[gpfsug-discuss] Compression question
Luis Bolinches
luis.bolinches at fi.ibm.com
Thu Nov 28 13:00:22 GMT 2019
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 <b.cregan at imperial.ac.uk> 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: b.cregan at imperial.ac.uk
> W: www.imperial.ac.uk/ict/rcs
> <Outlook-1505984389.png>
> @imperialRCS @imperialRSE
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>
>
> From: gpfsug-discuss-bounces at spectrumscale.org <gpfsug-discuss-bounces at spectrumscale.org> on behalf of Daniel Kidger <daniel.kidger at uk.ibm.com>
> Sent: 28 November 2019 12:30
> To: gpfsug-discuss at spectrumscale.org <gpfsug-discuss at spectrumscale.org>
> Cc: gpfsug-discuss at spectrumscale.org <gpfsug-discuss at spectrumscale.org>
> Subject: Re: [gpfsug-discuss] Compression question
>
> Caution - This email from daniel.kidger at uk.ibm.com 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
> daniel.kidger at uk.ibm.com
>
>
>
>
> ----- Original message -----
> From: "Alexander Wolf" <A.Wolf-Reber at de.ibm.com>
> Sent by: gpfsug-discuss-bounces at spectrumscale.org
> To: gpfsug-discuss at spectrumscale.org
> 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
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> a.wolf-reber at de.ibm.com
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>
> ----- Original message -----
> From: "Luis Bolinches" <luis.bolinches at fi.ibm.com>
> Sent by: gpfsug-discuss-bounces at spectrumscale.org
> To: gpfsug-discuss at spectrumscale.org
> Cc: gpfsug-discuss at spectrumscale.org
> 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
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> "If you always give you will always have" -- Anonymous
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>
>
> ----- Original message -----
> From: "Cregan, Bob" <b.cregan at imperial.ac.uk>
> Sent by: gpfsug-discuss-bounces at spectrumscale.org
> To: gpfsug main discussion list <gpfsug-discuss at 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.cregan at imperial.ac.uk
> W: www.imperial.ac.uk/ict/rcs
> <Image.f7e9a6a7-c4ab-4a2c-a979-4513774aa054.png>
> @imperialRCS @imperialRSE
> <Image.dbd2fd87-b9f2-4b15-903f-c74c7b7298d3.png>
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>
>
>
>
> From: Cregan, Bob
> Sent: 28 November 2019 09:43
> To: gpfsug main discussion list <gpfsug-discuss at 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.cregan at imperial.ac.uk
> W: www.imperial.ac.uk/ict/rcs
> <Image.ba38b5d6-12d6-476a-bcd8-7d647bfab5b3.png>
> @imperialRCS @imperialRSE
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>
>
>
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