Re: [gpfsug-discuss] Compression question

Alexander Wolf Thu, 28 Nov 2019 04:22:14 -0800

I just tested this. Compressing a file did free up space in the file system. Looks like our compression code does not trigger COW on the snapshot. You can test this yourself by looking into mmlssnapshot -d (please not on a large production fs, this command is expensive).

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----- Original message -----
From: "Luis Bolinches" <[email protected]>
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To: [email protected]
Cc: [email protected]
Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question
Date: Thu, Nov 28, 2019 11:47

Hi

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

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----- Original message -----
From: "Cregan, Bob" <[email protected]>
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To: gpfsug main discussion list <[email protected]>
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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,
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W: www.imperial.ac.uk/ict/rcs

@imperialRCS @imperialRSE

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

@imperialRCS @imperialRSE

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Re: [gpfsug-discuss] Compression question

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