Re: [gpfsug-discuss] Compression question

[Luis Bolinches]

Hi

 

Same principle COW. The data blocks do not get modified.

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----- Original message -----
From: "Cregan, Bob" <b.cre...@imperial.ac.uk>
Sent by: gpfsug-discuss-boun...@spectrumscale.org
To: gpfsug main discussion list <gpfsug-discuss@spectrumscale.org>
Cc:
Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question
Date: Thu, Nov 28, 2019 12:06 PM
 

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.cre...@imperial.ac.uk

W: www.imperial.ac.uk/ict/rcs

 @imperialRCS @imperialRSE

 

 

 

 

 

---

From: Cregan, Bob
Sent: 28 November 2019 09:43
To: gpfsug main discussion list <gpfsug-discuss@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.cre...@imperial.ac.uk

W: www.imperial.ac.uk/ict/rcs

 @imperialRCS @imperialRSE

 

 

 

 

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