev parameter it is possible to specify an external log
> device on a separate disk and partition.
> 
> What would be even more interesting for my special purposes is whether
> even the file system structure (inodes etc) could be placed on a
> different disk?

The realtime subvolume will indeed give you this split.  See xfs(5)
and mkfs/xfs(8) where most doco resides on this.

> Rationale being, when one wants to build a data recorder like a
> Linux personal video recorder that is using A/V harddisks (ATA Streaming
> Feature Set, or SmoothStream), one could use the A/V disk with A/V
> streaming enabled to unreliably(!) write or read all the user data, and a
> second disk to reliably store the actual log and file system structure on.
> 
> Yes, there is alreayd the realtime subvolume in XFS, but can it tolerate
> unreliable A/V read/write? (i.e., where drive has been told to disable
> and skip all read error correction and write verification?)

Not 100% sure what unreliable means here from a software POV... would
we be seeing errors at the filesystem layer on IOs to/from the driver?
If not, I think it would work just fine.

cheers.

-- 
Nathan


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