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Re: DMAPI: DM attribute/region space allocation

To: James A Goodwin <jagoodwi@xxxxxxxxxx>
Subject: Re: DMAPI: DM attribute/region space allocation
From: Nathan Scott <nathans@xxxxxxx>
Date: Mon, 9 Feb 2004 10:53:12 +1100
Cc: roehrich@xxxxxxx, linux-xfs@xxxxxxxxxxx
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On Fri, Feb 06, 2004 at 01:42:50PM -0600, James A Goodwin wrote:
> Dean,

I think Deans away for a few days, let me try to answer...
(and get overruled by Dean when he returns ;)

> We have a problem when managing an XFS filesystem using the DMAPI.  When we
> copy data from an XFS file to our archive, we cover the file with a DMAPI
> region to inform us of read, write, and truncate events.  However, if the
> filesystem is full, we cannot establish this region, and we don't have a
> good way to recover.
> 
> To fix the problem, we plan to preallocate a DM region to each file when it
> is created.  This region would not be set to generate any I/O events but
> would serve only to preallocate storage.  We could later modify the region
> to generate I/O events.  If we couldn't create the region due to space
> constraints, we could just abort the file create.
> I have two questions.  First, will this approach work?

Seems like a valid approach to me.

> Second, will the
> pre-allocated region also preallocate space that could be used by DM
> attributes?

No - preallocated space is for file data only.

> I'm guessing that it will allocate one block, stick the region
> in there, and have lots of extra space for other extended attributes (i.e.,
> DM attributes).  I'm hoping that it will be enough to ensure that I can
> create, modify, and remove some number of DM attributes without having to
> worry about space shortages, but I have no idea how much space I'll have to
> play with.

I believe SGIs HSM uses the XFS RESBLKS interface to reserve a set
amount of filesystem space, which is only available when allocating
extended attribute space for the root namespace (where the DMAPI
attributes reside).

HTH.

cheers.

-- 
Nathan


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