[gpfsug-discuss] Compression question

[Cregan, Bob]

Just to clarify this is SS compression, so

mmchattr --compression yes <filename>

or an ILM equivalent

So not a regular modification.

Bob



Bob Cregan
HPC Systems Analyst

Information & Communication Technologies

Imperial College London,
South Kensington Campus London, SW7 2AZ
T: 07712388129
E: b.cregan at imperial.ac.uk

W: www.imperial.ac.uk/ict/rcs<http://www.imperial.ac.uk/ict/rcs>

[1505984389175_twitter.png] @imperialRCS @imperialRSE

[1505983882959_Imperial-RCS.png]


________________________________
From: Cregan, Bob
Sent: 28 November 2019 09:43
To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Subject: Compression question

Hi All,
           Can someone answer the following question on compression in a snapshot context? We have tried various experiments and they are inconclusive - too tedious to go into the detail.

What happens to the snapshot when a file is compressed in SS? The logic as I see it is

####### In a non compressed situation ###############

1) create a file,
2) create a snapshot.
3) modify a file in a normal way - the blocks on disk are changed and the old blocks are then written to the snap.


######In a compressed situation ############


1) create a file,
2) create a snapshot.
3) Compress the file. Now IBM says the blocks are rewritten when the file is compressed. So do the old uncompressed blocks go into the snap? If so we now have 2 copies of the file and unless the compression > 50%  we have used more space until the snap is deleted.

You get the space back in the end, but if you are in a tight situation then potentially compression might not work for you in the short term.

Thanks

Bob


Bob Cregan
HPC Systems Analyst

Information & Communication Technologies

Imperial College London,
South Kensington Campus London, SW7 2AZ
T: 07712388129
E: b.cregan at imperial.ac.uk

W: www.imperial.ac.uk/ict/rcs<http://www.imperial.ac.uk/ict/rcs>

[1505984389175_twitter.png] @imperialRCS @imperialRSE

[1505983882959_Imperial-RCS.png]

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