Hi
5.0.2 with a hotfix for a lz4 compression bug
Bob
Bob Cregan
HPC Systems Analyst
Information & Communication Technologies
Imperial College London,
South Kensington Campus London, SW7 2AZ
T: 07712388129
E: [email protected]
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________________________________
From: [email protected]
<[email protected]> on behalf of Luis Bolinches
<[email protected]>
Sent: 28 November 2019 13:00
To: gpfsug main discussion list <[email protected]>
Subject: Re: [gpfsug-discuss] Compression question
Caution - This email from [email protected] originated outside Imperial
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<http://www.imperial.ac.uk/ict/rcs>
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@imperialRCS @imperialRSE
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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
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----- 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
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* Dr. Alexander Wolf-Reber
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[email protected]
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----- 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
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Mobile Phone: +358503112585
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----- 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<http://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<http://www.imperial.ac.uk/ict/rcs>
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@imperialRCS @imperialRSE
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