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: [email protected] 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 <[email protected]> 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: [email protected] 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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