Re: [gpfsug-discuss] Compression question

Alexander Wolf Thu, 28 Nov 2019 06:04:25 -0800

I see the same behavior of mmlsattr on my system (with some post 5.0.4 development build). Funny enough if I look at the file content in the snapshot it gets properly decompressed.

Mit freundlichen Grüßen / Kind regards

Dr. Alexander Wolf-Reber
Spectrum Scale Release Lead Architect
Department M069 / Spectrum Scale Software Development

+49-160-90540880
[email protected]

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----- Original message -----
From: "Luis Bolinches" <[email protected]>
Sent by: [email protected]
To: "gpfsug main discussion list" <[email protected]>
Cc:
Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question
Date: Thu, Nov 28, 2019 14:00

Which version are you running?

I was involved on a big for compressed file sets and snapshots that were related to what you see.

--
Cheers

On 28. Nov 2019, at 14.57, Cregan, Bob <[email protected]> wrote:

Hi

Sounds logical - except the snap metadata does not have the compression flag set. So if the inode now points to a set of compressed blocks how does the client know to decompress it?

After compression of an existing file we get in the snap

-bash-4.2$ mmlsattr -L .snapshots/@GMT-2019.11.27-19.30.14/UserGuide_13.06.pdf

file name: .snapshots/@GMT-2019.11.27-19.30.14/UserGuide_13.06.pdf

metadata replication: 2 max 2

data replication: 1 max 3

immutable: no

appendOnly: no

flags:

storage pool name: sata1

fileset name: userdirs

snapshot name: @GMT-2019.11.27-19.30.14

creation time: Tue Mar 5 16:16:40 2019

Misc attributes: ARCHIVE

Encrypted: no

and the original file is definitely compressed.

-bash-4.2$ mmlsattr -L UserGuide_13.06.pdf

file name: UserGuide_13.06.pdf

metadata replication: 2 max 2

data replication: 1 max 3

immutable: no

appendOnly: no

flags:

storage pool name: sata1

fileset name: userdirs

snapshot name:

creation time: Tue Mar 5 16:16:40 2019

Misc attributes: ARCHIVE COMPRESSION (library z)

Encrypted: no

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

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From: [email protected] <[email protected]> on behalf of Daniel Kidger <[email protected]>
Sent: 28 November 2019 12:30
To: [email protected] <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: [gpfsug-discuss] Compression question

Caution - This email from [email protected] originated outside Imperial

Alexander,

Can you then confirm then that the inodes in the snapshot will now point to fewer but compressed blocks ?

Daniel

_________________________________________________________
Daniel Kidger

IBM Technical Sales Specialist

Spectrum Scale, Spectrum NAS and IBM Cloud Object Store

+44-(0)7818 522 266
[email protected]

----- Original message -----
From: "Alexander Wolf" <[email protected]>
Sent by: [email protected]
To: [email protected]
Cc:
Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question
Date: Thu, Nov 28, 2019 12:21

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).

Mit freundlichen Grüßen / Kind regards

<Image.15749424191280.png>

<Image.15749424191281.png>

Dr. Alexander Wolf-Reber
Spectrum Scale Release Lead Architect
Department M069 / Spectrum Scale Software Development

+49-160-90540880
[email protected]

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IBM Data Privacy Statement

IBM Deutschland Research & Development GmbH / Vorsitzende des Aufsichtsrats: Matthias Hartmann / Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294

----- Original message -----
From: "Luis Bolinches" <[email protected]>
Sent by: [email protected]
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.

--
Ystävällisin terveisin / Kind regards / Saludos cordiales / Salutations / Salutacions
Luis Bolinches

Consultant IT Specialist
Mobile Phone: +358503112585
https://www.youracclaim.com/user/luis-bolinches

"If you always give you will always have" -- Anonymous

----- Original message -----
From: "Cregan, Bob" <[email protected]>
Sent by: [email protected]
To: gpfsug main discussion list <[email protected]>
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: [email protected]

W: www.imperial.ac.uk/ict/rcs

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@imperialRCS @imperialRSE

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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

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@imperialRCS @imperialRSE

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Re: [gpfsug-discuss] Compression question

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