ADSM-L

Re: Help! AIX HSM problem

2000-03-22 08:46:27
Subject: Re: Help! AIX HSM problem
From: Richard Sims <rbs AT BU DOT EDU>
Date: Wed, 22 Mar 2000 08:46:27 -0500
>  Large disk blocks require 32 contiguous 4KB blocks. If you write to
>  large files beyond the 4MB, file offset will fail with ENOSPC if the
>  file system does not contain 32 unused contiguous 4KB blocks.

John - Good detective work.  Based upon your finding I looked further in
       InfoExplorer to get additional perspective.  In the article
"File Space Allocation" I found the statement:

    A "large" fragment is made up of 32 contiguous 4096-byte
    fragments. Because of this requirement, it is recommended that file
    systems enabled for large files have predominantly large files in
    them. Storing many small files (files less than 4 MB) can cause free-space
    fragmentation problems. This can cause large allocations to fail with
    ENOSPC because the file system does not contain 32 contiguous disk
    addresses.

And of course, though it seems intuitive to set up a large file system to
accept the large files we'd populate them with, when HSM migrates them we end
up small stub files and the the fragmentation as described.  It would in this
case be "cute" if we could define the stub size to be 4 MB, but the limits are
much smaller.

The implication is that we should not enable Large Files in AIX file systems
to be used with HSM.  There is a dilemma, though: if the file system is *not*
enabled for Large Files, that leaves no way to get large files into it!  And
the compelling reason to use HSM in the first place is to save all that file
system space that large files would create if left there unmigrated.  The
upshot of all this is that HSM cannot be realistically used for large files,
and that in AIX at least, the maximum file size that can be used with HSM is
2 GB.  This is not a very satisfying reality.  But it's good to be fully aware
of this now.

I scanned the PDF version of the HSM manual for the word "large", and found no
advisory about this.  (Interestingly, in the TSM Publications web page, the
name of HSM is Tivoli Space Manager; but the manual you find there is the old
ADSM version 3.1 copy.)  Also, the HSM redbook has not been updated since it
was written in 1996, which predates AIX 4.2 Large Files support.  I'm going to
submit requests to the documentation people that the advisories be added to
these publications.

     Richard Sims, BU
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